Maybe massive minis matter mightly

Woohoo, looks like normal paper-reading services have been resumed...

Disclaimer : I work with the lead author on an unrelated project. 

The problem of whether ultra-diffuse galaxies (UDGs) are tiny but massive or just tiny and boring has never really gone away. So far it seems they're all over the shop. Some appear to be such lightweights that lack any dark matter at all, and it's hard to see how they could ever from. Others appear of the opposite extreme, being so massive and dark matter-dominated that they might not fit with standard theories of galaxy evolution. Most seem to be somewhere in the middle, but those extreme values are significant.

The difficulty is that to properly weigh a galaxy's total mass, you need to know how fast it's rotating : there are some clever alternatives, but none are really as good as direct measurements of the kinematics. There are only a handful of cases so far where this has been possible, so every new measurement helps.

This paper presents the atomic neutral hydrogen (HI) measurements for two UDGs in different environments. This is important, since in some scenarios UDGs are formed through environmental processes. The fact that some UDGs (such as one in this paper) are really quite isolated doesn't mean that the majority of UDGS, which are thus far found in clusters, couldn't be the result of some cluster-based process, though it would be a bit contrived and anti-Occam.

The paper is very dense but it's crammed full of science and not excessive details of the observational methods. Their main result is that both galaxies are probably dark matter dominated. Though the resolution isn't exactly exquisite, it's more than enough to see that both objects have ordered rotation.

Of course, there are caveats. The first galaxy has rather messy HI and might have interacted with something. This makes converting its measured velocity width to its true rotation speed more difficult. Their estimates range about 50 - 150 km/s, which means it's either lacking dark matter or highly dominated by it. But the latter estimate, they say, is probably more likely, and its dark matter content in that case (although high) would be consistent with other objects of similar baryonic mass.

The second galaxy is more robust. This is clearly close to edge-on, making the velocity correction smaller and less prone to errors. And this one fits exactly where you'd expect to find a regular dwarf in terms of baryonic and dark mass. But it too has an extension indicating an interaction of some kind - most probably the gobbling up of a smaller satellite, they say. It also has an extremely high ratio of HI to stellar mass. Neither of these features, however, is much of a problem for the kinematic measurements.

So what of those other UDGs that lack dark matter ? Their plot of baryonic mass as a function of dynamical mass is... unclear. If I have to describe it somehow, I'd say there are two distinct populations. One, of normal dwarf galaxies, shows a broad decline in baryonic mass with increasing dark mass. The other, the weirdo UDGs, is a quite distinct cloud. But it's not at all clear - discerning a pattern here is like being given a join-the-dots puzzle without any numbers and trying to work out if it's supposed to be Jesus or a dinosaur. I think the answer is only going to become clear with more and better data. So my only conclusion is that I'm sitting firmly on the fence as to whether these objects are truly strange or just difficult to measure.

Resolved HI in two ultra-diffuse galaxies from contrasting non-cluster environments

We report on the first resolved HI observations of two blue ultra-diffuse galaxies (UDGs)using the Giant Metrewave Radio Telescope (GMRT). These observations add to the sofar limited number of UDGs with resolved HI data. Within the limits of the observations' resolution, our analysis indicates that SdI-2 is dark matter-dominated within its HI radius and this is also likely to be the case for UDG-B1.

Lord Of The Heavy Metal Rings

A two month break from reading papers is getting excessive, so let's address this with a nice little letter about Leo.

The Leo Ring is a gigantic, 200 kpc ring-shaped gas cloud in the Leo group, about 11 Mpc away. With a mass of HI of over 2 billion solar masses, this is one of those weird features that won't go away because someone made a calibration error or something daft like that. The Ring is by and large optically dark, with no obvious bright galaxies that you could point to and say, "yep, the gas probably came out of that one there".

Now some rings are not that complicated to explain : head-on collisions between galaxies can do the job nicely. But ring galaxies are collisional features which tend to be smaller and with more vigorous star formation, and tidal encounters usually have a clearer connection between the stripped galaxy and its lost material. While detecting star formation in a feature this large could be difficult just because it's so spread out, it certainly isn't happening at the level seen in other such collisions objects. It's not that there isn't any at all - UV observations have found some occurring in a few places in the recent past - just that there's not much happening right now.

This letter presents new Hα observations showing that there is ongoing star formation happening in at least a few parts of the Ring (coinciding with the previous UV detections, though I think this is by design). By itself, this isn't terribly interesting. It's not unexpected that parts of this gigantic structure could be collapsing and forming new stars under gravity, thought it's nice to know. What's more surprising is that the metallicity measurements indicate the chemical composition is similar to that of a typical galactic environment, and can't be explained by the enrichment due to the observed star formation.

The strange thing about that is that previous observations (from absorption lines in background quasars) showed that metallicity was very much lower. That would point to a primordial origin of the Ring, with the material condensing out of low-density material in the general field. I'm always skeptical of such claims of accretion : my question is always, "why are we seeing this happening here and not everywhere else ?". So an origin by some stripping mechanism, though it would have other problems, would at least knock this one on the head.

How come the previous estimates were so much lower ? They say it's because it relies on estimating the density of the HI material in the Ring from low-resolution observations, which underestimates the true density. So higher resolution observations could help with this.

Disclaimer : Leo is of particular interest to me because we have AGES data there. Unfortunately it isn't of any higher resolution than the existing observations, just of higher sensitivity. It doesn't reveal any smoking gun, but it's still interesting. Watch this space.

Heavy elements unveil the non primordial origin of the giant HI ring in Leo

Taking advantage of MUSE (Multi Unit Spectroscopic Explorer) operating at the VLT, we performed optical integral field spectroscopy of 3 HI clumps in the Leo ring where ultraviolet continuum emission has been found. We detected, for the first time, ionized hydrogen in the ring and identify 4 nebular regions powered by massive stars.