This paper caught my eye for the wrong reasons, but it turned out to be interesting all the same.
It's time for another look at our old friends, Ultra Diffuse Galaxies : those faint, surprisingly large smudges which have been confusing us all for some years now. I've covered their dynamical masses umpteen times before, but their gas content is interesting not just in understanding their dark matter content, but for their star formation too. So when this study said it was using an HI survey to examine UDGs, I was hoping for more than one measly detection.
They start with a nice little overview of possible ways to form UDGs. They might be dwarfs which were spinning unusually fast, so distributing their gas over such a large area that its density was too low for much star formation. Or it might have been early star formation that drove the gas out to larger distances, with the effect being the same; likewise tidal encounters could do something similar. More dramatically, they might be as massive as Milky Way-sized galaxies, with something happening to quench their star formation in a process yet to be understood.
The authors are using the WALLABY survey on one of the SKA pathfinder telescopes, which gives decent resolution and good sensitivity over a very wide field of view. Their target is the "Eridanus Supergroup", which is fascinating in itself. Only a few such supergroups are known, and they're thought to be groups in the process of merging to form a full-on cluster.
And they really do blur the boundaries between groups and clusters. One individual group in Eridanus has hot X-ray detected gas despite its small number of constituent galaxies. I'm going to have to update my introductory talks to include stuff like this – the presence of X-ray gas is a common way to distinguish groups from clusters, besides the more-obvious parameter of sheer number of galaxies. A small group which has this hot gas is not at all typical. Though it would have been nice of the authors to give a table describing the properties of the various Eridanus sub-groups; the maps that they show are very crude and not useful.
The X-ray component seems to be quite substantial, since galaxies here have measurable levels of HI deficiency. In clusters this would usually be interpreted as a classic signature of ram-pressure stripping. Yet with only a few galaxies, rather than a few tens or few hundreds as in most clusters, it's anyone's guess where this hot gas actually came from in the first place. They also note that there are two "enormous HI clouds without optical counterparts" in Eridanus, although that's not quite how I would describe them.
Anyway, they make a search for UDGs using the usual criteria and find 78 candidates in the WALLABY HI survey region. Since they don't have redshifts, most of these are probably background misidentifications but that's okay. But because this catalogue comes from something designed to search at greater distances, they apply an additional size constraint which leaves them with just 6 candidates.
Then they do something a bit strange : they search the HI cube using an algorithm. I mean that's fine, perfectly fine... but... it's not ideal in this case. Far better, I would think, would be to extract individual spectra of each UDG and see if there's any hint of a detection. After all, in this case the search is specifically for UDGs, so there's no need to insist the HI catalogue be homogenous – this is unnecessarily restrictive. Searching cubes blindly is all well and good but you have a much better chance of detecting the faintest stuff if you already know where to look. You want a blind search if you're specifically interested in which objects are gassy, not if you want to know how much gas each object has.
This all makes it a bit odd to call this a WALLABY paper since hardly any of the rest is concerned with the HI at all (they don't describe anything about their resulting HI catalogue, leaving me wondering if they only detect the UDG but that would be remarkable in itself). I mean they do try, but I think all the rest of the interesting stuff is in their discussion of optical relations, with the HI very much being an aside.
And there is interesting stuff here to be sure. They calculate the expected number of UDGs in each subgroup, which apparently follows a power law according to the group mass. They also show that (from other studies) the slope of this power law is invariant with environment, suggesting that environment doesn't matter much to the formation of UDGs. Which is very interesting considering that UDGs as a whole are a very diverse bunch, with some being blue and structured and some red and smooth (and a couple here, interestingly, which are blue and smooth).
While the numbers of predicted and candidate UDGs are consistent for two of the subgroups, for the one with X-ray gas there's a discrepancy : 17 predicted, none found. They suggest that in this presumably more mature group, most of the UDGs have already been disrupted or merged with the central galaxy. In that case though, I'd want to know where this power law comes from exactly, since most UDGs have so far been found in clusters : it seems a bit paradoxical to find the biggest disagreement in the group that's more like a cluster than the others !
And they also say this isn't a surprise anyway because of the number of candidate UDGs there is small, but this makes no sense to me at all. I cannot really get my head around it. The claim seems to be that they can predict the number using the mass of the group (an independent value), but that there's a discrepancy is because there's not as many found as they expected... huh ? The reason for the disagreement is that the numbers are different ? That feels like a truly weird tautologous statement. I may well have missed something.
While they make a brave attempt to consider if tidal stripping or ram pressure was most likely responsible for the UDG formation, with six objects this is inevitably a forlorn hope. Though they do find that one of the candidates is at the end of a long stellar tail in a pair of interacting spiral galaxies. I'd have made a bigger deal out of this.
I can't escape the nagging feeling that this paper is doing all the right things in the wrong ways, or at least in the wrong context. A blind comparison of HI detections and UDG candidates is great if you have substantial samples of both. Examining the scaling relations of UDGs in different environments is useful when you have novel data sets to present.
As it is, they have two interesting things : the detection of a single UDG in HI (showing that these are still very unusual) and a UDG at the end of a tail. Personally I would have concentrated heavily on the details of these two individual objects. Show me the HI spectrum ! Tell me more about how that near-tidal UDG compares to other UDGs ! Leave the scaling relations for when you have more things to relate : individual objects are still themselves worthy of study; not every study has to be about whole populations.
No comments:
Post a Comment