The giants have failed... or have they ?

Quite a nice little letter about ultra diffuse galaxies, those huge ghostly galaxies with hardly any stars. Do they have a crapload of dark matter, as their huge size suggests, or not that much at all, in proportion to their pathetic stellar mass ?

Thus far the issue has been pretty one-sided. Van Dokkum et al. keep claiming that they're probably massive, "failed" galaxies that do a lousy job at forming stars (which would pretty much break the current paradigm of galaxy formation); everyone else seems to think they're not very massive objects but somehow became very spread out. This paper sees a new team join in on the side of the failed giants.

The authors have spectroscopic measurements of globular clusters in and around 3 UDGs in the Virgo cluster (for some reason, Virgo doesn't seem to have as many UDGs as other clusters). Unfortunately the numbers of globular clusters is very low in each case, around 10. Still, the velocity dispersion seems high enough to indicate that all of them are heavily dark matter dominated. One of them looks like a reasonable candidate for a failed giant, another might be but it's more tentative. With this low number of data points, constructing nice rotation curves just isn't possible yet, although they do try.

Another possibility for these objects is that they could just be tidal debris stripped out of galaxies and not galaxies themselves at all. In this case their high velocity dispersion would just mean that they're disintegrating rather than being bound by a dark matter halo. They say this is particularly likely for one of the (probably less massive) objects, which looks disturbed. Personally I'd be a bit more cautious about the others too, but in general they do a nice job of describing the alternative explanations.

The problem with these super-faint smudges is that you can't really get a proper mass measurement even in the relatively small regions where the globular clusters are detected - with a high error, it's more of an estimate. They're not even sure if the objects are rotating. But this isn't too bad; what I really wish they'd state more directly is that the inferred total mass (assuming the dark matter halo extends like in normal galaxies) is about 100x the mass estimate from observations.

It's great to see more people examining the possibility that these things are really weird objects that don't fit the standard models (and this team is an eminently reputable one, so let's here no more talk of scientific closed-mindedness, thank you). But realistically, we're going to need much better data to say anything more definitive about them.
https://arxiv.org/abs/1803.09768