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Why the Virgo Cluster Is Slowly Killing Every Galaxy Inside It
Added:Hey guys, the response recently has been amazing. Thank you. We are almost at a point where this channel can focus purely on content. If you want to help get there, Patreon link is in the description. There is a particular kind of quiet that settles in late at night.
Not silence exactly, more like the world pulling its attention inward, away from you, leaving you with whatever you are already thinking about. And if you were not thinking about anything specific, which is maybe the best state to be in, then something else tends to move in.
Something slow, something old, that is where this starts. Before we go any further, I want to say wow. Just wow.
Over the last few videos, I talked about where this channel stands, and I asked if anyone wanted to help make it stable.
What happened after that, I was not prepared for. Almost every day since then, a new member has joined on Patreon. Every morning, I wake up and see that notification and I just smile.
I cannot explain what that feels like.
And what made it even more surprising is that I had made the entry tier much more accessible. And I noticed many of you went for the higher tiers anyway. I was not expecting that at all. But what gets me even more than the sport itself is the cutter people who are part of this community.
I want to tell you about Jake. He is one of the biggest supporters of this project, works as a railroad signal maintainer, and I've had some genuinely amazing conversations with him. That is the kind of community this is. People who actually care and people worth talking to. The other major supporters are mentioned below. And I mean it when I say I love having conversations with you guys, so hit me up whenever you feel like it. Guys, we are moving at a pace I genuinely did not expect, but we are still behind and 200 still ahead of us.
What I will say is this. If you guys keep this up, we are going to get there.
And when we do, the sleepy physicist will no longer be struggling to survive.
Thank you to everyone already part of this. And if you want to be part of it, too, the Patreon link is in the description.
Now, the Virgo cluster, it is 53.8 8 million light years away. That number gets written down a lot in astronomy, spoken quickly, passed over as though the mind can actually hold it. It cannot. No mind can really. But there is something useful in sitting with it for a moment before moving on and not trying to picture it. Just letting it exist as a fact that is too large to fully enter the room. 53 million light years. The light arriving at your eye tonight from the brightest galaxies in that cluster left before our earliest human ancestors walked upright before the continents looked anything like they do now. The cluster was already old when that light departed and it has continued being old every moment since. That is the thing about the Virgo cluster. It does not wait. It sits in the direction of the constellation Virgo. not inside the constellation, which is just a pattern we draw on the sky, but far behind it in the actual three-dimensional universe that the constellation accidentally points toward. And it is in the plainest possible terms enormous, something in the range of 1,300 to 2,000 galaxies depending on where you draw the boundary, which is itself a messy question because the cluster does not have clean edges. It bleeds outward.
Groups of galaxies trail off into the surrounding structure like well like something dissolving at its margins.
That analogy will matter later. The mass of the Virgo cluster the gravitational weight of the whole system is roughly 1.2 quadrillion solar masses which is 1.2 * 10 15th power suns. The number is less important than what it implies.
That the cluster holds everything inside it through sheer gravitational insistence. That individual galaxies moving through it are not free that they belong to it. The way a stone dropped into deep water belongs to the water, falling inward, pulled, and unable to simply decide otherwise.
At the heart of the cluster sits M87.
Most of you have probably encountered it by now. The giant elliptical galaxy whose central black hole was photographed in 2019. The first black hole image humans ever made. M87 is enormous even by galactic standards. Its mass somewhere around 5* 10 to the 13th solar masses surrounded by a vast x-ray emmitting halo of hot gas that extends well beyond the visible galaxy itself.
It is not quite at the geometric center of the cluster. The Virgo cluster is irregular. And that center is a complicated word to use, but it is close and its gravitational influence is deeply heavily real. Other galaxies feel matey7. They orbit around it and around the larger cluster potential at speeds that if you pause and actually try to absorb them and not slow at all. But here is the thing that I keep returning to, the part that feels important before anything else is said. The Virgo cluster is the gravitational nucleus of something even larger, the local supercluster, a structure roughly 100 million lightyears across that contains our galaxy, the Milky Way, as one unremarkable member among more than 100 galaxy groups. The Virgo cluster sits at the center of this supercluster and everything else, including us, exists in its outer regions. We are not separate from it. We are periphery.
The local group, which is the small collection of galaxies the Milky Way belongs to, experiences the Virgo clusters gravitational pull as a slow drift, a gentle leaning in its direction that astronomers call the Virgocentric flow. The cluster is pulling us at roughly 250 km/s in the direction of Virgo. We are in the loosest and most geological sense of the word falling.
Not fast, not in any way you would feel, but falling. It is the closest large galaxy cluster to us. The nearest place where hundreds of galaxies are packed into a relatively tight gravitational embrace, bathed in hot plasma, moving through each other's presence at speeds that make the word moving feel insufficient. And because it is nearest, it is the one we can actually study in detail. The one where individual galaxies resolve clearly enough to see what is happening to them. The one where astronomers have been able to point instruments and ask specifically what does a dense cluster environment actually do to the galaxies inside it.
That question is what this is about. Not abstractly, not in general terms in Virgo specifically right now or rather 53.8 8 million years ago, which is the only version of now available to us at that distance. The cluster is not in equilibrium. That matters. It is dynamically young, still assembling, still pulling in groups of galaxies from the surrounding filaments of the cosmic web. It consists of several subclusters, regions with their own local gravitational concentrations, which have not yet fully merged into one settled system. New galaxies are arriving. First time arrivals. Galaxies that have never before encountered the conditions inside a massive cluster. Galaxies that formed in the relative calm of the field and are now crossing into something their entire physical structure was not built to handle. And the space between them, not empty, nothing in the universe is quite empty, is full of something hot and invisible and pressing. That is what the next chapter is about. Most of the Virgo cluster is invisible. Not dark matter invisible, though that is also present. Also unlit, also doing its quiet gravitational work beneath everything. This is a different kind of invisible.
The space between the galaxies, the actual physical space, the stuff filling the gaps is a plasma so hot and so thin that it produces no visible light at all. It radiates in X-rays and X-rays do not reach the human eye. So the largest component of the cluster's ordinary matter by mass, by volume, by sheer physical dominance, is something our eyes pass through without registering.
That feels like it should be more unsettling than it is. The plasma is called the intracluster medium, the ICM.
And the first thing to understand about it, the thing that took me a while to properly absorb actually, is that it is not a trace, not a residue, not the faint whiff of something left over. It contains more ordinary matter than all the stars in all the clusters galaxies combined. The galaxies, the brilliant visible parts. The things we photograph and name and point telescopes at are the minority. The ICM is the majority. It is the medium. The galaxies are objects suspended inside it. The temperature of this plasma in the Virgo cluster's core runs to roughly 2.5 kilo electron volts which translates because kilo electron volts is a unit of energy and temperature and energy are convertible in a plasma at this scale to somewhere around 29 million Kelvin. In the outer regions, the temperature climbs between 3 and 4 kilo electron volts in places hotter at the edges than in the middle, which is its own strange wrinkle in the story that does not need to be resolved right now. What matters is the number itself, 29 million°.
The surface of the sun is about 5,800 Kelvin. So, the ICM is not slightly hot.
It is hot in a category that does not have a comfortable everyday comparison.
And yet the density is almost nothing. A few thousandths of a hydrogen atom per cubic cm near the cluster core. Further out that drops further to numbers that require scientific notation to write without embarrassment. It is simultaneously the hottest and one of the most dilute gases in the observable universe.
Which creates a contradiction. No, not a contradiction exactly, more of a tension between how dangerous it sounds and how thin it actually is. Stand inside it and you would feel nothing. The plasma would pass through you without friction, without burn, without consequence.
Individual particles are moving fast, but there are almost none of them in any given handful of space. The consequence only becomes real at scale, at the scale of a galaxy. The ICM did not start this way. It was made when matter in the early universe began collapsing under gravity into what would become clusters.
Gas fell inward from all directions, picking up speed and collided. These were not gentle collisions. Infalling streams of gas crashed into each other and into existing material at velocities that could exceed thousands of kilome/s.
The kinetic energy of that motion had to go somewhere and it went into heat thermalized. The gas was shock heated to millions of degrees and could not cool fast enough to collapse further. It settled loosely into an expanding sense of that word into a hot pressure supported atmosphere sitting inside the cluster's gravitational well. That process left behind the ICM and what has accumulated inside it since then is in a chemical sense interesting.
The plasma is not primordial hydrogen and helium. It contains iron, silicon, sulfur, oxygen, calcium, metals in the astronomers loose use of that word meaning anything heavier than helium.
Recent observations of the Virgo cluster using the XRISM satellite, highresolution X-ray spectroscopy, the kind that can actually resolve individual spectral lines, found super solar abundance ratios of silicon and sulfur relative to iron in the cluster core, which means the ICM is carrying a chemical record, a history written in atomic abundances of everything that has been ejected into it, expelled from galaxies over billions of years, supernovi winds, galactic outflows, and something else, something more direct and more violent that comes later in the story. The X-ray emission itself is worth pausing on. When a plasma is this hot, electrons and ions interact through a process called thermal breed when charged particles decelerate around each other. that produces a continuous X-ray glow. On top of that, highly ionized heavy elements emit their own line radiation at specific energies. So, the ICM glows in X-rays with a spectrum that is both structural, showing temperature and density across the cluster, and chemical, encoding what the plasma is made of. This is how the ICM was discovered. Not by looking for it directly because no one knew it was there, but by pointing early X-ray satellites at galaxy clusters in the 1970s and finding them far, far brighter in X-rays than the galaxies alone could explain.
Something between the galaxies was emitting. The intracluster medium revealed itself through its own heat.
The Virgo clusters ICM extends out to the viral radius roughly a mega parseek from the cluster center which is somewhere around 3.2 million lightyear and possibly beyond thinning as it goes its density falling off steeply with distance.
At the outskirts it becomes clumpy inhomogeneous groups of galaxies falling into the cluster for the first time bring their own gas. And that gas has not yet been fully thermalized into the surrounding medium. The edges of the cluster are ragged in this way. Not a clean wall, a zone of ongoing accretion where the ICM is still being built. Inside that zone, galaxies are moving. Some of them, as was already noted, at speeds of hundreds to over 1,000 km/s relative to the cluster mean. And they are not moving through emptiness. They are moving through this through the plasma through 29 million degrees of fits of hot stubborn gas that has nowhere to go and no obligation to move aside. The galaxy brings its own gas cold dense sitting in its disc forming stars doing what gas in a spiral galaxy does and the ICM presses back. Not through any intention. Nothing here is intentional, just physics. just the mechanical consequence of two different gas phases occupying the same space at different temperatures, different densities, different velocities, and governed by the same set of equations that govern every fluid that has ever moved anywhere. What happens next depends on how fast the galaxy is moving, how deep into the cluster it has fallen, and how much gas it carried in. None of those numbers are gentle. The word drifting gets used a lot when people talk about galaxies. It is not accurate. Not here. Not inside a cluster. The galaxies in the Virgo cluster are not drifting. They are not floating in some slow majestic procession through space. They are moving the way objects move when something very massive has been pulling at them for a very long time. They are falling. Some of them have been falling for billions of years, picking up speed, looping through the clusters gravitational potential on orbits that are not circular and not clean, elongated, tilted, carrying them deep into the cluster core and back out toward the edges in paths that can take hundreds of millions of years to complete. And during all of that, they are moving fast. How fast? Spiral and irregular galaxies in the Virgo cluster show a velocity dispersion. The spread of their speeds relative to the cluster mean of roughly 900 km/s.
That is a standard deviation, not a ceiling. Individual galaxies can be moving at over a,000 km/s relative to the cluster's average motion. NGC 4556 Naduisin 9, which will come up in detail later, has a line of sight velocity of about 1,300 km/s relative to the cluster mean. NGC 4388 moves at over 1,400 km/s relative to M87 and more than 2,800 relative to the nearby M86 subgroup. These are not outliers in any alarming sense. They're just what orbital mechanics produces inside a gravitational well this deep.
1,000 km/s.
You can try to picture that. Most people find it does not quite land, which is fair, but it travels at 300,000 km/s.
So, a galaxy moving at 1,000 is not relativistic, nor even close. But compared to anything in ordinary experience, a plane, a satellite, a spacecraft, it is in a different category entirely. The fastest human-made object ever launched left Earth's gravity at roughly 16 km/s.
These galaxies are moving at 60 times that. And they are not small objects.
They are systems containing billions of stars spread across tens of thousands of light years. The whole thing, all of it moving through the ICM. That is the part that matters here. The galaxies are not moving through empty space. They are moving through the intracluster medium.
That thin hot sermitting plasma sitting in the gravitational potential of the cluster like water filling a bowl.
The ICM does not move with the galaxies.
It has its own bulk motions, its own turbulence, but it is not in any useful sense getting out of the way. So, a galaxy falling through the cluster at a thousand km/s is from the perspective of its own gas experiencing something like a wind, a headwind coming from the direction of motion pressing against the galaxy's interstellar medium which is sitting in the disc doing its ordinary business of being gas. This is the setup. Not a metaphor yet, just physics.
The ram pressure that a galaxy feels and feels is a loose word, but accurate enough depends on two things. The density of the ICM at the galaxy's current location and the square of the galaxy's velocity relative to that medium. The velocity term is squared, which means doubling the speed does not double the pressure. It quadruples it.
Speed matters more than almost any other variable in this interaction. A galaxy falling fast through even a relatively thin region of ICM can experience more RAM pressure than a slowmoving galaxy in a denser region. The speed is doing most of the work and the speed is not a choice. It is a consequence of gravity.
A galaxy that fell into the cluster from outside, picking up speed as it descended through the potential well, arrives at its first paracentric passage, its closest approach to the cluster core, moving at its fastest, which is exactly when the ICM is also at its densest. Both variables are maximized simultaneously and they multiply. That is not an accident of timing. It is the geometry of gravitational collapse.
The worst moment for the galaxy in terms of what is about to happen to its gas is precisely the moment the cluster's own structure has been building toward.
There is something almost designed about it except nothing is designed. It is just the shape of the equations. The Virgo cluster adds a complication to this picture. It is not a settled relaxed system with a smooth symmetric ICM sitting calmly in the center. It is dynamically young. It still has subclusters, infalling groups, bulk motions in the ICM itself from ongoing assembly. The motion of any individual galaxy through the cluster is not a clean path through a uniform medium. The density varies. The ICM has its own internal flows. A galaxy's effective velocity relative to the surrounding plasma can be different from its velocity relative to the cluster as a whole. The interaction is messier than the simple version, but the simple version is real enough. A galaxy moving through a medium at 1,000 km/s or even 500 is doing something that it gas cannot simply ignore. The outer edges of the disc where the gas is thin and the gravitational hold is weakest are the first to feel it. And what they feel is pressure. Real physical fluid dynamic pressure. The same kind that makes a flag extend in a strong wind. Except the flag here is made of hydrogen and helium and trace molecules. And the wind is a plasma at 29 million°.
And the flag has been hanging there for billions of years, quietly forming stars. It does not get to keep doing that. The cluster does not announce what is coming. There is no transition, no warning coded in the physics. The galaxy falls. The pressure builds and somewhere in the outer disc, something that was bound becomes unbound. And gas that was part of a galaxy begins to be something else. What happens in that moment? And why is the next thing to understand there is a particular physical stubbornness to moving air. You notice it most in small undignified ways. A car window opened at speed. A door that will not close against a headwind. The way a sheet of paper held loosely becomes briefly insistently a problem. The air is not aggressive in any dramatic sense.
It is just there and it has mass. And when you move through it quickly enough, that mass pushes back with a force that is proportional to your speed squared, which is the part that surprises people because as doubling your speed does not just double the resistance, it quadruples it. The physics of moving through a medium is not linear. It is in that specific way unforgiving.
Ram pressure is that applied at galactic scale. When a galaxy moves through the intracluster medium, the ICM does not part for it. The plasma has no obligation to move, and it does not. The galaxy's interstellar medium, the gas sitting in the disc, threaded between the stars, doing the slow accumulating work of star formation, encounters the ICM headon, the way your hand encounters air through an open window, except the hand here is hundreds of thousands of light years across, and the wind is 29 million°, and the speed involved makes the comparison feel almost too small to be useful. almost the mechanism is the same. The physics does not change cuz the scale is extreme. The quantitative version of this was written down in 1972 by James Gunn and Jay Richard got in a paper about the infall of galaxies into clusters almost as a side observation rather than the centerpiece. They were thinking about what happens to gas when a galaxy moves through the ICM. and they derived a condition, a simple inequality actually, two competing pressures asked to face each other for when the stripping becomes effective. On the left side of the inequality sits the RAM pressure itself, the density of the ICM multiplied by the square of the galaxy's velocity relative to the plasma. On the right side sits the gravitational restoring force per unit area of the galaxy's disc which depends on the surface density of the stars and the surface density of the gas and the gravitational constant binding them together. When the left side exceeds the right, gas is stripped. When the right side wins, gas stays. That is the whole mechanism. The elegance of it is real.
And I think it deserves a moment before moving on because the gunot criterion has turned out to be one of those surprisingly durable pieces of physics.
Something derived from basic principles in a few lines that has held up against decades of increasingly detailed observations and simulations with only refinements, not replacements. The core inequality still works. The core physical picture is still accurate. What it says in terms of the galaxy rather than the equation is this. At any given radius from the galaxy's center, the gas is held in place by gravity. The stellar disc above and below it pulls it inward.
The gas's own self-gravity contributes.
The combined effect is a restoring pressure. The gas would have to overcome this to escape. And under ordinary circumstances, it does not try. It just sits there, forms stars, cycles through supernova, sits there again. But the restoring pressure is not the same everywhere in the disc. At the center, where the stellar density is highest and the gravitational grip is deepest, the restoring force is strong. Out in the periphery, where the stellar disc is thinned and the gas surface density is low, the restoring force is weak. thin gas at large radius held loosely. That is where the stripping begins. When the ram pressure, the ICM pressing against the face of the disc, exceeds the restoring force at some radius, the gas beyond that radius is no longer bound, not displaced gently, not slowly encouraged outward, removed.
The outer gas is pushed off the disc, accelerated away from the galaxy in the direction opposite to the motion and it joins the ICM or trails behind as a tail depending on how the stripping is proceeding and what phase the gas is in.
The radius at which the ram pressure exactly balances the restoring force is called the stripping radius. Everything outside that radius in principle goes.
Everything inside in principle stays. In practice, the picture is grainier than that. The gas disc is not a uniform sheet. It has dense clumps, molecular clouds, regions where the local surface density is high enough to resist stripping even at large radi and thinner intercloud gas that is more susceptible.
The stripping is not instantaneous.
It takes time and during that time the galaxy continues moving. The ICM density changes as the galaxy moves to different parts of the cluster and the geometry of the disc relative to the ICM wind shifts. Simulations show the process is turbulent at the interface. Kelvin Helm halts instabilities developing at the boundary between the coldest gas and the hot ICM wind shredding and mixing gas at the edges in a way that the simple analytic estimate does not fully capture. But the analytic estimate captures the gross behavior well enough that it predicted what observers eventually found.
Now the stellar disc. This is the part that initially seems strange and then seems obvious once the physics settles in. The stars are not stripped. A galaxy actively undergoing ram pressure.
stripping can have its gas disc dramatically truncated with huge tails of ionized hydrogen streaming behind it while its stellar disc remains symmetric and undisturbed. The same shape it had before, the same radial extent, the same smooth elliptical isopotes at the edge.
You can look at the stars and see nothing unusual. You have to look at the gas to see that something violent is happening. The reason is that ram pressure is a fluid interaction. It acts on gas because gas is a fluid, a medium that the ICM wind can press against and drag. Stars are not a fluid in that sense. They are discrete point masses, each one on its own gravitational orbit.
And the ICM is so thin, remember a few thousandths of a hydrogen atom per cm, that it has essentially no capacity to exert drag on individual stars. The mean-free path between particle collisions in the ICM is enormous.
Stars pass through it without friction, without deflection, without any measurable interaction. They keep their orbits. They keep their disc. The gas does not get to keep anything. There is something almost, no, I want to be careful with this. There's something that registers when you sit with it about the fact that you can have a galaxy in the process of losing the material it needs to survive. And the part you would most naturally look at, the part that glows, the stars, shows you nothing. The damage is happening in the invisible component, the gas, the raw material, the thing that would have become future generations of stars now leaving. And the speed of the galaxy is doing most of the work. Not just because RAM pressure scales with the velocity squared, which it does and which matters enormously, but because a faster galaxy reaches its closest approach to the cluster core, where the ICM density is highest, while still accelerating, so the peak ram pressure experienced during a first infall can be severe enough to strip the majority of the outer gas disc in a single pass.
Simulations using several different codes, including the illustrous TNG and Eagle cosmological models, consistently find that a significant fraction of the gas stripping in massive clusters happens at or near this first perentric passage. Not gradually over many orbits, once hard at the worst possible moment.
The gas that is stripped is not immediately gone. It trails. It forms tails. It can remain detectable for tens to hundreds of millions of years as it gradually mixes into the surrounding ICM, cooling or heating depending on local conditions, occasionally doing something unexpected in that trailing region that nobody quite predicted from the simple version of the mechanism. But the disc itself, the part of the galaxy that the gas left behind, is permanently smaller than it was. The stripping radius that day during that parasentric passage became the new outer edge of the galaxy's gaseous disc. Stars can still form inside it for a while, but the clock has started. The supply of new gas from outside from the top of the galaxy's own extended halo has also been cut, which is a separate mechanism and a slower one. The disc burns what it has.
It cannot be replenished from outside.
And from the outside, looking at the stars, you would not know any of this had happened. A galaxy undergoing ram pressure stripping does not announce it.
From a sufficient distance, and with the wrong instruments, a stripped spiral can look entirely ordinary. The stars are where they should be. The disc has its familiar shape, its gentle brightening toward the center, its smooth fall off at the edges. The spiral arms, if they are still present, still curl in the direction they always curled.
If you were cataloging galaxies and this one passed through your survey, you might classify it as a normal late type spiral and move on without a second note. Nothing in the stellar structure would stop you. The gas is another matter. But gas in its cold atomic form requires a radio telescope to see. And in its ionized form, a narrowband filter tuned to a specific emission line.
Neither of these things is how most surveys of galaxies have historically been conducted. So for a long time, the damage being done to cluster spirals was invisible, not because it was subtle, but because the instruments pointed at it were looking at light that the damage did not produce.
The stripping works from the outside in.
That is the organizing principle of this chapter, and it is also the reason why a galaxy can be substantially gutted and still look in an optical image like nothing has gone wrong.
The outer regions of the gas disc go first. The inner regions resist longest and the stars distributed distributed across both regions keep orbiting through all of it indifferent.
Here is why the outside goes first. The restoring force, the gravitational grip that holds gas in the disc is not uniform. It depends on the local surface density of stars and gas, both of which are highest at the center and fall off with radius. Near the galaxy's core, the stellar disc is thick. The gravitational potential is deep, and even a gas parcel sitting at the moderate height above the midplane is pulled back strongly. That gas would require an enormous ram pressure to dislodge. Out in the periphery, the stellar disc has thinned considerably. The gas surface density out there is low. The gravitational hold is weak and the same ram pressure that the center shrugs off is more than sufficient to push the outer gas over the threshold and away from the disc. So the stripping radius, the distance from the center inside which gas remains bound outside which it does not starts somewhere in the outer disc and depending on how the ram pressure evolves as the galaxy moves through the cluster can migrate inward over time. At first, only the very outermost gas is lost. Then more, then more. The disc shrinks from the outside in, like a candle burning from the wick outward rather than from the tip. The stellar disc does not follow. It stays at its original size, which is the source of what becomes, when you actually look at highresolution multi-wavelength images of these galaxies, a very particular and slightly unsettling mismatch. The stars extend to one radius the gas extends to a smaller one. The ratio between these, the gas disc size divided by the stellar disc size is a direct measure of how much stripping has occurred. And in the most strongly affected Virgo cluster spirals, that ratio drops well below half. Gas truncated to 30 40% of the stellar extent. Stars still reaching out to where the gas used to be, orbiting through regions that are now mostly empty of the material that would, under normal circumstances, be forming the next generation of stars out there.
NGC4522 is a clear example of this. Its gas disc is truncated at roughly 35% of the stellar optical radius. The stellar disc is symmetric, undisturbed.
The gas in radio observations simply stops well inside the stellar boundary.
And beyond that radius, what little gas exists is extra planer lifted off the disc entirely, trailing behind the galaxy as it moves. The extra planer gas is worth noting because it is not simply gone. In the early and middle stages of active stripping, gas that has been pushed off the outer disc, but not yet fully mixed into the ICM, can be detected as filaments, clouds, or continuous tails extending away from the disc in the direction opposite to the galaxy's motion. This gas is no longer part of the galaxy in any gravitationally bound sense. It will not fall back, at least not most of it, but it has not yet dissolved into the surrounding medium. occupies a temporary state. Former disc material now a drift still cold enough in some cases to contain neutral hydrogen detectable at 21 cm still containing in some cases molecular gas still in some cases forming stars. That last point is one that took observations to establish rather than theory to predict and it will get its own space later. The truncated disc that remains after significant stripping is not simply a smaller version of what was there before. The gas that survives inside the stripping radius is the densest gas. The molecular clouds, the star forming regions, the material most tightly bound. In some stripped galaxies, the stripping has actually increased the star formation efficiency in the central disc, at least temporarily, partly because the compression at the leading edge of the ICM wind interaction can push gas to higher densities. The galaxy can briefly look in terms of its central star formation more active than it was before the stripping began. Busier at the center, emptied at the edges, which is a strange condition to be in. using up your remaining fuel faster than before while simultaneously unable to acquire more. The efficiency of the consumption going up at exactly the moment the supply has been cut. It does not help. The disc keeps shrinking as the galaxy continues its orbit through the cluster, encountering varying ICM densities, experiencing different ram pressure at different positions. If the first perentric passage was severe, most of the outer gas may already be gone.
Subsequent passes at lower speed and often through a somewhat different part of the cluster remove what remains of the intermediate regions. The stripping radius moves inward slowly or in episodes depending on orbital geometry.
Eventually, in the most extreme cases, only the dense molecular gas near the nucleus remains. And even some of that has been shown in ALMA observations to be susceptible to removal in the right conditions. From outside through an optical telescope, the galaxy still looks like a spiral. This is the part that I keep returning to not because it is counterintuitive. Once you understand the mechanism, it is not. It follows directly from the physics, but because it means the appearance of a galaxy, the thing you see in an image, can be almost completely decoupled from its actual physical condition.
A galaxy can be in the late stages of a process that will eventually quench all of its star formation that has already removed the majority of its gas that has already determined most of its future.
And it will look to the wrong instrument like a normal healthy spiral. The damage is already done. The stars just have not noticed yet. Not everything that kills does it quickly. Some processes work by removal. Not the violent kind. Not pressure tearing at a disc, not gas being pushed bodily off a spinning galaxy at 1,000 km/s.
This one works by subtraction, by the quiet elimination of a supply line that the galaxy did not know it was depending on until it was gone. And by the time the consequences become visible, the cause has already finished its work and moved on, leaving the galaxy to run down on its own.
This mechanism has a name that is for astrophysics unusually direct strangulation. The setup requires understanding something about how galaxies maintain themselves under ordinary circumstances. Field galaxies, isolated spirals, galaxies that have never been near anything like the Virgo cluster. A spiral galaxy is not a closed system. It does not form stars on a fixed budget that it received at birth and has been slowly spending ever since.
It replenishes. The disc gas that gets consumed in star formation is partly offset by gas falling inward from the galaxy's extended hot halo. The diffuse atmosphere of warm and hot gas that surrounds the stellar disc at large radi left over from formation and supplemented over time by various feedback processes. That halo cools slowly and as it cools it drips inward adding to the disc's cold gas reservoir.
The rate is not enormous but over billions of years it matters.
It is the difference between a galaxy that forms stars at a roughly sustained rate and a galaxy that simply burns through what it has and quiets. When a galaxy falls into a cluster, one of the first things the cluster environment can do before ram pressure reaches the disc before the full force of the ICM wind is felt in the cold gas is strip the hot halo. The hot halo is easier to remove than the disc gas. It sits at large radius held weakly and its density is low enough that even moderate ram pressure the kind present at the cluster out skirts well beyond the viral radius in some cases can push it away. The ICM does not have to be dense to do this.
The interaction between the hot halo and the surrounding ICM can also involve simple mixing, thermal conduction and heating. the halo gas warming up until it becomes part of the intracluster medium rather than remaining associated with the galaxy.
Whatever the specific mechanism, the outcome is the same. The extended hot reservoir around the galaxy is gone or reduced to a fraction of what it was before the galaxy has even fully entered the cluster. The disc does not notice immediately. The cold gas already in the disc continues forming stars. The rate does not drop overnight. The galaxy keeps doing what galaxies do. And if you were observing it in the early stages of this process, you would not necessarily flag it as disturbed.
Its star formation rate might be slightly below what you would expect for its mass, but only slightly and within the scatter of normal variation among field galaxies of similar type. This is what makes strangulation difficult to observe directly and correspondingly difficult to study.
The cause is gone before the effect is large enough to measure cleanly over time. And this is where the timebased framing becomes necessary because strangulation operates across time scales of 2 to 4 billion years. The disc gas is used up faster than it is replenished. The replenishment is zero or near it. The star formation rate begins to decline slowly at first.
The most efficient star forming regions keep going. The densest molecular clouds keep producing stars and the galaxy looks from a distance like a galaxy in moderate decline rather than one that is fundamentally running out of fuel. The star formation rate eventually falls by a factor of a few. Then more. The disc gas thins. The galaxy's color reens because new blue stars are no longer being made quickly enough to replace those aging into yellow and red. The galaxy is not dead. Not yet. But it has no way to recover and it knows this in the only way a galaxy can know anything which is to say it does not know it at all. The physics just continues. The framework that emerged from this understanding is called delay then rapid quenching. The name is descriptive in a way that scientific naming often is not.
A galaxy enters the cluster. Its halo is stripped and for a period of roughly 2 to four billion years it experiences a delay phase. Star formation declining but not catastrophically. The galaxy still identifiable as star forming if you look at it quickly. Then something tips it into a rapid phase. Ram pressure stripping of the disc gas which has been building as the galaxy moved deeper into the cluster. harassment contributing its slow gravitational damage, possibly a paracentric passage that delivers the full force of the ICM wind against what is now a depleted and weakened disc. The rapid phase can take less than a billion years, sometimes far less, to complete the quenching that the delay phase set up. The two phases are not independent.
Strangulation does not simply wait and then hand the galaxy off to RAM pressure. It weakens the disc. A galaxy that has been slowly depleted over 2 billion years enters its paracentric passage with less gas than it would otherwise have had, less to lose, and what remains is perhaps less well organized. The outer disc already somewhat thinned by the slow decline in replenishment.
Ram pressure then finds an easier job than it would have found in a freshly and falling gas-rich spiral.
This layering of mechanisms, strangulation setting the conditions and ram pressure completing the work is part of why it took so long to clearly separate their contributions observationally. They do not announce themselves. They leave traces in the chemical composition of the gas.
Strangulation produces a measurable increase in metallicity because the usual dilution of the ISM by fresh low metalicity inflow has stopped and so the gas that remains is more metalenriched than field galaxies of similar mass. But that signal requires precise spectroscopy to detect against background scatter.
Studies of galaxy clusters at intermediate red shift have found evidence for exactly this metallicity enhancement in cluster members compared to field galaxies of similar stellar mass which is one of the cleaner observational handles on a process that otherwise hides well. What strangulation leaves behind ultimately is a galaxy that looks like a spiral may still have a disc may still have some gas but is drawing on a reservoir with no inlet.
The fire is still there, still producing light, still warming whatever surrounds it, still doing what it was built to do.
It is just that nobody is adding wood.
And the wood that remains, the cold gas disc that has not yet been consumed, is being eaten through at whatever rate the remaining star formation demands. And no faster and no slower. And when it is gone, what remains will be whatever the stellar population looked like at that moment. Whether ram pressure finds the galaxy before that moment or after or exactly during depends on orbital geometry and cluster properties and timing that is different for every galaxy in the cluster. Strangulation does not care about any of that. It just removes the supply then waits for the consequences to accumulate and they do.
Nobody named it gently. Galaxy harassment. The term was introduced by Ben Moore and colleagues in 1996 and it has persisted in the literature with a kind of comfortable permanence that suggests astronomers found it accurate enough to keep. Not a metaphor exactly.
Or maybe it is, but one that has been used so long it no longer feels like one. The name points at something real.
that a galaxy moving through a cluster is not simply acted on by the ICM, not simply strangled of its supply, but is also subject to a relentless sequence of gravitational interruptions from every other galaxy it encounters along the way. These encounters are not slow.
Nothing in the Virgo cluster is slow.
Galaxies are moving at hundreds to over a thousand km/s relative to each other.
Which means that when two galaxies pass close to each other within 50 kilo parex say which is close in galactic terms the interaction is brief far too brief for the tidal forces to act over any extended period. Too brief for the kind of resonant coupling that drives spiral arm formation in isolated interacting pairs. Too brief really to do much at all in a single pass. And then it happens again and again at a frequency.
The typical estimate is around 10 close encounters per galaxy at impact parameters of 10 kilo parex or less over the course of its time in the cluster that accumulates into something the single encounter picture completely misses. Each individual passes too fast to be catastrophic. The combined effect of many passes is not. This is actually a different kind of physics from ram pressure stripping and the difference matters. Ram pressure is fluid dynamics.
It acts on gas because gas is a fluid because there is a pressure differential because the ICM wind physically pushes the interstellar medium off the disc.
Harassment is pure gravitational mechanics. It acts on stars as much as on gas because gravity does not distinguish between phases of matter.
Stars feel it. The stellar disc feels it. And because the encounters are impulsive, short, sharp gravitational kicks rather than sustained tugs, the effect is cumulative heating. Each encounter delivers a small injection of kinetic energy into the stellar orbits, slightly increasing the random motions of stars relative to the ordered rotation of the disc. The disc heats up in the dynamical sense, which means it thickens, becomes less organized, loses some of the sharp rotational structure that defines a spiral.
The gas responds differently than the stars and not always in the direction you might expect. The gravitational disturbance of a close encounter can drive gas inward toward the center of the galaxy rather than outward. The encounter perturbs the gas's angular momentum and angular momentum loss makes gas fall inward. So harassment can counterintuitively funnel gas toward the nucleus temporarily in Bitseng's central star formation rates possibly feeding the central black hole while simultaneously disrupting the outer disc structure and reducing the efficiency of star formation in the extended regions.
This is the selfcorrection the mechanism quietly requires. Harassment is not simply destructive. It redistributes. It moves material around in ways that are not uniform across the disc and not straightforward to predict from first principles without simulation. But the redistribution has a direction over long time scales. The outer disc loses. The outer disc is always the most susceptible. Lower surface density, shallower gravitational potential, less resistance to perturbation.
Over many encounters, the outer stellar disc is heated and thickened and eventually disrupted.
Tidal material is drawn off in streams that dissipate into the intergalactic space of the cluster. This galaxy becomes more centrally concentrated. Its outer regions thin and fade and in the most extreme cases of low surface brightness spirals essentially dissolve.
Now, massive and dwarf galaxies are not the same story here. A massive spiral and something the size of the Milky Way or larger has a deep gravitational potential. Well, a dense stellar disc, a substantial dark matter halo that provides structural resistance.
Harassment heats it, thickens it, disrupts the outer disc over billions of years. The transformation is real but slow and the galaxy retains its basic character through many encounters. It becomes perhaps less clearly spiral, more diffuse in the outer regions, more s0ike over time. The harassment is irritating to it in the way that to many small interruptions are irritating. None individually breaking anything, the total wearing something down. A dwarf galaxy is a fundamentally different situation. Low surface density, shallow potential, well dark matter, halo, less concentrated, less resistant to tidal forces.
For a low surface brightness dwarf falling through the Virgo cluster, a single close encounter with a massive galaxy can be enough to dramatically alter its structure. And the cumulative effect of several can be transformative in the morphological sense, stripping outer stars, thickening the system from a rotationally supported disc into something that looks more like a pressure supported spheroid.
More at all proposed that the excess population of dwarf elliptical galaxies seen in clusters like Virgo, there are more of them than simple models predict, might partly result from this transformation. low surface brightness dwarf irregulars and dwarf spirals falling in from the field harassed into rounder, redder, kinematically hotter systems over the course of their time in the cluster. The evidence for this remains partly circumstantial. The transformation is difficult to observe in progress because it is slow and because the intermediate stages look like several different things depending on viewing angle and the specific history of encounters.
simulation support it. Direct imaging occasionally catches what appear to be tidal streams around cluster dwarfs, but catching the full sequence from infalling dwarf irregular to dwarf elliptical requires either very long observational baselines or statistical approaches across large samples, neither of which provides the clean narrative that a single galaxy case study does.
What harassment contributes to the overall picture of cluster galaxy evolution is this a gravitational background noise that is always present, always accumulating, always slightly reorganizing the structural and kinematic properties of galaxies in the cluster regardless of what else is happening to them. Ram pressure strips the gas. Strangulation cuts the supply.
Harassment rearranges what remains, heats what cannot be removed, and leaves the stellar structure subtly different from what it was when the galaxy arrived. Together, these mechanisms do not take turns politely. They overlap.
They run simultaneously.
They strengthen each other's effects in ways that simulations are still working to fully characterize.
The galaxy somewhere in all of this keeps orbiting. It does not have another option. If you look at the right image for long enough, the name stops feeling like a metaphor. A jellyfish galaxy in a good deep observation has the body of a disc, the stellar component sitting there, more or less intact, roughly summar.
And then trailing away from that body in one direction, long filaments of gas, ionized hydrogen, mostly glowing in Halpha emission, which is a specific red wavelength produced when hydrogen atoms recombine after being ionized and release a photon in the process. The filaments are not uniform. They branch.
They thin and thicken along their length. They extend in the most dramatic cases to distances comparable to or greater than the diameter of the galaxy itself.
Trailing behind it in the direction opposite to its motion through the ICM.
It does look like a jellyfish. Not exactly, but close enough that when you first encounter one of these images, really encounter it, sitting with it rather than glancing past it in a paper figure. The name feels earned rather than decorative. The morphology is a direct read of the physics. The direction the tails point tells you which way the galaxy is moving. The length and brightness of the tails tells you something about how long and how intensely the stripping has been occurring.
The distribution of emission within the tails, whether it is smooth or clumped, whether there are compact knots of brighter emission embedded in the diffuse flow, tells you about whether star formation is happening in the stripped gas, which is a thing that happens and which will matter later. The image is not just a picture. It is a physical record frozen in the light that arrived at the detector of a process that is still ongoing.
The term itself, jellyfish galaxy, entered the literature informally before being adopted widely. Harold Ebling and the collaborators used it around 2014 to describe a class of morphologically disturbed cluster galaxies with obvious one-sided extensions of gas and sometimes stars. It was not the first time the morphology had been described or studied. Individual cases had been known for decades. NGC4522 and NGC 45569 in Virgo among them. But the term gave the class a name that made it searchable and discussible and worth treating as a population rather than a collection of oddities. The gasp survey gripping phenomena in galaxies with muse is where the systematic study of these galaxies became something more than case studies.
Gasp observed 114 galaxies using the MUSE integral field spectrograph on the very large telescope targeting RAM pressure stripping candidates in clusters at red shifts between 0.04 and 0.07 07.
The integral field approach matters here. Rather than taking a spectrum at a single point, Muse takes a spectrum at every spatial position in the field simultaneously.
Which means you get a full map of the ionized gas velocity, ionization state, star formation rate, and metallicity across the entire galaxy and its tail.
Not just a picture, a data cube. every pixel telling you something different about what is happening at that location.
From Gasp came a classification scheme for stripping stage that is worth understanding because it gives language to what would otherwise be a continuous and somewhat blurry sequence of morphological change. The most extreme cases J type one in the gasp scheme are those where the stripped gas tail extends at least as far as the galaxy's stellar disc diameter. These are the ones that look most unmistakably like jellyfish. The ones where the trailing material is long enough and bright enough to dominate the image.
Jype 2 galaxies have shorter or less continuous tails. Jype 3 galaxies show only moderate extra planer emission mostly confined to the inner disc with small external features. And then there are post stripping systems where the disc has been truncated and the tail is gone or fading and the galaxy looks simply smaller and quieter than it should for its stellar mass. This classification is not rigid. A galaxy's apparent stripping stage depends partly on viewing geometry. A tail extending behind a galaxy moving along the line of sight will look very different from the same tail extending across the plane of the sky. A galaxy observed during peak ram pressure at paracentric passage looks different from the same galaxy a few hundred million years later when the ram pressure has eased and the tail is dispersing.
The classification captures a snapshot and the snapshot depends on when and from which angle you took it. What the tails contain is in some ways more interesting than the tails themselves.
The ionized gas in the filaments is not simply disc material being passively dragged outward. It is interacting At the interface between the cold stripped gas and the hot ICM surrounding it, there are processes happening, mixing, heating, ionization from various sources, including shocks and the UV background and in some galaxies the central AGN that the simple picture of ram pressure pushing cold gas off a disc does not fully account for.
The metallicity of the tail gas has been measured in several gasp galaxies and it shows a gradient. Gas nearest the disc retains roughly the metallicity of the disc it came from while gas further along the tail mixed more with the surrounding ICM has lower metallicity.
The ICM is diluting the strip material as it goes. There are also cases and this is the part the narrator almost wants to set aside because it complicates the narrative but cannot be set aside because it is real. So where the tail gas is not being stripped passively at all but is being ionized partly by the AGN of the galaxy and it is trailing behind.
NGC4388, one of the Virgo clusters most studied examples, has a safer 2 nucleus whose ionizing radiation reaches outward into the extra planetar gas contributing to the emission. So the tail is lit partly by ram pressure physics and partly by the galaxy's own central engine. The two are not easy to disentangle and the line between them is not sharp. The one-sidedness of the tails is, when you think about it, the most diagnostic thing about them. A galaxy with a symmetric gas distribution, gas extending equally in all directions, is a galaxy in equilibrium. A galaxy with gas on one side only or more precisely a galaxy with a truncated leading edge where the ICM wind is compressing the gas inward and a trailing extended tail on the opposite side where the stripped material is flowing out is a galaxy in active interaction with its environment.
The asymmetry is the signature. It is what separates a ram pressure stripped galaxy from one that lost gas through internal processes through supernova driven winds through secular evolution.
Internal processes do not produce one-sided tails. Only an external wind does that. You can look at a jellyfish galaxy and know from the direction the tail points which way the galaxy is moving through the cluster. You are reading velocity from morphology, reading direction from shape. The galaxy has in the pattern of its own destruction encoded information about its trajectory. There is something about that which is hard to put precisely into words without it sounding more poetic than it is. So maybe leave it as a fact.
The tail points opposite to the motion.
The shape is the record. And the record is still being written in these galaxies right now 53.8 8 million years ago.
Three galaxies, each one a different version of the same general fate. Each one caught at a different point in the process, at a different velocity, from a different angle, with a different amount of itself already gone.
They are not the only stripped galaxies in the Virgo cluster. They are not even necessarily the most extreme depending on how you measure extreme. But they have been studied in enough detail across enough webs and across enough decades that they have accumulated something that most galaxies never accumulate. A reasonably complete account of what is happening to them and why. That account is worth sitting with.
NGC4522 is a medium-sized spiral not large by galactic standards. Its optical diameter is roughly 20 kilo parex. Its rotation velocity about 100 km/s which puts it at around half the luminosity of a galaxy like the Milky Way.
A small bulge, a disc that in isolation would be unremarkable. The kind of galaxy that appears in surveys without generating particular interest.
Its projected distance from M87 is just under a mega parseek. Not in the cluster core, not at the far outskirts, but in what you might loosely call the middle distance of the cluster's extent. The hi observations of GC4522 are where the story begins to have weight. The neutral hydrogen disc of this galaxy is truncated at approximately 3 kilo parex from the center. That is 35% of the optical radius. The radius out to which the stellar disc extends. The stars go out to 8.6 kilo parex. The gas stops at three. Beyond that radius in the disc plane, there is almost nothing. But above the disc, on one side only, the northwest side, the gas reappears. Extra planar emission distributed in a region that extends noticeably away from the disc surface. Roughly 40% of the galaxy's total hwan amounting to 1.5 * 10 8th solar masses is in this extra planet component. Not in the disc off the disc. Still detectable, still cold enough to emit at 21 cm, but no longer bound to the galaxy it came from. The stellar disc is symmetric. No peculiarities, no warps, no evidence of anything unusual in the old stellar population. You look at the stars and see a normal galaxy. You look at the gas and see something in the middle of being taken apart. NGC4522's line of sight velocity relative to the mean Virgo cluster velocity is approximately 1,300 km/s.
It is moving fast and the direction of the extra plan of gas entirely on one side trailing away in the direction opposite to the IDM wind the galaxy is experiencing is consistent with active ongoing stripping. Not a past event, not something that happened and left traces.
Something happening now or rather something that was happening 53.8 8 million years ago when the light now reaching our detectors was emitted which is the only now available at this distance. There is a detail about NGC4522 that is worth noting over too quickly.
His projected distance from M87 is roughly 800 kilo parex. At that distance, the standard estimates of the ICM density using the smooth spherically symmetric density profiles derived from X-ray observations suggest the ram pressure should not be quite sufficient to produce the stripping observed. The numbers do not quite add up with the simple model. The ICM at 800 kilo parex should be thin enough that the restoring force of the disc ought to win at the radi where the gas is actually being stripped. Various explanations have been proposed. The ICM is not actually smooth and symmetric. It has density fluctuations substructure. Possible enhanced density in some regions due to infalling groups. The galaxy's actual three-dimensional velocity relative to the ICM might be higher than the line of sight component alone suggests since we only directly measure the radial velocity.
Whatever the resolution, NGC4522 is actively being stripped at a location where the simple model says it should be marginally resistant, which is in a quiet way instructive. The simple model is not wrong. It is just simple. NGC4569 is the brightest latype galaxy in the Virgo cluster by optical luminosity.
Massive as spirals go, stellar mass around 10 to the power of 10.5 solar masses with a disc that in a field environment would be forming stars at a healthy rate for billions of years to come. It has lost more than 90% of its initial hy. That number deserves a moment. More than 9/10 of the neutral hydrogen this galaxy was born with or accumulated over its lifetime to that point is gone. The total mass of gas lost during its interaction with the cluster environment is estimated at approximately 1.9 * 10th solar masses comparable to the mass of the entire interstellar medium of the Milky Way.
That gas is not in the galaxy anymore.
Some of it is in the tail. Most of it has mixed into the ICM. The tail of NGC 4569 is extraordinary in its extent. Early observations detected ionized gas extending roughly 80 kilo parex from the galaxy disc. More recent deep emerging with mega cam at the Canada France Hawaii telescope part of the vestage survey detected a tail of diffuse halpha emission extending approximately 230 kiloparex over 700,000 lightyear trailing behind the galaxy. That is, to be direct about it, a tail of stripped material longer than the distance between the Milky Way and the large melanic cloud trailing behind a single galaxy as it moves through the Virgo cluster. The tail has no associated stellar component. It is purely gaseous. The emission at its most diffuse has a surface brightness of a few times 10 the minus8 urg extremely faint requiring deep dedicated observations to detect at all that such faint features exist and can be mapped is partly a statement about the sensitivity of modern instruments and partly a statement about how much gas is actually out there spread spread across an enormous volume slowly mixing into the surrounding plasma. NGC4569 has a negative recessional velocity. His heliocentric velocity is approximately -235 km/s meaning it is moving toward us and more significantly its velocity relative to the Virgo cluster mean puts it at around 1,300 km/s in the cluster rest frame. It fell into the cluster from behind, from our side, and encountered the densest ICM in the cluster's interior in a stripping event that models suggest began around 100 million years ago. The ram pressure at the closest approach was sufficient to begin stripping a galaxy with a stellar mass of 10 the^ of 10.5 solar masses, which is not a trivial galaxy to strip. The stripping has not quenched it entirely. Not yet. The central molecular gas reservoir is still present. Star formation continues in the inner disc at a reduced rate, but the disc is truncated. The outer star forming regions are gone and the rate is declining. NGC4569 is somewhere in the transition, past the delay phase of strangulation into the consequences, not yet fully quenched, but moving in only one direction. NGC 4388 is the different kind of case. It sits about 400 kilo parex in projection from ME87 well inside the cluster's inner regions. It hosts a safer two active galactic nucleus. Its central black hole is accreting and producing ionizing radiation which complicates the picture of its tail because some of the ionized emission in the extra painter region is lit not by the stripping process itself but by the nucleus.
Separating the two contributions requires careful spectroscopy and the separation is not always clean. The hi tail is unambiguous.
Observations by Stalu and Van Goram detected a plume of neutral hydrogen extending approximately 110 kyop parex north of NGC 4388 containing 3.4 * 10 8th solar masses of gas. The largest hi tail yet found in the Virgo cluster, extending in a single direction, pointing away from 87 in a manner consistent with the galaxy, having swept through the denser ICM near the cluster center and left this material behind.
NGC 4388 is moving at over 1,400 km/s relative to M87.
relative to the nearby M86 subgroup, which has its own substantial ICM and which NGC 4388 may have passed through.
The relative velocity exceeds 2,800 km/s. That velocity combined with the ICM density at its orbital location produces ram pressure that is genuinely severe. The HI deficiency of this galaxy is high. Its gas disc is heavily truncated. Star forming regions have been found in the strip tail itself at 35 and 66 kilo parex from the galaxy disc with stellar ages of around 6 million years. Those stars formed in intergalactic space in gas that was part of a galaxy and then was not. What these three galaxies share beyond the obvious fact of being stripped is that each one was caught at a moment that illuminated something the others did not. NGC4522 showed that stripping can be active at unexpectedly large cluster ccentric distances and that the simple density models of the ICM are insufficient to fully account for where stripping happens.
NGC4569 showed the sheer mass of material that can be removed from a massive spiral in a single stripping episode and revealed the extraordinary extent of the gas tales that such an episode produces. NGC 4388 showed that stripped gas can form stars in the intergalactic medium and that the presence of an AGN adds complexity to the interpretation of the emission without changing the fundamental reality of the stripping.
Three galaxies, three incomplete stories, each one still being written in the light currently traveling toward instruments that have not been built yet. The gas they lost is still out there somewhere in the cluster slowly becoming part of something that is not a galaxy at all. The assumption embedded in most descriptions of ram pressure stripping is that it end star formation that it is in net terms a subtractive process something that removes the raw material and therefore removes the possibility of what the raw material would have become. Gas leaves. Stars do not form where gas is not. the stripped galaxy quiets. This is true over long time scales and in the final accounting it is true. But in the shorter term and in the specific geometry of the interaction, something else happens first. Something that took observations to reveal because it was not clearly predicted from the simple analytic picture of ram pressure as a force that simply pushes gas off a disc and disperses it. The leading edge of a galaxy moving into the ICM wind does not immediately lose its gas. The ram pressure does not simply pick the outer disc up and remove it in one motion.
What happens at the leading edge, at least during certain orbital phases, and for certain disc orientations relative to the wind direction, is compression.
The ICM wind pressing against the front face of the galaxy's gas disc pushes the outermost gas inward, temporarily increasing the local gas surface density before the gas is ultimately removed.
Higher surface density means shorter gravitational freefall time. Shorter freefall time means faster star formation.
The galaxy's leading edge, the side being pressed into the ICM, can briefly produce stars at an elevated rate.
A compression-driven starburst, that is, in the context of what is about to happen, a final burst of productivity in a region that is weeks in galactic time from losing its fuel entirely. This happens inside the disc in the existing star forming regions in the molecular clouds that are being compressed before they are dispersed. It is brief. The simulations that reproduce it consistently show it lasting at most a few hundred million years before the stripped gas is gone and the star formation rate drops below what it was before the interaction began. But during that window, the galaxy is not quieting.
It is locally more active than normal.
The observational signal of this is subtle in individual galaxies and clearer in statistical samples. The Gasp survey found that jellyfish galaxies as a population have star formation rates in their discs elevated by roughly 0.2 decks compared to a matched control sample of non-stripped cluster galaxies at similar stellar mass. Not dramatically higher, but higher. The stripping is also quenching and the two effects run simultaneously which is part of why the signal is not larger. Enhanced central star formation and reduced outer disc star formation happening at the same time averaging out in the integrated measurement to something modest. That is the disc story. The tail story is different. The tail story is the one that sitting with it refuses to settle into something ordinary. Gas that has been stripped from the disc. Gas that is no longer gravitationally bound to the galaxy. Gas that is trailing behind in filaments through the intracluster medium.
This gas under the right conditions form stars.
stars born in intergalactic space in gas that was until recently part of a galaxy's disc and is now floating through the hot ICM cooling [snorts] in clumps where the local density happens to be high enough and collapsing under its own gravity in the way gas has always collapsed when given enough density and enough time. The first detections of this were tentative UV emission and Halpha knots in the tails of stripped galaxies that could arguably be explained by ionization from the galaxy's own radiation field rather than by insight you star formation.
But the detections have accumulated across multiple galaxies and multiple wavelengths and the case is no longer tentative.
In NGC 4388, star forming regions have been confirmed at 35 and 66 kilo parex from the galaxy disc. The stellar ages of those regions are around 6 million years. The stripping event that produced the tail began much earlier than that which means the stars did not travel to those locations from the disc. They formed there in the stripped gas in intergalactic space in NGC4569.
The 230 kilo parsect tail contains regions of ionized emission that spectroscopic analysis has struggled to attribute entirely to AGN ionization or to the UV background. Some of it appears to require local ionization. Young massive stars recently formed producing their own radiation field within the tail. The evidence is not as clean as in NGC 4388, partly because NGC 4569 is more massive and its tail contains more material that is harder to fully characterize. But the direction of the evidence is consistent. The Gasp survey produced HST imaging of six jellyfish galaxies at sub kilo parseek resolution resolving individual star forming clumps in the tails. The clumps have sizes, colors, and ionization properties consistent with young stellar populations, ages in the range of a few to a few tens of millions of years forming in dense condensations within the stripped gas. The star formation efficiency in these tail clumps appears comparable to what is seen in the outer discs of field galaxies, which is itself not a simple result. It means that when stripped gas manages to reach sufficient density, it forms stars with roughly the same efficiency as gas in a normal galactic environment. The location into a galactic space inside a hot ECM tens of kilo parex from the nearest stellar disc does not fundamentally change the local star formation process. Gas dense enough to collapse collapses. What happens to those stars afterward is a question the current data does not fully answer. Some of them will remain in the intergalactic medium, never accreted back onto any galaxy, contributing to the diffused population of intracluster stars that float freely through the Virgo cluster in loosely organized streams and halos. Others might eventually be swept up into a galaxy if they happen to be near one on a convergent orbit, but this is unlikely for most of the tail stars given the geometry and the velocities involved.
They form in the stripped gas. They age in the stripped gas and when they die through supernova, through stellar winds, through the various endings available to stars of different masses, they return their processed material not to a galaxy but to the ICM. The metals they synthesize go into the intracluster plasma. The enrichment that would in a filled environment have cycled back through the disc and contributed to the next generation of star formation goes instead into the hot medium. The tail stars are in this sense a one-way transaction. They form from gas that left the galaxy and they return their material to an environment the galaxy will never recover from. The galaxy does not get those metals back. It does not get those stars back. It loses the gas, watches the gas briefly produce something and receives nothing in return. Whether that constitutes tragedy in any meaningful sense is not a physics question, but it is the kind of thing that sits with you after you have followed the numbers carefully enough to understand what they're saying. The stripping takes. The tale gives briefly to nothing in particular. And the new stars burn in the dark space between galaxies in gas that was never supposed to be there. Doing exactly what gas does when the conditions allow it, which is in the end just physics. At the center of almost every large galaxy sits a super massive black hole. Most of them are quiet, not inactive in any absolute sense. They're still there, still massive, still warping a Bos time in their immediate vicinity, but not accreting at any significant rate, not producing the luminous outpouring of energy that defines an active galactic nucleus.
The Milky Way's central black hole, Sagittarius, a star, contains roughly 4 million solar masses and is, by AGN standards, essentially dormant. It flares occasionally, produces modest X-ray and infrared variability, but does not qualify as active in the way that term is used when discussing quazars or safer galaxies.
The fuel supply cold gas falling into the nucleus is not sufficient to drive sustained high luminosity accretion.
What changes that is gas reaching the center? Not just any gas. Hot diffuses gas at large radius does not easily lose enough angular momentum to fall all the way into the nuclear region. What feeds AGN efficiently is cold dense gas.
Molecular gas the same phase that forms stars that has somehow had its angular momentum reduced enough to fall inward rather than simply orbiting.
The mechanisms for this are various and in normal field galaxies they include bardriven inflows, galaxy merges and minor interactions.
None of these are peculiar to cluster environments, but ram pressure stripping introduces another mechanism and it is not the one you might first think of.
The obvious expectation would be that ram pressure reduces AGN activity. Ram pressure removes gas from the galaxy.
Less gas means less fuel. Less fuel means less accretion. This is a reasonable chain of logic and it is not entirely wrong and will minist in lower mass galaxies where the ram pressure is sufficient to strip gas from well inside the disc. A GN activity and star formation are both suppressed before the galaxy reaches the cluster core. The stripping depletes the fuel supply before it can be used for anything. But in more massive galaxies, something different happens. The outer disc gas is stripped. The inner disc gas where the gravitational restoring force is strong enough to resist the ICM wind remains.
And the stripping event, the turbulence at the ECM disc interface.
The compression waves moving through the disc, the perturbation of the gas distribution, can reduce the angular momentum of some of the inner disc gas, causing it to fall inward toward the nucleus rather than continuing to orbit at its original radius. The ram pressure event, which is removing gas from the outer disc, is simultaneously and through a different physical channel, funneling gas toward the center. The AGN lights up while the disc is being stripped. The observational evidence for this came most prominently from the GASP survey. In an early GSP paper, Poanti and colleagues found that five out of seven jellyfish galaxies in a specific highmass subsample hosted an AGN based on emission line diagnostics. That is a fraction of roughly 70% against a background AGN fraction into normal cluster galaxies of considerably less closer to 10 to 20% depending on the mass range and selection criteria. The number was immediately flagged as potentially significant and immediately flagged as potentially uncertain because seven galaxies is not a large sample and five out of seven is the kind of statistic that can shift substantially with a larger data set. The data set has since grown. Palo and colleagues extended the gasp AGN analysis to 82 jellyfish galaxies across a range of stellar masses and found an overall AGN fraction of around 24% lower than the initial result as expected when the sample grows beyond the most extreme cases but still elevated compared to control samples of non-stripped galaxies in the same clusters.
When the analysis was restricted to high mass jellyfish galaxies above 10 to the^ of 10.5 solar masses, the fraction rose again toward the higher values. Mass matters. More massive galaxies with deeper potential wells that can protect inner disc gas while outer disc gas is stripped show a stronger AGN stripping connection. The physical picture that emerges from simulations is consistent with the mass dependence.
In the Romulus C simulation, one of the higher resolution cosmological cluster simulations available, RAM pressure was found to suppress both star formation and black hole accretion in low mass galaxies. But in more massive galaxies near the cluster center, RAM pressure could actually enhance AGN activity by driving gas inward.
The stripping and the feeding are happening simultaneously to different parts of the same gas reservoir through different physical pathways operating at different radi. This creates a situation that is in terms of what is happening inside the galaxy genuinely strange to hold in mind. The outer disc is being removed. Stars in those outer regions will stop forming. The gas that once sat at intermediate radi is being displaced.
Some of it outward into the tail. Some of it poor the fraction that loses angular momentum in the turbulent interaction inward toward the nucleus.
The AGN receives fuel and brightens produces outflows radiation feedback.
That feedback can itself push gas further, potentially contributing to the quenching of the remaining star formation in the central disc. And meanwhile, the ram pressure continues stripping from outside. The galaxy is being pressured from two directions at once. The ICM pressing inward from outside. The AGN now active, pushing outward from within. Whether these two effects cooperate in quenching the galaxy faster than either would alone or whether they partly cancel each other in some orbital configurations is something the simulations have not fully resolved.
Different codes give somewhat different answers. The observations are consistent with both possibilities depending on which galaxies are examined. What is clear is that treating RAM pressure stripping as purely a gas removal mechanism misses something. In massive galaxies in particular, the stripping event reshapes the internal gas dynamics of the galaxy. Not just the external gas distribution, it does not only subtract, it also redirects. And what gets redirected toward the center has consequences that extend beyond the immediate stripping episode. The AGN activity triggered by stripping can outlast the stripping itself. A galaxy that passes through the cluster core loses most of its outer discass in a single paracentric passage and then moves back toward the outskirts can retain an active nucleus for some time afterward fueled by the gas that fell inward during the event. The outer disc is quiet. The tail is dispersing. The nucleus is still bright. It is a strange sequence to follow. The damage is done.
The gas is gone and the center is briefly more alive than it has been in a long time. Then the inner fuel runs out too and everything goes quiet. Science does not usually arrive as a single clear moment. It accumulates.
One observation adds a constraint.
Another removes an ambiguity. A third reveals something that the first two taken together made possible to see. The picture of what the Virgo cluster does to its galaxies was not assembled in an afternoon or a year or even a decade. It came together across roughly half a century of observations. Each generation of instruments enabling a question the previous generation could not ask. Each survey building on the residue of everything that came before it. And the picture is still not finished. The most recent surveys are still producing papers. The data is still being analyzed. The model is still being revised at its edges. But there are four surveys in particular that changed the shape of what was known, not incrementally.
In each case, the survey revealed something that reframed the question rather than simply answering the previous one. They arrived in a rough sequence which is the most natural way to follow them. The VLA imaging of Virgo spirals in atomic gas. Va was published in its primary form by Ary Chung and colleagues in 2009. Though the observations accumulated over several years before that, the instrument was the very large array, a radio interferometer array in New Mexico whose 27 antennas can be arranged in different configurations to achieve different spatial resolutions and whose sensitivity to the 21 cm hyperfine emission of neutral hydrogen made it the right tool for mapping cold atomic gas in nearby galaxies.
Viva observed 53 latype galaxies in the Virgo cluster, 48 spirals and five irregular systems selected to span a range of star formation properties and cluster centric distances from 0.3 to 3.3 mega parex from M87.
The resolution was approximately 15 arcsecond to just over a kilo parc which was sufficient to resolve the gas morphology of individual galaxies in some detail. What Viva found laid out across its atlas of hire maps was that the cluster had done different things to different galaxies depending on where they were and how they were moving. Some galaxies had truncated HYI discs smaller in radius than their stellar discs by the factors of two or more. Some had one-sided tails, asymmetric hy distributions with extended emission pointing away from the cluster center.
Seven galaxies out of the sample had long clearly defined H1 tails extending well beyond the stellar disc. All of them pointing away from 87. Some galaxies, particularly those in the outer cluster regions, looked largely undisturbed.
The spatial distribution of the disturbed galaxies was informative.
Hbumia truncation and asymmetry were not confined to the cluster core. They appeared at projected distances from M87 that under the simple models of ICM density should not have been sufficient to drive significant stripping, which was itself a result. The ICM was apparently more effective at larger radi than the smooth symmetric density models predicted or the galaxy velocities at those radar were higher than expected or the stripping had begun earlier in the orbit than the current position suggested.
Viva also established statistically that Virgo spiral galaxies are systematically hi deficient compared to field galaxies of similar type and size. Galaxies in the core regions contain on average only about a third of the HRI that field spirals of comparable optical size would be expected to have that deficiency gradient increasing toward the center was a direct statistical fingerprint of the cluster environment acting on the gas. But Viva could only see the cold neutral hydrogen, the ionized gas, the warm and hot phases, the molecular gas.
None of these are accessible at 21 cm.
The picture Viva produced was complete in Hway and blind to everything else.
Vestage, a Virgo environmental survey tracing ionized gas emission, filled part of that gap. A deep blind narrowband Halpha imaging survey of the Virgo cluster carried out with mega cam on the Canada France Hawaii telescope designed to cover the full cluster out to its viral radius, roughly 104 square degrees of sky at unprecedented depth and angular resolution.
The Halpha emission line at 656 nanome is produced by ionized hydrogen recombining after being excited by ultraviolet radiation from young massive stars or by shocks or by the AGN radiation field. It is a tracer of ionized gas and in star forming regions a direct tracer of recent star formation on time scales of around 10 million years. Vestage detected nearly 400 star forming galaxies in the Virgo cluster region. More importantly for the stripping story, it detected ionized gas tales. Sometimes faint, sometimes spectacular, always one-sided, associated with galaxies that were actively losing their gas to the ICM wind.
The survey revealed NGC 45569's tail at 80 kilo parex in its initial imaging and in deeper followup extended that detection to 230 kilo parex. It found dust being stripped alongside the gas confirming that the ICM interaction affects multiple components of the interstellar medium simultaneously rather than just the atomic hydrogen.
It showed across the population of Virgo cluster spirals that truncated star forming discs, star forming discs smaller in radius than the stellar disc are nearly ubiquitous across the cluster, including in regions well beyond the viral radius where galaxies are presumably falling in for the first time.
That last result quietly shifted the picture. The truncation was not confined to the cluster core. It was everywhere, which meant the cluster's influence on star forming discs begins earlier in the info process and reaches further out than the prevestage picture suggested.
Vertigo, the Virgo environment traced in CO, took the molecular gas layer that both Viva and Vestage could not directly access and mapped it across a statistically meaningful sample.
An ALMA large program using the Atakama compact array. Vertigo observed the CO2-1 emission line, a tracer of cold molecular hydrogen in 51 Virgo cluster galaxies at sub kilo parseek resolution.
Published initially in 2021 with subsequent papers continuing to extract results from the data set. The molecular gas is the immediate fuel for starport de formation. Hi is upstream the reservoir that cools and condenses into molecular clouds which then collapse into stars. Knowing what the cluster environment does to the molecular gas separately from the atomic gas matters because the two phases respond differently to ram pressure. The molecular gas is denser, more tightly bound, more concentrated at small radi.
It was expected to be more resistant to stripping. Vertigo found that the cluster environment does affect the molecular gas though not uniformly.
Hi, e deficient galaxies those already identified by vivea as having lost significant atomic gas tend to also have suppressed molecular gas content particularly in their outer regions. The environmental processing that truncates the atomic gas disc appears to eventually propagate inward to the molecular disc as well. Either through direct stripping of dense gas in the right orbital conditions or through the removal of the HI reservoir from which molecular gas would normally be replenished.
Star formation efficiency, the rate at which molecular gas is converted into stars, shows more scatter in Virgo cluster galaxies than in field galaxies of similar mass, with some stripped galaxies showing elevated efficiency and others reduced depending on the stripping stage and the specific orbital history.
Gasp covered the ionized gas and stellar populations with the integral field spectroscopy that imaging surveys could not provide.
The full spatial mapping of mission line ratios, gas kinematics as star formation rates at every position in the galaxy, and its tail, stellar population ages.
Gasp produced a physical characterization of individual stripped galaxies that turned case studies into something like detailed clinical records. Together, these four surveys did not simply add up. They triangulated the HI from Viva, the H alpha from Vestage, the CO from Vertico, the IFS from Gasp, each tracing a different phase of the gas, a different time scale of star formation, a different aspect of the stripping geometry. Where they overlap, they constrain each other.
where they diverge, they identify complexity that a single wavelength picture would have missed entirely.
The current model of how the Virgo cluster processes its galaxies is not the product of any single observation.
It is the residue of all of them, accumulated slowly, revised repeatedly, and still not quite finished, which is how it should be. The gas that leaves a galaxy does not disappear. This sounds obvious when stated plainly, but it is worth holding for a moment before moving past it because the intuitive picture of stripping, gas being removed, galaxy losing something and of that gas's story is not quite accurate. The gas moves. It joins a medium that is already there, already vast, already hot, and it becomes part of that medium in ways that are slow and chemical and in a very specific sense permanent. The intracluster medium is not primordial.
That is the first thing to establish. A plasma made entirely of hydrogen and helium in the proportions left by a big bang nucleioynthesis would be chemically featureless. No spectral lines from heavy elements, no iron emission, no silicon or sulfur or calcium contributing their signatures to the X-ray spectrum. What X-ray observations of galaxy clusters actually show is something different. The ICM contains metals not in enormous quantities. The overall metallicity is roughly a third to a half of solar abundance which means the heavy elements are present but diluted across an enormous volume of gas but measurably detectably consistently across every cluster that has been observed in X-rays with sufficient spectral resolution.
Those metals did not arrive there by themselves. They were made in stars, processed through stellar interiors over millions or billions of years, expelled through supernovi and stellar winds into the interstellar medium of galaxies and then removed from those galaxies by some combination of mechanisms and mixed into the surrounding plasma. The ICM's chemical composition is in this sense a record of stellar evolution, a distributed accounting of every generation of stars that ever lived and died in the galaxies that passed through the cluster. Ram precious stripping is one of the primary ways that processed material gets from galaxies into the ICM. The mechanism is direct. When the outer disc gas is stripped, so the gas that has been cycling through star formation and supernova for billions of years and has accumulated a metallicity that reflects that history, it carries its chemical content with it. The stripped HI, the ionized filaments, the molecular gas clumps that dissolve as they move outward from the disc, all of it contains the metals that were in the interstellar medium at the time of stripping. As this material mixes into the ICM, it raises the local metallicity of the surrounding plasma, the enrichment is not uniform. RAM pressure stripping deposits metals preferentially in the inner cluster regions because that is where the stripping is most intense and where most of the strip material ends up mixing into the surrounding plasma before it can travel very far. Galactic winds, by contrast, can be suppressed in high-press ICM environments and are more effective at enriching the cluster out skirts. The two mechanisms leave different spatial signatures in the metal distribution.
Iron. Iron deserves a pause here because it is the element most easily detected in cluster X-ray spectra and because its abundance in the ICM carries specific information about the history of star formation in the clusters galaxies. Iron is produced predominantly by type 1 supernova the thermonuclear explosions of white dwarfs in binary systems which have a delay time of hundreds of millions to billions of years after star formation. So, the iron abundance in the ICM is weighted toward old stellar populations, towards stars that formed early and whose white dwarf remnants have had time to explode. The silicon and oxygen produced more promptly by core collapse supernova in massive short-lived stars carry a different temporal signature. The ratio of these elements to iron in the ICM tells you something about the balance between prompt and delayed enrichment about how the star formation history of the clusters galaxy population is written into the medium.
Those galaxies have spent billions of years shedding material into the XRISM satellites resolve instrument in observations of the Virgo cluster core published in 2025 measured these abundance ratios with a precision that earlier instruments could not achieve.
It found super solar silicon to iron and sulfur to iron ratios in the cluster core higher than what is seen in most other cool core clusters which suggests that the enrichment history of the Virgo cluster has some specific character some particular balance of stellar populations and stripping events that is not universal.
The chemical fingerprint of the ICM is not a generic cluster fingerprint. It is Virgo's fingerprint. The total iron mass in the Virgo cluster ECM within roughly one viral radius has been estimated at around 4 * 10^ the 9th solar masses.
Some of this was already in the gas before it became the ICM. Some came from galactic winds and AGN outflows, and some came from stripping from the outer discs of spiral galaxies that fell through the cluster over billions of years were processed by the environment and left their chemical content behind as they quieted.
The Vestage Survey found in its dust stripping observations that RAM pressure removes dust from galaxy discs alongside the gas, injecting dust grains into the ICM, where they are eventually destroyed by the hot plasma through thermal sputtering, but not before they have contributed to the chemical composition of the medium. Dust grains carry carbon, silicon, iron, and other elements locked in solid form. And when those grains are destroyed in the ICM, those elements are released into the plasma. The stripping is not just a gas process. It is a material process, moving solid as well as gaseous matter from galaxies into the cluster medium. What accumulates in the ICM over cosmic time is a kind of sediment.
Not physical sediment. Nothing settles in a hot plasma in the way particles settle in water, but a chemical sediment, a layered record of what the cluster's galaxies contained and when and how they lost it. The early cluster, when its galaxies were gas-rich and newly infalling, contributed one kind of material. The later cluster, as its spirals were progressively stripped and quenched, contributed another. The current ICM composition reflects all of it, averaged and mixed and thermalized into the hot uniformish plasma that the X-ray observations show. The galaxies that were stripped are still there in one sense, not as galaxies, not as the spiral discs they were when they they first crossed the viral radius, but as chemistry, as iron lines in an X-ray spectrum, as a slight elevation in the silicon abundance of a plasma that extends for millions of light years in every direction. The cluster keeps what it takes, just not in a form the galaxy would recognize. Start with a galaxy that has not yet arrived. It exists in a group, a small collection of a few dozen galaxies, gravitationally bound to each other, moving through the large scale structure of the universe in the direction of the Virgo cluster. It has been in this group for billions of years. It is a spiral, moderately massive, with a gas-rich disc forming stars at a rate consistent with its mass and its age and the general state of galaxies at this point in cosmic history. It has a hot gas halo extending well beyond its stellar disc, slowly cooling, slowly dripping cold gas inward to replenish what the disc consumes. It is in the way that a galaxy can be said to be anything healthy. It is also falling.
The Virgo cluster's gravitational pull extends far beyond its viral radius. The group this galaxy belongs to has been responding to that pull for a long time.
its trajectory curving gradually toward the cluster, its velocity increasing as it descends through the large scale potential. At some point, not a sharp boundary, not a moment that announces itself, the group crosses into the cluster's sphere of influence in a meaningful way. The ICM, which thins continuously rather than ending cleanly, begins to interact with the group's own gas environment. The galaxy's hot halo sitting at large radius and held weakly begins to feel pressure from the surrounding medium. This is where strangulation begins, not with a dramatic event, with a boundary condition changing. The hot halo is not stripped all at once. It is gradually disrupted, compressed on the leading side, drawn out on the trailing side, mixed at its edges with the ICM that is now surrounding it. Over hundreds of millions of years, the halo loses coherence.
The cooling flow that was dripping cold gas inward to the disc slows. The replenishment rate drops. The disc does not notice immediately. It has its own cold gas reservoir, its molecular clouds, its ongoing star formation, and none of that changes overnight because the hot halo has been compromised. The disc burns what it has. Meanwhile, the group itself is falling apart. Galaxy groups do not survive clustering fall intact. The tidal forces of the cluster potential, the individual encounters between group members and cluster galaxies, the ram pressure acting on each group member individually. These break the group apart over roughly 1 to2 billion years. The galaxy we are following separates from its companions, each on a slightly different orbit, each now falling through the cluster potential individually rather than as a bound unit.
This separation matters because the group's own gravitational potential had been providing some additional resistance to environmental effects.
Once the group is gone, the galaxy is on its own. The star formation rate in the disc has been declining for perhaps a billion years by this point. Slightly, not catastrophically.
The outer disc, deprived of new cold gas from the halo, has thinned at its edges.
The star forming disc is a little smaller than it was at infall. The galaxy's color has shifted marginally.
Bluer stars not quite replacing the older ones at the same rate. A careful observer comparing this galaxy now to what it was didn't fall would see a difference. A quick survey might miss it. The orbit continues. The galaxy falls deeper into the cluster. Its velocity increasing as the gravitational potential steepens. The ICM density rising with every kilo parseek of infall. And somewhere along this orbit, the exact location depending on the orbital parameters, the mass of the galaxy, the geometry of the approach, the ram pressure crosses the threshold at the outer disc. The stripping begins.
Not the slow stripping of the hot halo, the direct stripping of the cold disc gas, the ICM wind pressing against the disc's outer face, exceeding the gravitational restoring force at the outermost radi, pushing gas off the disc and into the trailing wake behind the galaxy's motion. The stripping radius, the radius inside which gas remains bound, moves inward. The outer disc empties. The extra plan of gas that was pushed off but not yet dispersed forms a tail. Detectable in Hway at 21 cm.
Detectable in H alpha if the stripped gas is being ionized by shocks or UV radiation. Detectable in CO if molecular gas has been dislodged along with the atomic gas in the denser outer regions.
The galaxy from the outside with the right instruments now looks like a jellyfish or the early stages of one.
The stellar disc is intact. The gas disc is truncated and asymmetric. The tail points away from the cluster center. At perentric passage, the closest approach to the cluster core where the ICM density peaks and the galaxy's velocity is highest. The ram pressure is at its maximum. The stripping in a single parasentric passage can be severe.
simulations consistently find that a large fraction of the outer disc gas can be removed in this single event. The galaxy emerges from the core with a gas disc that is substantially smaller than what it carried in. The tail it leaves behind during the passage disperses into the ICM over hundreds of millions of years. What the galaxy looks like after paracentric passage is something between what it was and what it is becoming. The inner disc is still there. Star formation continues, elevated briefly by the compression effects at the leading edge during the passage, then declining as the reduced reservoir reasserts itself. The central molecular gas is largely intact. If the galaxy is massive enough, an AGN may have been triggered by gas funneled inward during the stripping event, and that nucleus is now active, contributing its own feedback to the inner gas dynamics. The galaxy moves back outward toward the cluster out skirts on its first return orbit. Now slower than all it was on infall. Not because anything has decelerated it, because the outward leg of an orbit is always slower than the inward leg. The ram pressure drops as the ICM thins. The stripping slows, but it does not stop and it does not reverse. The gas that was removed is not returning. The replenishment from the hot halo is gone.
What remains in the disc is what remains. Harassment has been operating throughout. Every encounter with another cluster galaxy, brief, impulsive, gravitationally jarring, has added a small increment of kinetic energy to the stellar orbits. The disc has thickened slightly. The outer stellar regions, never dramatically disturbed, are nonetheless kinematically warmer than they were at infall. The spiral arms dependent on the discs cold gas for their density wave contrast have become less defined. The galaxy is becoming something quieter in its structure. The second paracentric passage, if it occurs, finds a different galaxy from the one that made the first. The gas disc is already reduced. The stripping this time removes what was left of the intermediate radius gas. The remaining star formation is now confined to the central region, drawing on the molecular gas that the gravitational restoring force has been strong enough to protect through two close approaches to the cluster core. Over the following billions of years, that central reservoir is consumed.
Slowly, star formation rates that were once substantial are now a small fraction of the original. The color continues to reen. The disc is still there.
Structurally, the stars that were formed over the galaxy's lifetime are still orbiting, will continue to orbit for billions of years. But nothing new is being built in the outer regions. At some point, and there is no sharp line to identify it, the galaxy crosses from star forming to quenched, not by any single event. By the exhaustion of the last usable cold gas in the nuclear region, the final star forming clumps use up their molecular clouds. The clouds are not replenished and the hi regions around the last generation of massive stars fade as those stars age and are not replaced.
What remains is a disc of old stars.
reddish, dynamically warmer than a spiral. Still rotating, but without the cold gas that made the rotation meaningful in terms of producing anything new. Structurally, it resembles a lenticular galaxy, a disc without the activity that made it a spiral. It will orbit the cluster for billions more years in this condition, slowly fading its stellar populations aging toward dimmer and redder configurations. A record of something that was once gas-rich and is no longer. The cluster did not break it. It just ran it down.
The Virgo cluster is young. That word young requires a moment when applied to something 53.8 8 million lighty years away that contains over a thousand galaxies and has a binding mass of more than a quadrillion suns relative to what exactly and by whose accounting.
But in cosmological terms in terms of how relaxed and settled a galaxy cluster ought to be after sufficient time has passed. Virgo is genuinely young. It has not reached dynamical equilibrium. Its substructures have not fully merged. Its galaxies are distributed in a way that reflects ongoing infall rather than a viralized system that has been sitting quietly for 10 billion years. The cluster is still being assembled right now in the light currently traveling toward instruments that will detect it decades or centuries from now. This matters for the galaxies inside it and for the galaxies not yet inside it at this moment, which is to say 53.8 8 million years ago, the only moment available to us at this distance. There are spirals crossing the viral radius of the Virgo cluster for the first time.
Gas-rich, star forming, structurally intact galaxies that have spent their entire existence in the relative calm of the field or in small groups and are now entering an environment their entire physical structure was not built to handle. The hot halo stripping has already begun for some of them, well outside the viral radius. The strangulation clock started without announcement.
They do not yet show the truncated hi discs or the one-sided tails that the deeper cluster members display. A survey catching them at this moment might classify them as normal. They are not normal anymore. The process has started.
Virgo's irregular morphology, the subclusters, the infalling groups, the asymmetric distribution of galaxies around the core is itself a record of this ongoing assembly. The cluster did not form all at once. It grew hierarchically, the way structure forms in the universe generally. Small things first, then larger things accreting and merging over time. the M87 subcluster, the M49 subcluster in the south, the M86 concentration. These are components that have not yet fully relaxed into a single smooth gravitational potential. They are still distinguishable, still maintaining some coherence as separate concentrations. Even as the larger cluster potential draws them together, the ICM reflects this. It is not smooth.
It has temperature variations, density inhomogeneities, bulk motions from the ongoing assembly, sloshing cold fronts, sharp edges in the X-ray surface brightness where cooler, denser gas has been displaced by the motion of infalling substructure, then sloshed back and forth in the gravitational potential have been detected in the Virgo cluster ECM and used to constrain the effective viscosity of the plasma.
The ICM is turbulent in ways that a fully relaxed cluster's ISCM would not be. And that turbulence affects the stripping process, making the RAM pressure experienced by infalling galaxies more variable and less predictable than the smooth analytic model suggest. Over the next several billion years, if the broad cosmological picture is approximately right, the Virgo cluster will continue to grow.
surrounding groups and their member galaxies will continue to fall in. The subclusters will merge more completely.
The ICM will gradually thermalize toward a smoother, hotter, more homogeneous state as the energy from mergers and infall dissipates into the plasma. The cluster will become in the parlance of observational cosmology more relaxed.
And as it relaxes, it will have processed more of its galaxies. The spirals currently falling in for the first time will over the next 2 to four billion years pass through the strangulation phase, cross the stripping threshold, complete their first paracentric passages. The ones that have not yet shown hi tails will show them.
The ones currently classified as mildly hi deficient will become severely deficient. The jellyfish morphologies visible in the current cluster population, the trailing tails, the truncated discs, the asymmetric gas distributions will appear in galaxies that currently look ordinary. And the galaxies currently in the jellyfish phase will move on to the post stripping phase, their tails dispersing, their discs shrinking to their final quenched configurations.
The fraction of early type galaxies in the cluster ellipticals and lenticulars galaxies that have already completed the transformation will increase. It has been increasing since the cluster formed. This is the morphology density relation observed across galaxy clusters. Generally denser environments contain higher fractions of early type galaxies because denser environments have been processing their spirals for longer and more intensely. Virgo being dynamically young has a lower early type fraction than older more relaxed clusters of similar mass. Give it a few billion years and that fraction will be higher. What will not change is the process itself. New galaxies will continue falling in. The mechanisms strangulation, harassment, ram pressure stripping will continue operating on each new arrival in the same sequence with the same physics, producing the same basic arc from gas-rich infall to quenched remnant.
The cluster will be older and more settled, but it will not have finished.
Clusters do not finish, they accrete.
The people who support this project through Patreon are part of why this kind of work gets made. quiet, unhurried, trying to do the science justice.
If you are one of them, thank you genuinely for keeping this going. There is something particular about studying a system that is still in process. Most objects of astronomical interest are either so fast evolving that we catch them in a single dramatic state, a supernova, a gammaray burst, a stellar merger, or so slow evolving that what we observe is essentially a fixed condition, a snapshot of something that will not look meaningfully different for billions of years. The Virgo cluster sits in a strange middle ground. The individual stripping events happen on time scales of hundreds of millions of years, which means we do not watch them unfold in real time. We see snapshots of different galaxies at different stages and assemble the sequence from the population statistics, but the cluster scale assembly, the merging of subclusters, the thermalization of the ICM, the long-term shift in galaxy population toward earlier types. These are happening on time scales that overlap meaningfully with the age of the universe itself. The cluster we observe today is a specific moment in a process that is genuinely ongoing. We are not watching a finished thing. We are watching something in the middle of becoming what it will be. And what it will be no one knows precisely. the details of which galaxies will fall in on what orbits, carrying how much gas, interacting with an ICM that will itself be different from what it is now. None of this is specified by the current observations. The broad strokes are clear enough. The specifics are not. The light leaving the Virgo cluster right now, the light that will reach Earth in 53.8 million years, is carrying information about galaxies currently in the middle of their infall sequences.
Galaxies being strangled, being stripped, being harassed. Galaxies whose hi tales are at this moment in the cluster's actual present trailing behind them as they fall. We will not be here to receive that light, but it is
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