Another day, another paper on how exciting Ultra Diffuse Galaxies are.
At first, these large, faint galaxies were just wholly unexpected, and that was weird enough by itself. But the natural question was how massive they were. Could they be as massive as the Milky Way, having loads of dark matter but hardly any stars for some reason ? Or could they be relative lightweights, more like dwarf galaxies but much more extended ?
Answers have varied over time, but it seems like the picture is at least beginning to clear. Ultra Diffuse Galaxies do seem to fit some scaling relations of typical dwarf galaxies, but it now looks like their dynamics are markedly different. Rather than being "failed" Milky Way galaxies, they're equally surprising but in the exact opposite sense : they have much less dark matter than expected rather than more, in extreme cases even consistent with having none at all. See that last link for a more detailed overview.
Today's paper is another twist in the saga. It says, "hold on there just a minute. Actually some of them might be really massive after all !".
In principle, both of these outcomes are excitingly weird. Nobody ever predicted "failed" Milky Way galaxies, or galaxies having strongly varying dark matter fractions - especially those found in isolation, where we can't just claim it was removed by tidal interactions and suchlike.
I do have some minor quibbles with this paper. I don't remember there being all that much "enthusiasm" for the failed Milky Way hypothesis : sure, everyone would like this to be true because it would punch a massive hole in the standard model, but it always seemed to me that this claim was being made pretty much by one group and nobody else. I also have some issue with saying that the large dwarf hypothesis is a natural "prediction" of CDM; it may well be explicable in CDM, but nobody predicted this population prior to its discovery. Lastly, I don't like the way they keep insisting this work is the first kinematic evidence for very massive UDGs. Yes, the famous Dragonfly 44 is no longer thought to be so massive, but it's stretching the language a bit to then say that this paper is therefore now the first such evidence. Just make the claims, they're exciting enough without the forced superlatives.
That said, I have no issue with the claims at all. I think it's a very detailed, compelling work, and though it unavoidably has issues with small number statistics, I think the authors have done as good a job as possible in trying to mitigate this.
The problem in measuring the rotation speeds of UDGs, which you need to get their total mass, is that they're bloody faint. So here they select UDGs in the Virgo Cluster which have plenty of globular clusters. These have much higher surface brightness so they're easier to measure. Nevertheless, the observations took a long time, beginning in 2017 but delayed due to, "unusual events such as an earthquake, the Kilauea volcanic explosion, a snow blackout, and the pandemic." If that's not sufficient excuse then I don't know what is.
The upshot is that about half their sample of galaxies has about the typical expected velocity dispersion (i.e. total mass) given their optical luminosity, but the rest extends to very much higher velocities. The only potentially significant criticism I can think of here is that it's unclear if their comparison sample (of normal galaxies) is also measured using globular clusters, or if that's done through the stars in the main body of the galaxies. However, it's been previously shown that this method works, though I've not checked the cited paper for this.
It also seems unlikely that they could have misidentified enough globular clusters to have got the measurements wrong. They've been quite conservative in their cluster selection, only choosing those close to the galaxy and at similar velocities, so the chance of interlopers is small. Also some of the galaxies are at quite different velocities to the cluster mean, so the chance of interlopers at the same velocities is even lower in those cases.
Could they just be galaxies which are being disturbed as they fall through the cluster ? In principle this could perturb them from equilibrium and so the equation calculating the dynamical mass would be invalid. But weirdly, there's a neat anti-correlation between density of the environment and velocity dispersion : the galaxies with the highest dispersions have the fewest neighbours ! If their high dispersions were a result of some cluster-based process, you'd expect to see this more strongly in the densest parts of the cluster.
I'm putting this one firmly into the "intriguing" category. As they say, no cosmological simulations predict the existence of failed giant galaxies.
What does this mean for our theory of galaxy formation, then ? At the moment, nothing much. The sample is too small and in a cluster, which are complex places full of weird stuff anyway. It's just not possible to say at the moment if these are "exotica" - weird, anomalous objects, interesting by themselves but not indicative of anything more broadly - or genuinely indicating that something really very strange indeed is going on. We'll see.
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