Back in the halcyon days of March 2018 there was a very interesting paper about the discovery of an isolated gas cloud in Virgo. We know of a few of these, of course, and they're all interesting and most are hard to explain. What's remarkable about this one was that while it's quite isolated, it has (apparently) an entirely young stellar population.
Now, gas clouds that don't do anything are weird enough : what stops star formation in some objects but not in others ? This cloud makes things even worse. Accepting that there is indeed some star formation family planning mechanism at work, we now have to understand why this spontaneously fails. Why did this cloud wander around in the void before, for no apparent reason at all, it just decided to go THWOOP and start forming stars all of a sudden ?
At least the survival of the cloud seems a bit clearer, with the previous paper showing that this could be a result of pressure confinement by the intracluster gas. My own work has shown that this basically doesn't work for clouds with strong enough internal motions, but this little cloud (dubbed SECCO 1) has much more well-behaved gas. So it could indeed move quite slowly through the cluster (it's in a region where the velocity dispersion of the galaxies is a lot lower than the average) and survive for a billion years without being torn apart.
This new paper builds on that with a series of new, more advanced, 3D simulations. Again they examine the behaviour of such a cloud after its formation, and don't look at the formation process itself (which would be complicated and require a very different setup). They confirm the previous findings, and to be honest, a large chunk of the paper is given over to extremely laborious descriptions of what the simulations show. They can be summarised thus : the cloud gets disrupted but survives. It gets very slightly more or less disrupted depending on the exact choice of parameters. To be honest, at times it gets downright tedious.
But it's worth slogging through this one, as there are at least four interesting results here. First, they can't explain the sudden onset of star formation, which personally I think they should make a much bigger deal out of : this cloud is weird. It naturally lends itself to clickbait : "This gas cloud just started forming stars and no-one knows why", "Watch as this gas cloud moves through the Virgo cluster - you won't believe what happens next !" and so on.
To be fair, modelling the cause of the onset of star formation isn't really their goal. Instead they try to model the overall star formation history, and that raises the second interesting result : they just can't get this right. If they have the correct current star formation rate then the stellar mass is much greater than the real cloud, whereas if they have the correct stellar content then they underestimate the current star formation activity. This seems to be closely related to the first point though, as in their model star formation begins immediately, whereas in reality it seems to have started only very recently. More observations could help reveal if there is a faint, old population hiding here. And the lack of modelling the formation scenario is perfectly understandable, but means we're missing all the corner pieces of the puzzle. And the edges. And quite a lot of the inside ones too.
The third interesting result is that motion through the intracluster gas actually helps the cloud survive. It's not just the static, thermal pressure keeping the cloud from flying apart, but also the dynamic ram pressure. Rather than creating a big horrible mess, what this does is increase the cloud's density so that it can cool faster, becoming even denser and thus be less vulnerable to ram pressure stripping. I wouldn't have expected that.
The fourth result is that the cloud may point towards a lack of a clear threshold for the onset of star formation. Observations traditionally indicate a distinct break in the relationship between gas density and star formation rate, although when volumetric effects are taken into account this might disappear. The average density of the cloud in their simulations is substantially below this value, and although the density might be higher on the very smallest of scales, it's at least a valid challenge to the idea of a threshold. Still, since they don't get the star formation history right, this one needs to be treated with a bit of caution. In any case, the cloud is more than strange enough to deserve more attention.
Hydrodynamic simulations of an isolated star-forming gas cloud in the Virgo cluster
We present a suite of three-dimensional, high-resolution hydrodynamic simulations that follow the evolution of a massive (10^7 M_sun) pressure confined, star-forming neutral gas cloud moving through a hot intra-cluster medium (ICM).
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