Will No-One Rid Me Of This Turbulent Sphere ?

This is my ninth paper as first author. The title was suggested by the ever-astute Robert Minchin (see the acknowledgements section, and if you've never heard of Thomas Becket you should consult wikipedia), and the hard work of the simulations was done by the second author.

While there's a full blog post on this for for the enthusiasts who despise reading dry academic texts, here's the short version. Back in 2012 we discovered these eight dark clouds of hydrogen floating around in the Virgo cluster. At first glance there's nothing too odd about that, quite a few of these are already known - some of which are a lot more impressive than ours. But ours are weirder. They're quite compact but six have high velocity dispersions, meaning they should be rapidly disintegrating. This is especially odd considering they're literally miles away from normal galaxies - if they were ripped out of galaxies, as is the conventional explanation for such clouds, then where are the expected streams of gas ? It's hard to see how you could rip out a compact gas cloud from a galaxy without a lot more debris spread all over the place, and we didn't find any.

Subsequently we confirmed this in great and tedious detail with a long series of simulations looking at tidal encounters between galaxies. We found that this mechanism has extreme difficultly in producing features matching the three key parameters of the clouds : their physical size, isolation, and high velocity dispersion. Oh, isolated clouds with dispersion < 50 km/s are common as muck, and those of < 100 km/s are just a bit unusual, but those > 100 km/s are so rare that it's just not a sensible interpretation for the real objects. Even with the important provision that weird objects imply weird physics - the rate at which they should form is just too dang low.

Then in 2016, another explanation was proposed by a certain Burkhart & Loeb. Their idea was that the clouds are in pressure equilibrium with the hot, thin intracluster gas. This gas pervades a large fraction of massive galaxy clusters and isn't bound to any individual galaxy. Though it's very low density, it's also extremely hot, so its pressure can be significant. They made an analytic calculation which uses the measured velocity dispersion of the clouds to estimate their internal pressure, X-ray data to get the external pressure of the surrounding gas, and then by equating the two they can find the expected size of the clouds. Which, interestingly, is in good agreement with the observations (limited though they are).

Now, if the velocity dispersion of the clouds was lower, that would be fine and we would have stopped there. The problem is that it's so high that the internal pressure of the clouds, in that model, has to come from bulk motions - i.e., turbulence (if the dispersion came from thermal pressure the clouds would have to be so hot they couldn't possibly remain atomically neutral, as they actually are). And that's a completely different state from thermal pressure, which acts uniformly. Turbulence implies that different bits of the clouds are moving in different directions, so it's hard to see how the external pressure could ever help them remain stable.

So we set up this series of simulations to test how long the clouds could match the observations in this scenario. Our models have spherical gas clouds set to be compatible with the observations, embedded in a hot, low-density gas to mimic the intracluster medium. We varied the mass of the gas clouds, their velocity dispersion, and more detailed parameters of the structure of their velocity field. Given the mass of the clouds, we know that their velocity is so high they can't be gravitationally bound, so we don't expect any equilibrium state to develop - what's much harder to predict is just how long they survive, given the external pressure helping to hold them together. It's even harder to guess how long they'd maintain that very interesting high velocity dispersion.

Our result is... not very long at all. Well, they do survive in the crude sense that there's still gas there, but the dispersion drops very rapidly. The clouds tend to either immediately explode, initially collapse and then slowly disperse, or are rapidly heated by their own self-interactions to the point where they'd no longer be detectable (you can watch one of the simulations here : https://plus.google.com/u/0/+RhysTaylorRhysy/posts/Buvd5QH9NR3). The high velocity dispersion lasts no more than about 100 Myr, which is so short in this context that it's safe to say the surrounding gas hasn't really helped at all. At most, all it's done is change the manner of their demise, not postpone it.

Could the model be saved or is something else at work ? Well the original model is dead in the water : turbulence is just not at all the same as thermal pressure. But in combination with other physics - rotation, or that chronic astronomical bogeyman known as magnetic fields maybe it would play a more significant role. It might help, but personally I doubt it : there's no good mechanism to explain where the velocity dispersion comes from, or what maintains it.

The most radical possibility of all is currently the front-runner to explain these objects : they're rotating, and bound together by a massive dark matter halo, and their gas density is so low they're not able to form any stars. That naturally explains the high "dispersion" of the objects and how it's maintained, and fits with chronic problem that simulations predict far more galaxies than are observed. Of course, it has its own problems, like how such low-density cool gas could survive as it moves through the hot cluster gas, but that's for a future paper.
https://arxiv.org/abs/1805.03414