Today's paper revisits a very minor but interesting storm in a teacup.
Back in 2021, Junais et al. reported on a possible Ultra Diffuse Galaxy losing gas in the Virgo Cluster. At face value it all looked very convincing. The HI gas detection was very clear, nicely offset from the UDG-candidate (basically an especially faint, fluffy sort of galaxy if you aren't keeping up with things – shame on you !) but still overlapping it. At the very centre of the gas detection was a sort of ragged line of blue starlight, plus there were some patches of stars scattered about as well. It's all very much as you'd expect if this was star formation occurring in the stripped material.
Okay, ram pressure is old hat. But to find evidence of this occurring in a UDG would be especially interesting : it would allow us to start investigating whether UDGs in clusters (which seem to be pretty common) are the same as those in the general field (which are known to exist, but we don't know how many there are). In particular, there's this whole controversy over whether they lack dark matter or not, in which case the effects of stripping might be quite different since the gravitational forces involved would be much less. And also it would show whether both cluster and field UDGs form by the same process, or whether there are multiple ways to form the same sort of objects.
All this was strongly challenged by Jones et al. 2021 They said, no, hang on, the distances are all wrong. Using high-resolution Hubble data, they were able to show that the distance to the UDG-candidate is actually much closer than the Virgo Cluster, and it also seemed to be linked to another, much brighter galaxy (VCC 2034) by a giant bridge of HI, so presumably that would also be at the same distance. The patchy starlight, however, could well be in the Cluster, in which case it would require a different explanation because it doesn't look anything like a galaxy.
You may or may not remember that I'm moderately skeptical about all this. It's not that I don't believe the distance estimate... it's that I'm wary about them after that whole "ping-pong" series of papers concerning some other UDGs – a debate which apparently still isn't fully settled. That suggests we shouldn't take any single value as definitive but should wait for multiple analyses.
And the HI stream... although I did send Jones some of our deeper (WAVES) data, and he was able to find the stream by taking a slice through at the right angle... it feels very off to me. Given that our HI data is about 3-4x deeper than the original ALFALFA observations, I'd expect it to be immediately obvious in our data. It isn't. My suspicion is that the analysis and source-finding package SoFiA (which is hella powerful) is oversmoothing here, creating the appearance of a bridge because of the degraded resolution, as is used for increasing sensitivity.
I don't know for sure though. I'm moderately skeptical, but no more than that.
Enter today's paper by Yu-Zhu Sun and friends. They use a combination of new deep data from FAST and high resolution data from the VLA. And this paints a pretty convincing picture that the patchy starlight is not the result of gas stripping from VCC 2034, even if they agree that it's nothing to do with the UDG candidate. This is my favourite sort of paper in that it doesn't actually solve the mystery but just demonstrates that things were even weirder than initially thought.
This is all quite complicated : there are several different galaxies in this region, plus the fuzzy stars, plus the possible gas bridge, and conflicting distance claims for all of them. Let's try a few simple diagrams to illustrate. I'll start with the main hypotheses proposed by Junais and Jones. But, since both groups quite rightly caveat their conclusions and aren't definitive, and don't all deal with the same objects, I'm going to try and standardise and simplify things a little bit. This should be enough to get a general sense of what's going on, but this is very much a limited guide. Needless to say, these are not to scale !
Hypothesis 1
The second scenario relies on a number of additional observations. Direct distance estimates suggest that both UDG-X and VCC 2037 are at 10 Mpc, much closer than the Virgo Cluster (17 Mpc, estimated elsewhere to be be 1-2 Mpc deep). However the Fuzz seems still to be at the cluster distance, and there's a much larger bridge of HI apparently connecting it to VCC 2034. So essentially, the Fuzz results from gas stripping of the cluster member VCC 2034, whereas UDG-X is so close to us that's actually not a cluster member at all : it may or may not relate to VCC 2037 instead. This would make UDG-X an uninteresting normal dwarf galaxy, but the Fuzz becomes very interesting as a rare example of star formation in a gas tail.
A tricky question indeed, one which they understandably don't commit to answering. Their main conclusion is that the Fuzz is likely not stable and in the process of disintegration, but as to what formed it in the first place, they don't (can't) say.
Disclaimer : I know a few of the co-authors very well, have published with them, and certainly hope to do so again ! They raise many excellent points, but there are a few with which I disagree. For example, they say that the HI cloud around the Fuzz has a "well-defined" velocity gradient of 10 km/s, but that's the width of the HI line itself so I'm very skeptical that this can be in any sense meaningful.
They do, however, have both new, extremely sensitive FAST data (even slightly deeper than WAVES), and new VLA data which should be of even higher resolution than the earlier observations. The FAST data fails to show the large HI envelope, as does WAVES – and taken together this seems to quite reasonably disprove its existence. I had in mind a simple project to see if this could really result from how the data was processed... maybe one day I'll have the time to try it, as it would be nice to know exactly how this happened if indeed it doesn't exist.
What about UDG-X ? The FAST data is highly sensitive but low resolution, and can't distinguish gas associated with the Fuzz (which definitely does exist) from UDG-X. The Sun et al. VLA data, however, shows much less of a head-tail morphology than the earlier data, now appearing to only be associated with the Fuzz. That makes it unlikely that Fuzz is the result of gas stripping from UDG-X, though it can't be said with too much confidence. There could still be diffuse gas in UDG-X which the VLA wouldn't detect, or the entire gas of the Fuzz might have been displaced wholesale from UDG-X.
And when they say they detect a velocity gradient in this case, it looks a lot more like a very sudden change to me. Their dynamical mass estimates – how much mass is needed to keep the system stable – are, I think, stretching things beyond the quality that the data can sustain, given how narrow the velocity width of the object is. That said, they say the total amount of dark matter that would be present is so low that this is unlikely be a dark/dim galaxy candidate : more likely it's some form of debris. That seems entirely reasonable from the low line width, even if I'd be skeptical about the exact dark matter mass estimate.
But is the debris stable ? That's much harder to answer. A lot of recent work has found candidates for so-called "blue blobs", which are interpreted as gravitationally-bound clumps of gas and stars that formed by the removal of gas from ordinary galaxies by ram pressure. In essence this would be a new class of stellar system, not really galaxies in the classical sense (since they'd have no dark matter) but not star clusters either (being very much larger and formed by a totally different mechanism).
Personally I rather like this idea, but here they place a few well-aimed holes in the scenario. The high metallicity of the clouds seemed in Jones like strong evidence that the clouds originated from within galaxies, as otherwise their chemistry should be basically hydrogen and bugger all else – you need prolonged star formation to cause significant enrichment, which isn't going to happen at their current pathetic levels of star formation activity. But here they say it could happen through mixing with the gas in the cluster itself. On the other hand, the paper they cite in support of this says that metallicity should drop with distance from the parent galaxy, whereas all the blue blobs have essentially the same high metallicity value. So this is an interesting critique, but not a fully convincing one.
Similarly, they're rather skeptical of the whole pressure confinement scenario for blue blobs – the idea here being that the gas within the cluster helps prevent them from disintegration. Now when we simulated this for dark clouds with very high velocity dispersions, we found it flat-out didn't work. But we were investigating rather exceptional systems, and simulations of low velocity dispersion systems have found very much more favourable results (as you'd expect anyway : with a low dispersion, things can only expand more slowly by definition). So I think their toy model is overthinking things. In any case, given the extremely low dispersion of the Fuzz's gas cloud, it would only expand by 10 kpc in a billion years... even if it is technically disintegrating, it's doing so so slowly that it might as well not be.
Finally, I don't agree at all with their interpretation regarding the location of the blue blobs within the cluster. The previous paper by Dey suggested that they're found in regions of modest cluster gas density because this is where they can both form and survive for a while; they avoid the denser core because this would rapidly destroy them. But Sun et al. claim that a "more natural" suggestion is that actually these objects are all outside the cluster in 3D space and only appear projected against it. Surely, though, if that were the case, we'd be equally likely to see such objects projected against the core ! To me, that the distribution of the objects relates to the geometry of the cluster feels like extremely compelling evidence that they are indeed within the cluster.
The long and short of it is that this is a very complex system, and it all serves to underscore that even observations don't always get the last word. It's particularly interesting that the new VLA data looks markedly different to the earlier findings, showing distinctly different structures. Likewise, I have to wonder why everyone is treating the distance estimates with such high confidence, given recent prominent debacles about how damn difficult it is to get these right.
As it stands, it now looks a lot less likely that the origin of the Fuzz can be explained by a giant gas stream from VCC 2034. But I, for one, am by no means convinced that we can rule out the original suggestion of stripping from a UDG, and I downright disagree that we can be so confident that it's a disintegrating gas cloud rather than a ram pressure dwarf. It's likely not a dark galaxy, however.
Which leaves the usual question of : what would it take to resolve all this ? This is very tricky. Well, the question of the long gas stream could be easily answered by running SoFiA over data sets with artificial signals injected of similar configurations to the current system; if the long stream results from oversmoothing, this ought to be reproducible. Distance measurements are much harder to resolve unambiguously, but at a minimum, another team need to try this independently, preferably using different data. As to why the various VLA data of the same objects looks so different, however, I'm at a loss. It's definitely a weird system, but certainly an interesting weird.
No comments:
Post a Comment