Heh, that's every pseudoscience email ever.
Found on the internet :
[AUTHOR] is [BASICALLY, ENTIRELY, PRECISELY] [WRONG / RIGHT].
To be fair, [CONCESSION]. But [UNCHARITABLE RETRACTION OF THAT CONCESSION]. In this case, [SPECIAL PLEADING].
Presuming [THING WHICH IT IS INSANE TO PRESUME], then [OVERSTATED CONCLUSION.] But if [INSANE MISSTATEMENT OF OPPOSING ARGUMENT], then [UNCONDITIONAL RETRACTION OF MY BASIC POINT].
When [THING WHICH WILL NEVER HAPPEN] happens, then I will be proven conclusively right. But until that time, no one can criticize me for having made unfalsifiable conclusions, because they were merely conditional at the time I made them.
[SHORT STATEMENT WHICH IS LESS PITHY THAN I INTENDED]
Friday, 30 March 2018
Thursday, 29 March 2018
The Coolest Galaxy In The Universe
The press releases about a galaxy without any dark matter are doing the rounds, but are they accurate ? For once, yes ! This is a really genuinely strange object.
"To our knowledge this is the coolest known galaxy outside of the Local Group", say the authors. Sadly this galaxy isn't decked out with bling - they just mean it has a low velocity dispersion (which is sort-of equivalent to temperature). Measuring individual stars at this 20 Mpc (65 million light years) distance isn't possible, so they use globular clusters instead. These are much brighter, so they're easier to measure. Galaxies with large dark matter components (which is pretty nearly all of them) have their globular clusters orbiting around them like a swarm of normally very fast-moving bees.
In this case the bees are much more lethargic. The galaxy is of comparable size to the Milky Way, which rotates at about 220 km/s, but the globular clusters in this object are moving at only about 10 km/s (or even less). Speed measurements let you estimate total mass without needing to know anything about how bright the stars are, and usually masses measured in this way are 10x more than estimated by their brightness (because of the invisible dark matter). Not in this case though : its total mass is about 400 times less than typical galaxies of this size, and the motions can be explained entirely from the stellar mass. It doesn't seem to have any dark matter at all, which is almost unprecedented.
Unfortunately even the number of available globular clusters is small, just ten. Normally I wouldn't put much stock in such small numbers, but in this case the velocity of the clusters is so close to that of the galaxy it's difficult to believe this could happen by chance. There's no obvious reason for any selection effects here : globular clusters should be at random positions and velocities around their parent galaxy.
Of course there are some caveats. Not quite all galaxies lack dark matter. Tidal dwarf galaxies, produced by galaxies tearing bits off each other, seem to be pretty nearly devoid of the stuff. So could that explain this oddball ?
Probably not. Tidal dwarfs are well-accepted when you can actually see the tearing in action, e.g. denser clumps within long stellar streams, but much more controversial for other objects. This galaxy is in an unhappy middle ground : at only 80 kpc (260,000 light years) from the nearest bright galaxy, with other objects nearby, there's certainly scope for tidal interactions to have occurred. It's not close enough to say it's probably tidal, nor isolated enough to say that it probably isn't.
But against this, it's a very smooth, regular looking object, with no signs of long extended features. The encounters could have happened in the distant past and the fainter streams dispersed, but this galaxy is very large and very low-mass : it should be vulnerable to being disrupted itself by further encounters. It's tough to see how it could survive for very long. Actually this is true regardless of its origins, which would make for some fun simulations.
Another possibility is that the distance has been measured incorrectly, but that doesn't seem plausible either : the distances look solid, and if it was significantly different then it would have a very strong peculiar velocity. That can happen in massive groups and clusters, where the gravity of the group as a whole can accelerate individual galaxies to tremendous speeds. But this group is little, so that shouldn't happen. It would be weird, but, as the authors put it, "it is difficult to argue that it is less likely than having a highly peculiar globular cluster population and a lack of dark matter." Weird objects do tend to require weird explanations, after all... the key point is to check if this weird explanation is at least possible in this environment.
Could it just be a galaxy that's rotating but we're seeing it face-on ? In that case we wouldn't be able to measure the rotation since we can only do that for line of sight movement. The authors don't like this because discs normally have visible structures like spiral arms and whatnot, whereas this one doesn't. I'd caution that some lenticular galaxies look very smooth indeed - although they often do have some structure, it can be very low contrast.
And then there's the second, less-reported paper on the globular clusters themselves - they're a bit odd as well. Like other UDG's, the galaxies got literally tonnes* of globular clusters, but in this case they're generally more elongated and about four times brighter. So a weird galaxy surrounded by weird star clusters. Fun times !
* OK, lots of tonnes.
So what's going on ? Does this point to galaxy formation occurring by multiple mechanisms, with some requiring huge amounts of dark matter but some without ? Dunno. To answer that, we need more statistics both on this object and others. Lots of ultra-diffuse galaxies are known, but dynamical mass estimates are still very rare.
Does it challenge our ideas of gravity ? Well, the authors say it disfavours popular alternatives to dark matter. They rely on reproducing the dark matter evidence just from the stars and gas alone, so in those models all galaxies should appear to have dark matter. There's no reason some galaxies should have a fake appearance of missing mass while others don't. In contrast, if dark matter does exist then it's a bit easier to believe that maybe some galaxies just don't have very much of it - there's no problem in principle here, at least.
A caveat is that some alternative theories of gravity are bloody complicated, and the external gravity of nearby galaxies can change the dynamics. This "external field effect" is routinely invoked to explain just about any problems with such theories, which is extremely irritating. But in this case, since there are nearby massive galaxies, the effect can probably be at least estimated.
So for once the press release matches the paper, and neither seem unreasonable. But, as usual, this raises more questions than answers.
Related reading :
https://arxiv.org/abs/1803.10237
https://arxiv.org/abs/1803.10240
http://astrorhysy.blogspot.cz/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html
https://arxiv.org/abs/1803.10237
"To our knowledge this is the coolest known galaxy outside of the Local Group", say the authors. Sadly this galaxy isn't decked out with bling - they just mean it has a low velocity dispersion (which is sort-of equivalent to temperature). Measuring individual stars at this 20 Mpc (65 million light years) distance isn't possible, so they use globular clusters instead. These are much brighter, so they're easier to measure. Galaxies with large dark matter components (which is pretty nearly all of them) have their globular clusters orbiting around them like a swarm of normally very fast-moving bees.
In this case the bees are much more lethargic. The galaxy is of comparable size to the Milky Way, which rotates at about 220 km/s, but the globular clusters in this object are moving at only about 10 km/s (or even less). Speed measurements let you estimate total mass without needing to know anything about how bright the stars are, and usually masses measured in this way are 10x more than estimated by their brightness (because of the invisible dark matter). Not in this case though : its total mass is about 400 times less than typical galaxies of this size, and the motions can be explained entirely from the stellar mass. It doesn't seem to have any dark matter at all, which is almost unprecedented.
Unfortunately even the number of available globular clusters is small, just ten. Normally I wouldn't put much stock in such small numbers, but in this case the velocity of the clusters is so close to that of the galaxy it's difficult to believe this could happen by chance. There's no obvious reason for any selection effects here : globular clusters should be at random positions and velocities around their parent galaxy.
Of course there are some caveats. Not quite all galaxies lack dark matter. Tidal dwarf galaxies, produced by galaxies tearing bits off each other, seem to be pretty nearly devoid of the stuff. So could that explain this oddball ?
Probably not. Tidal dwarfs are well-accepted when you can actually see the tearing in action, e.g. denser clumps within long stellar streams, but much more controversial for other objects. This galaxy is in an unhappy middle ground : at only 80 kpc (260,000 light years) from the nearest bright galaxy, with other objects nearby, there's certainly scope for tidal interactions to have occurred. It's not close enough to say it's probably tidal, nor isolated enough to say that it probably isn't.
But against this, it's a very smooth, regular looking object, with no signs of long extended features. The encounters could have happened in the distant past and the fainter streams dispersed, but this galaxy is very large and very low-mass : it should be vulnerable to being disrupted itself by further encounters. It's tough to see how it could survive for very long. Actually this is true regardless of its origins, which would make for some fun simulations.
Another possibility is that the distance has been measured incorrectly, but that doesn't seem plausible either : the distances look solid, and if it was significantly different then it would have a very strong peculiar velocity. That can happen in massive groups and clusters, where the gravity of the group as a whole can accelerate individual galaxies to tremendous speeds. But this group is little, so that shouldn't happen. It would be weird, but, as the authors put it, "it is difficult to argue that it is less likely than having a highly peculiar globular cluster population and a lack of dark matter." Weird objects do tend to require weird explanations, after all... the key point is to check if this weird explanation is at least possible in this environment.
Could it just be a galaxy that's rotating but we're seeing it face-on ? In that case we wouldn't be able to measure the rotation since we can only do that for line of sight movement. The authors don't like this because discs normally have visible structures like spiral arms and whatnot, whereas this one doesn't. I'd caution that some lenticular galaxies look very smooth indeed - although they often do have some structure, it can be very low contrast.
And then there's the second, less-reported paper on the globular clusters themselves - they're a bit odd as well. Like other UDG's, the galaxies got literally tonnes* of globular clusters, but in this case they're generally more elongated and about four times brighter. So a weird galaxy surrounded by weird star clusters. Fun times !
* OK, lots of tonnes.
So what's going on ? Does this point to galaxy formation occurring by multiple mechanisms, with some requiring huge amounts of dark matter but some without ? Dunno. To answer that, we need more statistics both on this object and others. Lots of ultra-diffuse galaxies are known, but dynamical mass estimates are still very rare.
Does it challenge our ideas of gravity ? Well, the authors say it disfavours popular alternatives to dark matter. They rely on reproducing the dark matter evidence just from the stars and gas alone, so in those models all galaxies should appear to have dark matter. There's no reason some galaxies should have a fake appearance of missing mass while others don't. In contrast, if dark matter does exist then it's a bit easier to believe that maybe some galaxies just don't have very much of it - there's no problem in principle here, at least.
A caveat is that some alternative theories of gravity are bloody complicated, and the external gravity of nearby galaxies can change the dynamics. This "external field effect" is routinely invoked to explain just about any problems with such theories, which is extremely irritating. But in this case, since there are nearby massive galaxies, the effect can probably be at least estimated.
So for once the press release matches the paper, and neither seem unreasonable. But, as usual, this raises more questions than answers.
Related reading :
https://arxiv.org/abs/1803.10237
https://arxiv.org/abs/1803.10240
http://astrorhysy.blogspot.cz/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html
https://arxiv.org/abs/1803.10237
Wednesday, 28 March 2018
The giants have failed... or have they ?
Quite a nice little letter about ultra diffuse galaxies, those huge ghostly galaxies with hardly any stars. Do they have a crapload of dark matter, as their huge size suggests, or not that much at all, in proportion to their pathetic stellar mass ?
Thus far the issue has been pretty one-sided. Van Dokkum et al. keep claiming that they're probably massive, "failed" galaxies that do a lousy job at forming stars (which would pretty much break the current paradigm of galaxy formation); everyone else seems to think they're not very massive objects but somehow became very spread out. This paper sees a new team join in on the side of the failed giants.
The authors have spectroscopic measurements of globular clusters in and around 3 UDGs in the Virgo cluster (for some reason, Virgo doesn't seem to have as many UDGs as other clusters). Unfortunately the numbers of globular clusters is very low in each case, around 10. Still, the velocity dispersion seems high enough to indicate that all of them are heavily dark matter dominated. One of them looks like a reasonable candidate for a failed giant, another might be but it's more tentative. With this low number of data points, constructing nice rotation curves just isn't possible yet, although they do try.
Another possibility for these objects is that they could just be tidal debris stripped out of galaxies and not galaxies themselves at all. In this case their high velocity dispersion would just mean that they're disintegrating rather than being bound by a dark matter halo. They say this is particularly likely for one of the (probably less massive) objects, which looks disturbed. Personally I'd be a bit more cautious about the others too, but in general they do a nice job of describing the alternative explanations.
The problem with these super-faint smudges is that you can't really get a proper mass measurement even in the relatively small regions where the globular clusters are detected - with a high error, it's more of an estimate. They're not even sure if the objects are rotating. But this isn't too bad; what I really wish they'd state more directly is that the inferred total mass (assuming the dark matter halo extends like in normal galaxies) is about 100x the mass estimate from observations.
It's great to see more people examining the possibility that these things are really weird objects that don't fit the standard models (and this team is an eminently reputable one, so let's here no more talk of scientific closed-mindedness, thank you). But realistically, we're going to need much better data to say anything more definitive about them.
https://arxiv.org/abs/1803.09768
Thus far the issue has been pretty one-sided. Van Dokkum et al. keep claiming that they're probably massive, "failed" galaxies that do a lousy job at forming stars (which would pretty much break the current paradigm of galaxy formation); everyone else seems to think they're not very massive objects but somehow became very spread out. This paper sees a new team join in on the side of the failed giants.
The authors have spectroscopic measurements of globular clusters in and around 3 UDGs in the Virgo cluster (for some reason, Virgo doesn't seem to have as many UDGs as other clusters). Unfortunately the numbers of globular clusters is very low in each case, around 10. Still, the velocity dispersion seems high enough to indicate that all of them are heavily dark matter dominated. One of them looks like a reasonable candidate for a failed giant, another might be but it's more tentative. With this low number of data points, constructing nice rotation curves just isn't possible yet, although they do try.
Another possibility for these objects is that they could just be tidal debris stripped out of galaxies and not galaxies themselves at all. In this case their high velocity dispersion would just mean that they're disintegrating rather than being bound by a dark matter halo. They say this is particularly likely for one of the (probably less massive) objects, which looks disturbed. Personally I'd be a bit more cautious about the others too, but in general they do a nice job of describing the alternative explanations.
The problem with these super-faint smudges is that you can't really get a proper mass measurement even in the relatively small regions where the globular clusters are detected - with a high error, it's more of an estimate. They're not even sure if the objects are rotating. But this isn't too bad; what I really wish they'd state more directly is that the inferred total mass (assuming the dark matter halo extends like in normal galaxies) is about 100x the mass estimate from observations.
It's great to see more people examining the possibility that these things are really weird objects that don't fit the standard models (and this team is an eminently reputable one, so let's here no more talk of scientific closed-mindedness, thank you). But realistically, we're going to need much better data to say anything more definitive about them.
https://arxiv.org/abs/1803.09768
Friday, 23 March 2018
A huge hydrogen stream in an galaxy group no-one's ever heard of
Very interesting paper today about the discovery of an enormous, 500 kpc (about 1.6 million light years) hydrogen stream in an otherwise obscure galaxy group. Earlier observations with ATCA (Australian Telescope Compact Array) had found something, but observations with the shiny new Karoo Array Telescope (KAT-7) revealed a much larger, fainter structure. KAT-7 is only a precursor to the larger MeerKAT, which is itself a prototype for the even larger Square Kilometre Array, giving some indication of what we'll find with the next generation of radio telescopes.
For comparison, as far as extended gas goes Arecibo is about ten times more sensitive than KAT-7, which is itself about ten times more sensitive than the VLA. The other advantage of KAT-7 and other SKA "pathfinder" instruments is that the field of view is much larger than in the older facilities. Getting that sensitivity increased by another factor of 10, though, is a formidable challenge indeed, and new instrumentation on Arecibo could expand its field of view by a factor of a few. The VLA also possesses superior resolution. In short, radio instrumentation is fiendishly complicated, and anyone who thinks it isn't does not know what they're talking about.
Anywho, this new giant gas stream (see their figure 2) is in a fairly small galaxy group. In galaxy clusters, the main gas loss mechanism is believed to be ram pressure stripping, where galaxies move through the hot, thin surrounding gas. That can't really be the explanation here, since a) there isn't much in the way of external gas; b) the galaxies are moving too slowly. Which means the most likely explanation is that the galaxies are gravitationally interacting. Unlike ram pressure, that's actually more effective because of the low velocities, since it gives the gravitational forces more time to act.
This is sort of plausible for this case, but there are some interesting oddities. Their figure 4 shows higher resolution (but lower sensitivity) observations with the VLA of the likely parent galaxy of the stream. It does seem to be interacting with a nearby companion, in that they both have one-sided gaseous extensions. But normally such interactions produce two tails, characteristically on opposite sides. That doesn't seem to be the case here. The authors say there's a twin-tail structure, but I don't see it.
A lot of the other galaxies in the group also seem to be interacting. Taken together they form a single, coherent, giant structure. The velocity of the gas shows pretty convincingly (figure 3) that this is likely a single entity, not a chance alignment of different features. I suppose that's possible though, if the galaxies were all falling into the group along a filament. It also shows a strange bifurcation, with one part of the stream at one velocity but (at the same spatial location) other parts are at quite different velocities. And the velocity changes look to be pretty sharp. They also say that there's a cloud nearby with no optical counterpart, though it isn't clear to me which feature they mean by this or its velocity gradient.
In context this is especially interesting to me because we've shown how it's possible to make this "kinky" velocity structures in galaxy clusters (http://astrorhysy.blogspot.cz/2017/01/check-out-my-kinky-curves.html). But clusters are much more massive than groups, so I wouldn't necessarily expect the same effects in groups. Naively, I would expect the lower speed of the interactions to be better at drawing out long gas streams, but worse at causing kinks. Only more simulations will answer that one.
https://arxiv.org/abs/1803.08263
For comparison, as far as extended gas goes Arecibo is about ten times more sensitive than KAT-7, which is itself about ten times more sensitive than the VLA. The other advantage of KAT-7 and other SKA "pathfinder" instruments is that the field of view is much larger than in the older facilities. Getting that sensitivity increased by another factor of 10, though, is a formidable challenge indeed, and new instrumentation on Arecibo could expand its field of view by a factor of a few. The VLA also possesses superior resolution. In short, radio instrumentation is fiendishly complicated, and anyone who thinks it isn't does not know what they're talking about.
Anywho, this new giant gas stream (see their figure 2) is in a fairly small galaxy group. In galaxy clusters, the main gas loss mechanism is believed to be ram pressure stripping, where galaxies move through the hot, thin surrounding gas. That can't really be the explanation here, since a) there isn't much in the way of external gas; b) the galaxies are moving too slowly. Which means the most likely explanation is that the galaxies are gravitationally interacting. Unlike ram pressure, that's actually more effective because of the low velocities, since it gives the gravitational forces more time to act.
This is sort of plausible for this case, but there are some interesting oddities. Their figure 4 shows higher resolution (but lower sensitivity) observations with the VLA of the likely parent galaxy of the stream. It does seem to be interacting with a nearby companion, in that they both have one-sided gaseous extensions. But normally such interactions produce two tails, characteristically on opposite sides. That doesn't seem to be the case here. The authors say there's a twin-tail structure, but I don't see it.
A lot of the other galaxies in the group also seem to be interacting. Taken together they form a single, coherent, giant structure. The velocity of the gas shows pretty convincingly (figure 3) that this is likely a single entity, not a chance alignment of different features. I suppose that's possible though, if the galaxies were all falling into the group along a filament. It also shows a strange bifurcation, with one part of the stream at one velocity but (at the same spatial location) other parts are at quite different velocities. And the velocity changes look to be pretty sharp. They also say that there's a cloud nearby with no optical counterpart, though it isn't clear to me which feature they mean by this or its velocity gradient.
In context this is especially interesting to me because we've shown how it's possible to make this "kinky" velocity structures in galaxy clusters (http://astrorhysy.blogspot.cz/2017/01/check-out-my-kinky-curves.html). But clusters are much more massive than groups, so I wouldn't necessarily expect the same effects in groups. Naively, I would expect the lower speed of the interactions to be better at drawing out long gas streams, but worse at causing kinks. Only more simulations will answer that one.
https://arxiv.org/abs/1803.08263
Wednesday, 21 March 2018
I am super famous now
I haven't read the article yet. Stephen Hawking used one of my graphics !
http://www.rhysy.net/other-1.html
Confirmation 7 minutes in :
https://www.youtube.com/watch?v=QF9jAGyL1fg
Excuse me while I go and stroke my ego with a nice cup of tea.
“The questions that they’re attempting to answer are still valid, open questions, and the best this paper can do — if it’s correct and relevant, and it may be neither — is provide suggestions towards an answer. The approach is largely based off of work that Hartle, Hawking, and Hertog have done in the past, the dS/CFT connection pioneered by Chris Hull and others, along with string-inspired work done by Andrew Strominger and his collaborators. None of this is based off of any realistic cosmological models; these are toy models that they are calculating in, and then reasoning-by-analogy with what we actually know exists. Like most theoretical work in the very early stages, there are interesting ideas that are presented, the work and calculations are highly speculative, and there is not necessarily a connection with reality. But there’s a non-zero chance that one is real. And in theoretical physics, a novel idea with a chance is worth infinitely more than no new ideas at all.”
There have been a lot of incredible claims floating around the media about what’s going on with Stephen Hawking’s final paper, which was submitted earlier in March, less than two weeks before he died. Some are claiming it will help us detect the multiverse, others claiming that it will tell us how the Universe will end. The truth is much more sobering, however: it discusses issues involving the dynamics of inflation. There are incredible questions we’re trying to understand about the Universe, such as: did inflation begin, or was it eternal; will it continue indefinitely into the future; does it inevitably lead to a multiverse; did time and space begin with a singularity? These are very important, and Hawking’s final paper was the construction of a toy model that argued “yes” for the final question. But it has nothing to do with the hype surrounding it.
Let’s not deify our heroes; let’s allow their good work to stand on their own merits. And most importantly, let’s be honest about what they did. Here’s the truth."
https://www.forbes.com/sites/startswithabang/2018/03/21/i-am-an-astrophysicist-heres-what-stephen-hawkings-final-paper-was-actually-about/
http://www.rhysy.net/other-1.html
Confirmation 7 minutes in :
https://www.youtube.com/watch?v=QF9jAGyL1fg
Excuse me while I go and stroke my ego with a nice cup of tea.
“The questions that they’re attempting to answer are still valid, open questions, and the best this paper can do — if it’s correct and relevant, and it may be neither — is provide suggestions towards an answer. The approach is largely based off of work that Hartle, Hawking, and Hertog have done in the past, the dS/CFT connection pioneered by Chris Hull and others, along with string-inspired work done by Andrew Strominger and his collaborators. None of this is based off of any realistic cosmological models; these are toy models that they are calculating in, and then reasoning-by-analogy with what we actually know exists. Like most theoretical work in the very early stages, there are interesting ideas that are presented, the work and calculations are highly speculative, and there is not necessarily a connection with reality. But there’s a non-zero chance that one is real. And in theoretical physics, a novel idea with a chance is worth infinitely more than no new ideas at all.”
There have been a lot of incredible claims floating around the media about what’s going on with Stephen Hawking’s final paper, which was submitted earlier in March, less than two weeks before he died. Some are claiming it will help us detect the multiverse, others claiming that it will tell us how the Universe will end. The truth is much more sobering, however: it discusses issues involving the dynamics of inflation. There are incredible questions we’re trying to understand about the Universe, such as: did inflation begin, or was it eternal; will it continue indefinitely into the future; does it inevitably lead to a multiverse; did time and space begin with a singularity? These are very important, and Hawking’s final paper was the construction of a toy model that argued “yes” for the final question. But it has nothing to do with the hype surrounding it.
Let’s not deify our heroes; let’s allow their good work to stand on their own merits. And most importantly, let’s be honest about what they did. Here’s the truth."
https://www.forbes.com/sites/startswithabang/2018/03/21/i-am-an-astrophysicist-heres-what-stephen-hawkings-final-paper-was-actually-about/
Tuesday, 20 March 2018
The latest lowdown on ultra diffuse galaxies
Couple of things on ultra diffuse galaxies, those super-faint galaxies as large as the Milky Way.
The first, linked below, is a two-page research note indignantly pointing out that these things are nothing new. The author's being working on these for years, but never thought to give them a fancy name or make silly claims as to discovering a new class of galaxy. Well, I think we kindof all knew that anyway, it's just now they're being discovered en masse and in many different environments. I don't remember them ever being found in such numbers - they always seemed more like unusual objects rather than a huge component of the general galaxy population; a few objects here and there might be collectively significant, but far more thinly spread than the latest discoveries. With the caveat that small faint galaxies were indeed known to be common, and probably unfairly neglected, but not the bigger guys as far as I'm aware.
The major issue with these things is how much dark matter they contain. Everyone (who believes in dark matter at all) accepts that they're more strongly dominated by dark matter than most other systems of the same mass or size. The question is, how much ? If it's just a little, then they're just a bit more massive than normal dwarf galaxies, and fully compatible with standard models of galaxy formation. If it's a lot, then they pose a real problem, because galaxies this massive ought bloody well to be forming lots of stars.
That's where the second paper comes in. To get rotation, you normally measure the velocity of the gas or stars at many different points. This is extremely difficult when the system's 100-1,000 times fainter than usual, so what they do instead is to measure motion at just three points in each of five galaxies : the centre, and two points either side. This can give an estimate of the maximum rotation velocity, which is what you need to constrain total mass. They find that they're more like biggish dwarves and see no evidence of a more massive population.
In other ways the finger seems to be pointing to extreme dwarfs rather than stealthy giants. The chemical composition is more similar to dwarfs than giants, their stellar masses are similar to dwarfs, and it's unclear if they're rotating (like giant disc galaxies) or if their stars move on more random orbits (like low-mass dwarfs).
And yet I wouldn't write-off the stealthy giants just yet. The first paper says that these UDGs are more common in clusters, but so is every sort of galaxy. The main mechanism proposed to explain these objects, if they didn't start off with their unusual appearance, is that interactions with other galaxies stripped out most of their stars. The first authors point out that they have no correlation between their size and stellar mass, unlike normal galaxies, which is what you'd expect if they were formed by such a random series of events.
The problem is that we know such objects are also found not only in massive, rich clusters, but also in smaller groups and even in pretty extreme isolation. This harassment process simply can't work in such cases. Similarly, there's not enough discussion on the gas observations : the authors of the second paper say it's unclear if their galaxies are rotating (which to be fair, it is) but the gas measurements of at least some look exactly like they're rotating. And, so far as I know, no-one has seen any evidence of the extended stellar tails you'd expect to see if this things really were formed by harassment. When they have gas, they look for all the world like normal star-forming galaxies, just much fainter; when they don't, they look like normal elliptical, quiescent galaxies. Their chemical composition might be similar to dwarfs only because they have similar stellar masses, i.e. similar amounts of chemical processing going on. Furthermore, those with gas sometimes have as much gas as normal bright galaxies. I don't see how such objects can be explained in the harassment model - the gas is easier to remove than stars, so you'd expect them to be severely gas depleted as well, but they're not.
Almost certainly, there are different populations of these galaxies. Calling them UDGs to distinguish them from the general population of faint galaxies is maybe somewhat misleading though; we don't yet know what really sets these things apart or not. Conceivably they could be formed by different mechanisms in different environments, with some being formed more or less as they appear now and others via some evolution of normal galaxies. We still don't really know what's going on with these things.
More information on UDGs here.
https://arxiv.org/abs/1803.06927
The first, linked below, is a two-page research note indignantly pointing out that these things are nothing new. The author's being working on these for years, but never thought to give them a fancy name or make silly claims as to discovering a new class of galaxy. Well, I think we kindof all knew that anyway, it's just now they're being discovered en masse and in many different environments. I don't remember them ever being found in such numbers - they always seemed more like unusual objects rather than a huge component of the general galaxy population; a few objects here and there might be collectively significant, but far more thinly spread than the latest discoveries. With the caveat that small faint galaxies were indeed known to be common, and probably unfairly neglected, but not the bigger guys as far as I'm aware.
The major issue with these things is how much dark matter they contain. Everyone (who believes in dark matter at all) accepts that they're more strongly dominated by dark matter than most other systems of the same mass or size. The question is, how much ? If it's just a little, then they're just a bit more massive than normal dwarf galaxies, and fully compatible with standard models of galaxy formation. If it's a lot, then they pose a real problem, because galaxies this massive ought bloody well to be forming lots of stars.
That's where the second paper comes in. To get rotation, you normally measure the velocity of the gas or stars at many different points. This is extremely difficult when the system's 100-1,000 times fainter than usual, so what they do instead is to measure motion at just three points in each of five galaxies : the centre, and two points either side. This can give an estimate of the maximum rotation velocity, which is what you need to constrain total mass. They find that they're more like biggish dwarves and see no evidence of a more massive population.
In other ways the finger seems to be pointing to extreme dwarfs rather than stealthy giants. The chemical composition is more similar to dwarfs than giants, their stellar masses are similar to dwarfs, and it's unclear if they're rotating (like giant disc galaxies) or if their stars move on more random orbits (like low-mass dwarfs).
And yet I wouldn't write-off the stealthy giants just yet. The first paper says that these UDGs are more common in clusters, but so is every sort of galaxy. The main mechanism proposed to explain these objects, if they didn't start off with their unusual appearance, is that interactions with other galaxies stripped out most of their stars. The first authors point out that they have no correlation between their size and stellar mass, unlike normal galaxies, which is what you'd expect if they were formed by such a random series of events.
The problem is that we know such objects are also found not only in massive, rich clusters, but also in smaller groups and even in pretty extreme isolation. This harassment process simply can't work in such cases. Similarly, there's not enough discussion on the gas observations : the authors of the second paper say it's unclear if their galaxies are rotating (which to be fair, it is) but the gas measurements of at least some look exactly like they're rotating. And, so far as I know, no-one has seen any evidence of the extended stellar tails you'd expect to see if this things really were formed by harassment. When they have gas, they look for all the world like normal star-forming galaxies, just much fainter; when they don't, they look like normal elliptical, quiescent galaxies. Their chemical composition might be similar to dwarfs only because they have similar stellar masses, i.e. similar amounts of chemical processing going on. Furthermore, those with gas sometimes have as much gas as normal bright galaxies. I don't see how such objects can be explained in the harassment model - the gas is easier to remove than stars, so you'd expect them to be severely gas depleted as well, but they're not.
Almost certainly, there are different populations of these galaxies. Calling them UDGs to distinguish them from the general population of faint galaxies is maybe somewhat misleading though; we don't yet know what really sets these things apart or not. Conceivably they could be formed by different mechanisms in different environments, with some being formed more or less as they appear now and others via some evolution of normal galaxies. We still don't really know what's going on with these things.
More information on UDGs here.
https://arxiv.org/abs/1803.06927
Tuesday, 13 March 2018
Destroying the Milky Way for funzies
More visualisation experiments with SILCC, a project to simulate the evolution of a small section of the Milky Way's disc (https://hera.ph1.uni-koeln.de/~silcc/). This one shows the actual behaviour rather than just a simple snapshot. Unfortunately, because of the vast size of the data, only every 10th output is stored. This means the low density gas, which gets blasted by supernovae to extremely high velocities, is very hard to follow. The denser gas (shown in blue) is a bit better.
Only one component of the gas is shown here. The simulations include multiple phases of the gas (molecular, atomic, ionised, different elements, etc.) but that's for next time.
In this particular run the disc blasts itself apart. I'm guessing there's too much energy injected by the supernovae (either too many supernovae or too much energy per explosion). Other runs are much more stable.
Monday, 12 March 2018
Milky Way simulation in FRELLED
A little visualisation test of the SILCC data, which simulates a small part of the Milky Way's gas disc using a whole bunch of physics.
https://hera.ph1.uni-koeln.de/~silcc/
Wednesday, 7 March 2018
A new dark hydrogen cloud in the Virgo cluster
The other paper I read today on the train. This one's about an almost dark hydrogen cloud in the Virgo cluster, a.k.a. my personal obsession.
This particular cloud has been detected some time ago, but this new analysis adds more observations and simulations. It's got about 30 million solar masses of hydrogen and maybe as many as 100,000 solar masses of stars, although no-one is quite sure exactly. In any case, it's got way more gas than stars. Earlier papers hinted that it might be rotating, but very slowly. It's also very compact, perhaps only 1/30th the size of the Milky Way. And it's really far from any other galaxies, so it's not easy to see how the little blighter formed.
Although the thing is so bloody faint it's hard to be entirely sure, the authors found no evidence of an old stellar population : stars don't seem to have started forming until about 50 Myr ago. Which would make it an incredibly young galaxy, and that would fit quite well with its overall properties (mass, size, velocity dispersion). But an old stellar population would be hard to detect, so the limits on that seem to be pretty crappy really.
More secure are the author's measurements of the cloud's chemical composition. If this really was a young galaxy, it should be nearly pristine, primordial gas. But it isn't - it's got the composition of a far more massive galaxy. With this few stars, it's impossible to produce such a composition in the 50 Myr time frame. What this means, the author's say, is that it's far more likely this cloud was ripped out of a galaxy.
But, given the distance of the nearest galaxy this cloud could have originated in, this means the itty-bitty thing would have to survive a billion years of travel through the hot, oppressive intracluster gas. Can it ? Their simulations say yes, quite happily - the intracluster gas actually helps confine it and prevents it from flying apart. As you may have seen (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/SbWeh4rJapA), we're running our own simulations of such an idea in Prague, with blackjack, and hookers... but we get a totally different result. The reason seems to be that the clouds we're interested in have a much greater velocity dispersion, so blast themselves apart in a great big splooch. The cloud in this paper doesn't have that problem : its thermal pressure can balance out nicely with the surrounding gas pressure, and it stays as a happy little gas cloud for a billion years or more.
All well and good, but of course the mystery isn't solved yet. As the authors note, it's difficult to explain why the cloud should have gone along quietly minding its own business for a billion years and then VERY SUDDENLY DECIDED IT WANTED TO FORM STARS for some reason. It's not near any galaxies today, so tidal encounters can't explain it. While clouds like this one have been produced in lots of different studies, they're always part of a much larger star-forming "wake" of stripped material. In this case, that material is completely absent. Where's it gone ? Why did this lonely little guy survive, all alone on the wide wide sea ?
No-one knows. But if the bloomin' Arecibo Time Allocation Committee would just bloomin' well realise the value of big surveys, we could detect a whole bunch more of these critters and maybe start to figure something out.
http://adsabs.harvard.edu/doi/10.1093/mnras/sty467
This particular cloud has been detected some time ago, but this new analysis adds more observations and simulations. It's got about 30 million solar masses of hydrogen and maybe as many as 100,000 solar masses of stars, although no-one is quite sure exactly. In any case, it's got way more gas than stars. Earlier papers hinted that it might be rotating, but very slowly. It's also very compact, perhaps only 1/30th the size of the Milky Way. And it's really far from any other galaxies, so it's not easy to see how the little blighter formed.
Although the thing is so bloody faint it's hard to be entirely sure, the authors found no evidence of an old stellar population : stars don't seem to have started forming until about 50 Myr ago. Which would make it an incredibly young galaxy, and that would fit quite well with its overall properties (mass, size, velocity dispersion). But an old stellar population would be hard to detect, so the limits on that seem to be pretty crappy really.
More secure are the author's measurements of the cloud's chemical composition. If this really was a young galaxy, it should be nearly pristine, primordial gas. But it isn't - it's got the composition of a far more massive galaxy. With this few stars, it's impossible to produce such a composition in the 50 Myr time frame. What this means, the author's say, is that it's far more likely this cloud was ripped out of a galaxy.
But, given the distance of the nearest galaxy this cloud could have originated in, this means the itty-bitty thing would have to survive a billion years of travel through the hot, oppressive intracluster gas. Can it ? Their simulations say yes, quite happily - the intracluster gas actually helps confine it and prevents it from flying apart. As you may have seen (https://plus.google.com/u/0/+RhysTaylorRhysy/posts/SbWeh4rJapA), we're running our own simulations of such an idea in Prague, with blackjack, and hookers... but we get a totally different result. The reason seems to be that the clouds we're interested in have a much greater velocity dispersion, so blast themselves apart in a great big splooch. The cloud in this paper doesn't have that problem : its thermal pressure can balance out nicely with the surrounding gas pressure, and it stays as a happy little gas cloud for a billion years or more.
All well and good, but of course the mystery isn't solved yet. As the authors note, it's difficult to explain why the cloud should have gone along quietly minding its own business for a billion years and then VERY SUDDENLY DECIDED IT WANTED TO FORM STARS for some reason. It's not near any galaxies today, so tidal encounters can't explain it. While clouds like this one have been produced in lots of different studies, they're always part of a much larger star-forming "wake" of stripped material. In this case, that material is completely absent. Where's it gone ? Why did this lonely little guy survive, all alone on the wide wide sea ?
No-one knows. But if the bloomin' Arecibo Time Allocation Committee would just bloomin' well realise the value of big surveys, we could detect a whole bunch more of these critters and maybe start to figure something out.
http://adsabs.harvard.edu/doi/10.1093/mnras/sty467
Explaining the MDAR
Since the MDAR is back in the news on arXiv again, and I had a 5-hour train trip without wifi, I decided to catch up on my reading. I read this paper last year when it came out, but I never got around to annotating it or translating it into a slightly friendlier form.
For those who've forgotten all about it, there was a great furore last year because of a discovery in galaxy dynamics that didn't seem to fit the standard model. Specifically, there's a relationship between the acceleration predicted by the measured mass and distribution of stars and gas and the acceleration they're observed to be experiencing. Why is that weird and why would anyone care about some obscure piece of mathematical analysis ? Wouldn't they rather be eating cake ?
Possibly. But the MDAR* is quite interesting**. A whole slew of observations and theory have pointed to the need for dark matter to dominate galaxy rotation, typically by a factor of a few to a few tens. Ordinary (baryonic) matter is just the icing on the rich, deliciously dark cake***. So there ought to be a strong discrepancy between the acceleration predicted just from normal matter (without accounting for dark matter) and its observed value. Naively, we might not expect there to be much of a relationship at all, with galaxies lying all over the shop. The mass of baryonic matter shouldn't really tell us anything about the dark matter.
* Mass Discreprency Acceleration Relation. Or maybe it's RAR, the Radial Acceleration Relation. Different authors seem to prefer different things, so I'm going with MDAR.
** So interesting that people started calling it a new law of nature, which made a lot of people very upset.
*** I'm hungry.
But that's not what was found. Although there isn't a neat 1:1 correlation, there is a very tight relation between the normal and dark matter. In effect, the mass of the normal matter allows you to predict with a fair precision the mass of dark matter. This was exactly what everyone's favourite alternative gravity theory (MOND) has long predicited.
In response to this there was a slew of papers back and forth by the various sides trying to work out if this was a really cool discovery or if it was all just a bit silly. Almost instantly, papers came out showing that the MDAR happened in standard model simulations without any tweaking whatsoever. No-one gave a clear explanation as to why it happened, much to the disgust of the MOND supporters, but happen it certainly did.
This paper attempts to provide that explanation. And it's not exactly simple, so bear with me.
In the standard model, galaxies which form in isolation can only do so in a quite narrow range of dark matter "halo" masses. Or more specifically, detectable galaxies. Too small and supernovae blast out all the gas, or hot young stars blow it out with strong winds, preventing any more star formation. Dark matter halos might well exist below this lower limit, but they can't host detectable galaxies. Going in the other direction, galaxies which become too massive have powerful feedback from active galactic nuceli (supermassive black holes with accretion discs and jets and whatnot) and the gas takes longer to cool and form stars. So galaxy growth is heavily restricted.
That means, in effect, that you can't get galaxies outside a certain, reasonably narrow acceleration window : no discs ever rotate faster than ~300 km/s, and none slower than ~30 km/s. Since their size doesn't vary all that much, neither does their acceleration.
Above a characteristic acceleration, all galaxies are dominated by their baryons - making the relationship of predicted/observed acceleration steeper the closer you get to this acceleration value. The baryon/dark matter ratio, which varies with total mass, is something that's reasonably (though not brilliantly) already predicted by the cold dark matter (CDM) model.
In addition, CDM makes a very specific prediction for how the dark matter is distributed within each halo, whereas stars, we know from observations, are distributed quite differently. All of this combines to give a very tight behaviour of the acceleration of matter within a galaxy. In the inner regions it tends to be dominated by the dense baryonic matter, whereas in the outer regions it's more about the dark matter. And you can't observe the acceleration beyond a certain radius (where acceleration is very low and dark matter dominated) because the detectable matter just doesn't extend that far - or equivalently, below a certain characteristic acceleration.
This lower acceleration is pretty close to the value where dark matter dominates. So for galaxies of very low baryonic content, at any radius they will be more strongly dominated by dark matter than baryons - and so the MDAR becomes shallower (and you only get galaxies of very low baryon content at low total masses), and galaxies tend to deviate a bit more from the standard MDAR relation. Moreover, if you consider the acceleration variation within individual (faint, low mass) galaxies , it will still be dominated by dark matter at all points, giving the same characteristic slope. Hence the MDAR is visible if consider either entire galaxies (choosing some well-defined point at which to measure their acceleration) or the acceleration at many points within each galaxy.
Another, more crude way to think about this is to imagine you're sitting at the edge of a galaxy's stellar disc, inasmuch as it ever has one. You'll be experiencing some acceleration due to the stars and some due to the dark matter. The more massive the galaxy, the more important the stars are : CDM explains pretty well why larger galaxies are better at forming stars; at any given galaxy mass, you must, therefore, be experiencing some very particular acceleration. But if you move around within any galaxy, your acceleration varies in a very similar way to changing the galaxy's mass, because the shape of the dark matter profile and stellar distribution are closely connected. It's the dark matter controlling things, not the stars : but that still means you can go the other way around and use the stars to examine the dark matter.
So there's no magic here. It's not a silly discovery, but it doesn't point to any new physics either. MOND can do it, but CDM can do it too. Lately I've been witnessing a lot of abuse of the word "consistency", which is necessary but not sufficient for evidence to favour one position over another. Taken purely by itself, if your information is consistent with theory X, that by no means automatically makes it inconsistent with theory Y - which may be radically different or even opposite to theory X ! A dead cow is consistent with alien mutilation, but it's also consistent with El Chupacabra, a werewolf, and a very hungry carnivorous ostrich named Derek.
Could the MDAR still prove interesting ? Well, if more regimes of acceleration could be probed, then possibly. There should be more deviations at the low-mass end, where dark matter dominates - and that seems in fact to be the case. Low baryonic mass galaxies in massive dark matter halos would also be interesting, but they don't seem to exist. Galaxies which are forming in the early universe might be so rich in baryons that they might probe much higher accelerations than the ones we see today, but we don't have good data for those yet. What would be especially interesting would be an isolated galaxy (galaxies which are interacting with each other can do whatever fancy shennangigans they like; all of these predictions are concerned with isolated objects) that deviates from MDAR, which CDM would find problematic. It would also be pretty awful for MOND supporters, so we'd all be up the proverbial creek together, merrily singing a jaunty tune as the canoe sinks beneath us, and nobody would get any cake at all.
Well anyway, that's my attempt to make this paper understandable without (hopefully) being woefully innacurate. For more on the MDAR and the assorted angry rants it triggered, see this post.
http://adsabs.harvard.edu/abs/2017MNRAS.471.1841N
For those who've forgotten all about it, there was a great furore last year because of a discovery in galaxy dynamics that didn't seem to fit the standard model. Specifically, there's a relationship between the acceleration predicted by the measured mass and distribution of stars and gas and the acceleration they're observed to be experiencing. Why is that weird and why would anyone care about some obscure piece of mathematical analysis ? Wouldn't they rather be eating cake ?
Possibly. But the MDAR* is quite interesting**. A whole slew of observations and theory have pointed to the need for dark matter to dominate galaxy rotation, typically by a factor of a few to a few tens. Ordinary (baryonic) matter is just the icing on the rich, deliciously dark cake***. So there ought to be a strong discrepancy between the acceleration predicted just from normal matter (without accounting for dark matter) and its observed value. Naively, we might not expect there to be much of a relationship at all, with galaxies lying all over the shop. The mass of baryonic matter shouldn't really tell us anything about the dark matter.
* Mass Discreprency Acceleration Relation. Or maybe it's RAR, the Radial Acceleration Relation. Different authors seem to prefer different things, so I'm going with MDAR.
** So interesting that people started calling it a new law of nature, which made a lot of people very upset.
*** I'm hungry.
But that's not what was found. Although there isn't a neat 1:1 correlation, there is a very tight relation between the normal and dark matter. In effect, the mass of the normal matter allows you to predict with a fair precision the mass of dark matter. This was exactly what everyone's favourite alternative gravity theory (MOND) has long predicited.
In response to this there was a slew of papers back and forth by the various sides trying to work out if this was a really cool discovery or if it was all just a bit silly. Almost instantly, papers came out showing that the MDAR happened in standard model simulations without any tweaking whatsoever. No-one gave a clear explanation as to why it happened, much to the disgust of the MOND supporters, but happen it certainly did.
This paper attempts to provide that explanation. And it's not exactly simple, so bear with me.
In the standard model, galaxies which form in isolation can only do so in a quite narrow range of dark matter "halo" masses. Or more specifically, detectable galaxies. Too small and supernovae blast out all the gas, or hot young stars blow it out with strong winds, preventing any more star formation. Dark matter halos might well exist below this lower limit, but they can't host detectable galaxies. Going in the other direction, galaxies which become too massive have powerful feedback from active galactic nuceli (supermassive black holes with accretion discs and jets and whatnot) and the gas takes longer to cool and form stars. So galaxy growth is heavily restricted.
That means, in effect, that you can't get galaxies outside a certain, reasonably narrow acceleration window : no discs ever rotate faster than ~300 km/s, and none slower than ~30 km/s. Since their size doesn't vary all that much, neither does their acceleration.
Above a characteristic acceleration, all galaxies are dominated by their baryons - making the relationship of predicted/observed acceleration steeper the closer you get to this acceleration value. The baryon/dark matter ratio, which varies with total mass, is something that's reasonably (though not brilliantly) already predicted by the cold dark matter (CDM) model.
In addition, CDM makes a very specific prediction for how the dark matter is distributed within each halo, whereas stars, we know from observations, are distributed quite differently. All of this combines to give a very tight behaviour of the acceleration of matter within a galaxy. In the inner regions it tends to be dominated by the dense baryonic matter, whereas in the outer regions it's more about the dark matter. And you can't observe the acceleration beyond a certain radius (where acceleration is very low and dark matter dominated) because the detectable matter just doesn't extend that far - or equivalently, below a certain characteristic acceleration.
This lower acceleration is pretty close to the value where dark matter dominates. So for galaxies of very low baryonic content, at any radius they will be more strongly dominated by dark matter than baryons - and so the MDAR becomes shallower (and you only get galaxies of very low baryon content at low total masses), and galaxies tend to deviate a bit more from the standard MDAR relation. Moreover, if you consider the acceleration variation within individual (faint, low mass) galaxies , it will still be dominated by dark matter at all points, giving the same characteristic slope. Hence the MDAR is visible if consider either entire galaxies (choosing some well-defined point at which to measure their acceleration) or the acceleration at many points within each galaxy.
Another, more crude way to think about this is to imagine you're sitting at the edge of a galaxy's stellar disc, inasmuch as it ever has one. You'll be experiencing some acceleration due to the stars and some due to the dark matter. The more massive the galaxy, the more important the stars are : CDM explains pretty well why larger galaxies are better at forming stars; at any given galaxy mass, you must, therefore, be experiencing some very particular acceleration. But if you move around within any galaxy, your acceleration varies in a very similar way to changing the galaxy's mass, because the shape of the dark matter profile and stellar distribution are closely connected. It's the dark matter controlling things, not the stars : but that still means you can go the other way around and use the stars to examine the dark matter.
So there's no magic here. It's not a silly discovery, but it doesn't point to any new physics either. MOND can do it, but CDM can do it too. Lately I've been witnessing a lot of abuse of the word "consistency", which is necessary but not sufficient for evidence to favour one position over another. Taken purely by itself, if your information is consistent with theory X, that by no means automatically makes it inconsistent with theory Y - which may be radically different or even opposite to theory X ! A dead cow is consistent with alien mutilation, but it's also consistent with El Chupacabra, a werewolf, and a very hungry carnivorous ostrich named Derek.
Could the MDAR still prove interesting ? Well, if more regimes of acceleration could be probed, then possibly. There should be more deviations at the low-mass end, where dark matter dominates - and that seems in fact to be the case. Low baryonic mass galaxies in massive dark matter halos would also be interesting, but they don't seem to exist. Galaxies which are forming in the early universe might be so rich in baryons that they might probe much higher accelerations than the ones we see today, but we don't have good data for those yet. What would be especially interesting would be an isolated galaxy (galaxies which are interacting with each other can do whatever fancy shennangigans they like; all of these predictions are concerned with isolated objects) that deviates from MDAR, which CDM would find problematic. It would also be pretty awful for MOND supporters, so we'd all be up the proverbial creek together, merrily singing a jaunty tune as the canoe sinks beneath us, and nobody would get any cake at all.
Well anyway, that's my attempt to make this paper understandable without (hopefully) being woefully innacurate. For more on the MDAR and the assorted angry rants it triggered, see this post.
http://adsabs.harvard.edu/abs/2017MNRAS.471.1841N
Monday, 5 March 2018
Objectivity is nice if you can get it
There are of course counter-examples. "I had a dream last night about ostrich racing" (which, as it happens, is absolutely true). "I thought Black Panther was excellent" (also absolutely true). Can I prove it ? No. Does that make me a liar or a fraud ? Also no.
Of course the meme is about positions for which things can actually be measured and objectively tested. And there are things which we know which we cannot measure or objectively test, which is annoying but true anyway.
No, it's not a Jackson Pollock painting
This is a simulation of the evolution of a cool, dense gas cloud embedded in a hot, thin, surrounding medium. The cool gas starts off as a nice neat sphere with a randomised velocity field, so different parts of the cloud are moving at different speeds in different directions. The idea (not mine !) is that the pressure from the surrounding gas, given the specific numbers that we're dealing with, will be able to keep the cool gas in place, so the cloud shouldn't evolve too much. And that would explain why we detect gas clouds in the Virgo cluster that aren't forming any stars.
... which is of course not what happens. The disordered motions of the gas causes it to fragment, allowing the hot, high-pressure gas to penetrate the cloud and help blast it apart. Now if the motion was due to thermal pressure, which would act uniformly and push back neatly against the surrounding gas, it might work. But turbulent bulk motions have a completely different effect.
A proper animation showing how the cloud evolves should be ready quite soon.
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