The latest lowdown on ultra diffuse galaxies

Couple of things on ultra diffuse galaxies, those super-faint galaxies as large as the Milky Way.

The first, linked below, is a two-page research note indignantly pointing out that these things are nothing new. The author's being working on these for years, but never thought to give them a fancy name or make silly claims as to discovering a new class of galaxy. Well, I think we kindof all knew that anyway, it's just now they're being discovered en masse and in many different environments. I don't remember them ever being found in such numbers - they always seemed more like unusual objects rather than a huge component of the general galaxy population; a few objects here and there might be collectively significant, but far more thinly spread than the latest discoveries. With the caveat that small faint galaxies were indeed known to be common, and probably unfairly neglected, but not the bigger guys as far as I'm aware.

The major issue with these things is how much dark matter they contain. Everyone (who believes in dark matter at all) accepts that they're more strongly dominated by dark matter than most other systems of the same mass or size. The question is, how much ? If it's just a little, then they're just a bit more massive than normal dwarf galaxies, and fully compatible with standard models of galaxy formation. If it's a lot, then they pose a real problem, because galaxies this massive ought bloody well to be forming lots of stars.

That's where the second paper comes in. To get rotation, you normally measure the velocity of the gas or stars at many different points. This is extremely difficult when the system's 100-1,000 times fainter than usual, so what they do instead is to measure motion at just three points in each of five galaxies : the centre, and two points either side. This can give an estimate of the maximum rotation velocity, which is what you need to constrain total mass. They find that they're more like biggish dwarves and see no evidence of a more massive population.

In other ways the finger seems to be pointing to extreme dwarfs rather than stealthy giants. The chemical composition is more similar to dwarfs than giants, their stellar masses are similar to dwarfs, and it's unclear if they're rotating (like giant disc galaxies) or if their stars move on more random orbits (like low-mass dwarfs).

And yet I wouldn't write-off the stealthy giants just yet. The first paper says that these UDGs are more common in clusters, but so is every sort of galaxy. The main mechanism proposed to explain these objects, if they didn't start off with their unusual appearance, is that interactions with other galaxies stripped out most of their stars. The first authors point out that they have no correlation between their size and stellar mass, unlike normal galaxies, which is what you'd expect if they were formed by such a random series of events.

The problem is that we know such objects are also found not only in massive, rich clusters, but also in smaller groups and even in pretty extreme isolation. This harassment process simply can't work in such cases. Similarly, there's not enough discussion on the gas observations : the authors of the second paper say it's unclear if their galaxies are rotating (which to be fair, it is) but the gas measurements of at least some look exactly like they're rotating. And, so far as I know, no-one has seen any evidence of the extended stellar tails you'd expect to see if this things really were formed by harassment. When they have gas, they look for all the world like normal star-forming galaxies, just much fainter; when they don't, they look like normal elliptical, quiescent galaxies. Their chemical composition might be similar to dwarfs only because they have similar stellar masses, i.e. similar amounts of chemical processing going on. Furthermore, those with gas sometimes have as much gas as normal bright galaxies. I don't see how such objects can be explained in the harassment model - the gas is easier to remove than stars, so you'd expect them to be severely gas depleted as well, but they're not.

Almost certainly, there are different populations of these galaxies. Calling them UDGs to distinguish them from the general population of faint galaxies is maybe somewhat misleading though; we don't yet know what really sets these things apart or not. Conceivably they could be formed by different mechanisms in different environments, with some being formed more or less as they appear now and others via some evolution of normal galaxies. We still don't really know what's going on with these things.

More information on UDGs here.

https://arxiv.org/abs/1803.06927