Ultra Diffuse Galaxies, "Still A Thing", Study Finds

Ultra-diffuse galaxies are, broadly speaking, about the same size as the Milky Way but a thousand times fainter. Although a few such objects have been known since the bygone days of yore (e.g. the 1980s), it's only in the last couple of years that they've been discovered in large numbers. Probably the biggest question is how massive these things are. It's easy enough to find their stellar mass - we can basically measure that directly from their brightness - but much harder to work out how much dark matter they have. For that, we need to measure the motions of their stars and/or gas, which is much harder than just measuring how bright they are.

There have been a handful of previous attempts. Beasley et al. 2016 looked at VCC 1287 and concluded it was of low mass, but still a very strange object because it doesn't fit the normal rotation-brightness relation. van Dokkum et al. 2016 looked at Dragonfly 44 and concluded it was of high mass, and though their estimate of the total mass is an extrapolation it's certainly more massive than VCC 1287.

This latest paper uses the ALFALFA hydrogen survey to search for UDGs. This means the sample is biased towards UDGs that are hydrogen-rich (which might not be the case for the population in general), but the advantage is that the the hydrogen measurements easily allow for estimate of the rotation velocities. Because the survey is very large, this means they have a decent sample of 115 UDGs. ALFALFA doesn't have a high resolution, so they also have VLA observations of 3 UDGs where they can measure the rotation curve accurately rather than just estimating it.

They find that the UDGs appear to be huge dwarfs. That is, they have very low masses but are very extended - they're around the same diameter as the Milky Way, but rotating 4-7x less quickly (30-50 km/s compared to 220 km/s), meaning they don't need nearly as much dark matter to hold themselves together. Many measurements in astronomy are not all that precise and errors of a factor of a few are not all unusual. Rotation velocities are an exception - this is much too large to be dismissed as an error, especially given their large sample size.

It's still too early to say what this means. In principle, the more dwarf galaxies found the better, since the standard model predicts far more than observed. Other papers have indicated that these UDGs are nowhere near numerous enough to explain the difference, but their existence implies that there are even more galaxies out there which are even fainter and harder to detect. But at the moment we only have these mass estimates for these ~100 gas-rich galaxies, whereas most UDGs may not have gas at all. So the conclusions are still suffering a huge bias, and another recent paper suggests that there might be populations of different types of UDGs. Further research is required... as in, years of further research, not, "it'll be done by Tuesday".

I like this paper very much, I think it does a good job of declaring the biases, uncertainties, definitions (importantly noting that there's no hard definition of UDG yet - the IAU's arm is not that long) and methodologies. The one thing I wish they'd done is compare their galaxies to the normal rotation-brightness relation (Tully Fisher) because it looks to me like there might be an interesting offset. There's a sample of the full table but I'm not sure where the entire thing is, perhaps it's not available until after acceptance. That's certainly something I'd sit on, since it easily has the potential to warrant another paper.
https://arxiv.org/abs/1703.05293