Ultra Diffuse Galaxies remain a very hot topic in astronomy. You know the drill by now : great big fluffy things with hardly any stars and sometimes little or no dark matter, not really predicted in numerical simulations. I'm not going to recap them again because I've done this too many times, so I leave it as an exercise for the reader to search this blog and learn all about them. Get off yer lazy arses, people !
UDGs were first found in clusters but have since been found absolutely everywhere. Why clusters ? Well, because they're so faint, getting redshift (i.e. distance) measurements of them is extremely difficult. This means their exact numbers are fiendishly difficult to characterise : without distance you can't get size, which is one of their distinguishing properties – so without size you can't even count them. And if you can't count them, you can't really say much about them at all.
Getting distances in clusters, however, is much easier. There the distance to the whole structure is anyway known. The first studies found lots of UDG candidates in clusters but very few in control fields, so most of those are certainly cluster members rather than just being coincidentally aligned on the sky. Of course it's always possible that a small fraction (at the few percent level or less) weren't really in the cluster and therefore not truly UDGs, but statistically, the results were definitely reliable.
The SMUDGES project (Systematically Measuring UDGs) is a major effort to begin to overcome the limitation of relying on clusters for distance estimates*. In essence, they try to develop a similar procedure for clusters but which can be applied to all different environments. They want results which are at least statistically "good enough" to estimate the distance, even if there's some considerable margin of error.
* The main alternative thus far has been gas measurements, which give you redshift without relying on the much fainter optical data. This, however, has its own issues.
This paper is mainly a catalogue, and to be honest I rarely bother reading catalogue papers. In fact I only read this one to see what low-level methods they used to do the size estimates, since we have some possible candidate UDGs of our own we want to check. But as it turned out, they also present some interesting science as well, so here it is.
Most of the paper is given to describing these methodologies and techniques. It's pretty dry but important stuff, and like with the first cluster-based studies, they can't be sure that absolutely every candidate they find is really a UDG. Actually these measurements are, inevitably, quite a lot less reliable than the cluster studies, but they're careful to state this and the results are still plenty good enough to identify interesting objects for further study.
One interesting selection effect they note early on is that studies of individual objects tend to overestimate their masses (compared to studies of whole populations), since these tend to be particularly big, bright, and prominent. This at least helps begin to explain why some division has arisen in the community regarding the nature of UDGs : the objects studied by different groups are similar only at a broad-brush level, and in detail they may have significant differences. That's not an explanatory bias that was obvious to me, but maybe it should have been. It seems perfectly sensible with hindsight, at any rate.
And, once again, this is another study where the authors resort to flagging dodgy objects by eye, in another example of how important it is to actually look at the data. The machines haven't replaced us yet.
I won't do a blow-by-blow description of their procedures this time, but their final catalogue comprises about 7,000 objects, which they supplement with spectroscopic data where available. One of the main topics they address is the big one : what exactly are UDGs ? Are they galaxies with normal, massive dark matter halos but few stars, or do they instead have weird dark matter distributions ?
They conclude... probably the former. But this is not to say that they are "failed Milky Way" galaxies that have just not formed many stars for some reason : at the upper end they're probably still a few times less massive than that, and at the lower end that might be more than a factor ten difference. So mostly dwarf galaxies, but with normal dark matter distributions and very few stars. They get mass estimates from a combination of counting the number of globular clusters, which correlates with the total halo mass in normal galaxies, and their own statistical method to estimate other galaxy properties (which I don't fully understand).
These relations don't always work well, however, sometimes experiencing "catastrophic failure", by which they mean errors of an order of magnitude or more. Why this should be is impossible to say at this stage, but, intriguingly, might point to the dark matter distribution being indeed different in UDGs compared to normal galaxies, at least some of the time. Overall though this appears unlikely, because to make this work with the observed scaling relations, the dark matter would have to be more concentrated than expected, even though the stars are the exact opposite : much more spread out than usual.
Bottom line : they think UDGs are mainly dwarf galaxies (though a few may be giants), with normal dark matter contents but very poor star formation efficiency for whatever reason. I'm not so sure. They say the distribution of some parameters (e.g. stellar mass within a given radius) is the same for both UDGs and other galaxies but to me they look completely different; it doesn't help that the figure caption states two colours when there are clearly three actually used. What's going on here I don't know, but very possibly I've missed something crucial.
Of course this paper won't solve anything by itself, but it gives a good solid start for further investigations. As with the previous post, this is another example of how important it is to classify things in a homogenous way. At least one SMUDGES object is found within our own AGES survey fields, and was in fact known to much earlier studies. Sometimes what can look at first glance to be a normal object actually turns out to be something much more unusual, but it's only when you have good, solid criteria for classification that this becomes apparent.
Which is all very good news for AGES. I suspect there are actually quite a lot more UDGs lurking in our data. All we need is a team of well-armed and angry postdocs to track them down... i.e. a great big healthy grant. Well, a man can dream.
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