The little gas cloud that could

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).

Filamentary faff

I've been slacking on reading papers lately, but as I download 560 GB of data from my old Arecibo account (in the unlikely but possible event that the telescope collapse could result in data loss), I managed to read this one about galaxy filaments.

Filaments are rather under-appreciated things. Clusters, being very dense and full of galaxies doing all kinds of interesting things, tend to get all the glory. And it's not entirely unfair : with lots of galaxies all at a fixed distance, and all kinds of crazy environmental effects at work, clusters are both interesting and observationally advantageous places to study. But it's in filaments, the largest structures in the Universe, where most galaxies actually live. Clusters are interesting but they're also weird, and not typical of how galaxy evolution proceeds in general.

Arguably filaments aren't real. They aren't gravitationally bound, so in some sense they're transitory. But this rather misses the point : galaxies (it's generally thought) grow mainly via mergers, so they too could be deemed to be transient; being gravitationally bound today is no guarantee that will be true tomorrow. The key feature is that the processes at work to form a filament could very well influence the evolution of galaxies within them, and this evolution outside of galaxy clusters is usually described with the catch-all term preprocessing. The extent to which this happens is controversial and poorly understood.

This paper looks at preprocessing by studying the major properties of galaxies in seven different filaments. It does this is the classic way of seeing how the galaxies properties vary with distance from the spine (what they call the vertical distance for some reason, and never properly define). The trouble is that the authors don't do this very well.

For starters the sample size isn't clearly defined. They distinguish between galaxies which are in groups and which aren't, but it isn't clear how many are in each. The number of 289 is bandied around but I honestly don't know if this refers to group members or non-members, and whichever it is, I don't know how many are in the other. It's a bit odd that the value of such an important parameter isn't made a lot more obvious. Likewise, the filaments on their sky plot look extremely narrow, and it's a shame they didn't try something three-dimensional, which would really help in showing how good their filamentary membership criteria really are.

The first thing they try is to plot how galaxy density varies with distance from the spine of each filament. In general this decreases and then levels off as the population merges into the general field. But there's one clear exception to this, which shows a decrease to the edge of the filament and then a sharp rise before levelling off. Nowhere do they comment on this.

Then they look at the parameters of the galaxies themselves, but the results are similarly uninformative. Most galaxies in their sample are dwarves, but this is true in general so doesn't tell us anything. There's a very, very weak trend (0.05 magnitudes) in galaxies becoming bluer (indicating more star formation) and less massive with greater "vertical" distance, but I wouldn't call this at all convincing : for comparison, the colour difference between a typical blue and red galaxy is more like 0.5 magnitudes. 

More interesting would be to check how colours vary in a fixed mass range. They do try this, but only divide their sample into two huge mass bins, so this doesn't really show anything. They also plot how the mass varies as a function of vertical distance within the mass bins, which I found distinctly odd. You can't use mass as the control parameter when you're dividing your sample by mass - that just doesn't make much sense. Why would there be a trend in mass within a given mass range ? A trend in diameter, now that might be more interesting.

A similar confusion afflicts their plot of how the gas fraction varies. For this they simply take the mass ratio of gas to stars. The problem is that this (innately) varies strongly as a function of galaxy mass, so by itself this doesn't tell you anything : you have to control for total mass as well. Deficiency is a much better parameter to use, but the intrinsic scatter is very large so any trends will still be hard to see even with a very large sample.

All in all, I don't think this paper has any useful conclusions at all. Certainly filaments could have important levels of preprocessing (at least some of them probably do), but this analysis doesn't tell us whether they do or not. They need a much more detailed analysis and probably a very much larger sample size. Just because a trend is weak or unclear doesn't make it any less real or important, but it does make it a lot harder to verify.

Properties of Galaxies in Cosmic Filaments around the Virgo Cluster

We present the properties of galaxies in filaments around the Virgo cluster with respect to their vertical distance from the filament spine using the NASA-Sloan Atlas catalog. The filaments are mainly composed of low-mass, blue dwarf galaxies. We observe that the g - r color of galaxies becomes blue and stellar mass decreases with increasing vertical filament distance.