The Coolest Galaxy In The Universe

The press releases about a galaxy without any dark matter are doing the rounds, but are they accurate ? For once, yes ! This is a really genuinely strange object.

"To our knowledge this is the coolest known galaxy outside of the Local Group", say the authors. Sadly this galaxy isn't decked out with bling - they just mean it has a low velocity dispersion (which is sort-of equivalent to temperature). Measuring individual stars at this 20 Mpc (65 million light years) distance isn't possible, so they use globular clusters instead. These are much brighter, so they're easier to measure. Galaxies with large dark matter components (which is pretty nearly all of them) have their globular clusters orbiting around them like a swarm of normally very fast-moving bees.

In this case the bees are much more lethargic. The galaxy is of comparable size to the Milky Way, which rotates at about 220 km/s, but the globular clusters in this object are moving at only about 10 km/s (or even less). Speed measurements let you estimate total mass without needing to know anything about how bright the stars are, and usually masses measured in this way are 10x more than estimated by their brightness (because of the invisible dark matter). Not in this case though : its total mass is about 400 times less than typical galaxies of this size, and the motions can be explained entirely from the stellar mass. It doesn't seem to have any dark matter at all, which is almost unprecedented.

Unfortunately even the number of available globular clusters is small, just ten. Normally I wouldn't put much stock in such small numbers, but in this case the velocity of the clusters is so close to that of the galaxy it's difficult to believe this could happen by chance. There's no obvious reason for any selection effects here : globular clusters should be at random positions and velocities around their parent galaxy.

Of course there are some caveats. Not quite all galaxies lack dark matter. Tidal dwarf galaxies, produced by galaxies tearing bits off each other, seem to be pretty nearly devoid of the stuff. So could that explain this oddball ?

Probably not. Tidal dwarfs are well-accepted when you can actually see the tearing in action, e.g. denser clumps within long stellar streams, but much more controversial for other objects. This galaxy is in an unhappy middle ground : at only 80 kpc (260,000 light years) from the nearest bright galaxy, with other objects nearby, there's certainly scope for tidal interactions to have occurred. It's not close enough to say it's probably tidal, nor isolated enough to say that it probably isn't.

But against this, it's a very smooth, regular looking object, with no signs of long extended features. The encounters could have happened in the distant past and the fainter streams dispersed, but this galaxy is very large and very low-mass : it should be vulnerable to being disrupted itself by further encounters. It's tough to see how it could survive for very long. Actually this is true regardless of its origins, which would make for some fun simulations.

Another possibility is that the distance has been measured incorrectly, but that doesn't seem plausible either : the distances look solid, and if it was significantly different then it would have a very strong peculiar velocity. That can happen in massive groups and clusters, where the gravity of the group as a whole can accelerate individual galaxies to tremendous speeds. But this group is little, so that shouldn't happen. It would be weird, but, as the authors put it, "it is difficult to argue that it is less likely than having a highly peculiar globular cluster population and a lack of dark matter." Weird objects do tend to require weird explanations, after all... the key point is to check if this weird explanation is at least possible in this environment.

Could it just be a galaxy that's rotating but we're seeing it face-on ? In that case we wouldn't be able to measure the rotation since we can only do that for line of sight movement. The authors don't like this because discs normally have visible structures like spiral arms and whatnot, whereas this one doesn't. I'd caution that some lenticular galaxies look very smooth indeed - although they often do have some structure, it can be very low contrast.

And then there's the second, less-reported paper on the globular clusters themselves - they're a bit odd as well. Like other UDG's, the galaxies got literally tonnes* of globular clusters, but in this case they're generally more elongated and about four times brighter. So a weird galaxy surrounded by weird star clusters. Fun times !

* OK, lots of tonnes.

So what's going on ? Does this point to galaxy formation occurring by multiple mechanisms, with some requiring huge amounts of dark matter but some without ? Dunno. To answer that, we need more statistics both on this object and others. Lots of ultra-diffuse galaxies are known, but dynamical mass estimates are still very rare.

Does it challenge our ideas of gravity ? Well, the authors say it disfavours popular alternatives to dark matter. They rely on reproducing the dark matter evidence just from the stars and gas alone, so in those models all galaxies should appear to have dark matter. There's no reason some galaxies should have a fake appearance of missing mass while others don't. In contrast, if dark matter does exist then it's a bit easier to believe that maybe some galaxies just don't have very much of it - there's no problem in principle here, at least.
A caveat is that some alternative theories of gravity are bloody complicated, and the external gravity of nearby galaxies can change the dynamics. This "external field effect" is routinely invoked to explain just about any problems with such theories, which is extremely irritating. But in this case, since there are nearby massive galaxies, the effect can probably be at least estimated.

So for once the press release matches the paper, and neither seem unreasonable. But, as usual, this raises more questions than answers.

Related reading :
https://arxiv.org/abs/1803.10237
https://arxiv.org/abs/1803.10240
http://astrorhysy.blogspot.cz/2017/07/ultra-diffuse-galaxies-revenge-of-ghosts.html

https://arxiv.org/abs/1803.10237