Where do ultra diffuse galaxies live ?

Another paper on ultra diffuse galaxies, those weird faint smudgy things which are about as extended as the Milky Way but a thousand times fainter. This one looks at a new sample from the ALFALFA survey of atomic gas. That's important because measuring the gas tells us about a) the star formation activity of a galaxy and b) the kinematics of the galaxy, i.e. how fast it's rotating and thus its total mass.

The majority view seems to be leaning heavily towards a low total mass for these objects. Discovering a huge population of massive "failed" galaxies - in the sense that they suck at forming stars - would be really interesting,but it just wouldn't fit with everything else. Or so the legend goes.

I was a bit disappointed that this paper doesn't really tackle this issue much at all. While star formation is interesting in its own right, it wouldn't be at all surprising to find a bunch of dwarf galaxies with low star formation rates. It would be interesting to learn how such galaxies get to become so very extended, but that's just not as fun as failed giant galaxies. Personally I'm a little bit skeptical about ruling out the failed giants hypothesis just yet, but we'll see.

This paper doesn't really tackle the star formation issue much either. Instead, it takes a more oblique analysis by looking at the environment where gas-rich UDGs are found. That's a clever line of attack. One idea has been that dwarf galaxies in clusters could become more extended through all the tidal interactions with other cluster members. Obviously that mechanism doesn't work for isolated galaxies, which is one of the reasons this made this team's previous paper so interesting. Here they extend their previous analysis to UDGs which are not isolated, looking at how their properties change in different environments.

Or rather, doesn't. Their conclusion seems to be that environment doesn't make any different to the properties of gas-rich UDGs. That's not to say that gas-poor UDGs might not be found preferentially in different places, but if they're gas rich, then they seem equally happy to be in denser environments as in isolation. Their colours and stellar masses don't seem to vary much, though it does seem that gas-rich UDGs avoid the very densest regions but that's true of gas-rich galaxies in general. Annoyingly, though they do have line width measurements (i.e. how fast the galaxies are rotating), they only comment that these are rather low, and don't describe if or how they vary with environment.

What does this mean for theories of UDG formation ? Here too I felt the paper could be more bolshy. They say, "This environmentally independent behaviour is consistent with a formation scenario wherein UDGs evolve slowly because of low star formation efficiency and do not require an interaction with a cluster to become diffuse." Which is fine, but what specific models does it favour and which does it disfavour ? I commend the authors for sticking so ruthlessly to the facts, but I wanna know what they think this means. It feels like they've got a fantastic data set but then chosen to ignore all the most interesting stuff.

The environment of HI-bearing ultra diffuse galaxies in the ALFALFA survey

We explore the environment of 252 HI-bearing Ultra Diffuse Galaxies (HUDs) from the 100% ALFALFA survey catalog in an attempt to constrain their formation mechanism. We select sources from ALFALFA with surface brightnesses, magnitudes, and radii consistent with other samples of Ultra Diffuse Galaxies (UDGs), without restrictions on their isolation or environment, more than doubling the previously reported ALFALFA sample.