A Gigantic Stealthy Dwarf With Lazy Stars

This really needs a press release when it's accepted for publication.

Astronomers love three-word acronyms, preferably containing the word "ultra" because it makes us feel ultra-important. Also we're hugely unimaginative at naming things, as the Very Large Array testifies. Anyway, while I'm especially interested in Ultra Diffuse Galaxies - big, fluffy star systems that may or may not be chock-full of dark matter - Ultra Faint Galaxies are interesting too. Not so much my speciality though, so bear that in mind.

Ultra Diffuse Galaxies are defined as having few stars per unit area. But because their total area can be very large, overall they can be quite "bright", at least in the sense of radiating lots of energy. Imagine if you could make a light that sent out the same total power of a floodlight but was ten metres on a side - close up, it'd look pretty dim to the eye, even though the total amount of energy per second was the same as a smaller floodlight.

Ultra Faint Galaxies, in contrast, are defined simply by the total amount of light they emit. They can be small and compact or big and fluffy.

This UFG is of the big and fluffy variety (not as big and fluffy as UDGs mind you). The paper is unusually thorough and complete, describing the discovery, follow-up observations, significance, and modelling. They even comment on the chemistry and possible gamma-ray emission of the object. And the icing on the cake is they actually manage to make the paper readable, so huge kudos to them for that.

While UDGs can be detected at large distances, UFGs are really only detectable in our Local Group. They're important because they might help understand the missing satellite problem (that models predict that we should find more small, nearby galaxies than we actually do) and also for studying galaxy dynamics. One such recent discovery (Crater II) was found to have unusually slow-moving stars, which, taken at face value, contradicts the standard model where galaxies are all dominated by massive dark matter halos - generally their stars are moving much more quickly.

It would be a mistake to think that Crater II is definitive evidence against the dark matter model though. While such an object is indeed compatible with alternative theories of gravity, it's also possible that it's simply lost much of its dark matter through tidal encounters. With pathetically small statistics, every object discovered in this class is significant.

That's where Antila 2 comes in. The authors discovered this using Gaia data. Gaia provides direct distance measurements to nearby stars but also proper motion (that's motion across the sky) data as well. In this case, it was by looking at the proper motions that the authors noticed a group of stars that hadn't been seen before. Gaia also makes this much easier in this region, where the density of stars, gas and dust towards the plane of the Galactic disc makes it difficult to spot anything at all. And by the standards of dwarf galaxies, Antila 2 is a biggie - much bigger than Crater II, and even comparable in size to the Large Magellanic Cloud (which has been known since prehistoric times). Only its incredible faintness - it's 4,000 times fainter than the LMC ! - and crowded location have kept it hidden for this long. That's no match for Gaia, however.

Antila 2 is also very cool. That is, like Crater II, its stars aren't moving very quickly. Unlike larger galaxies it doesn't seem to be rotating at all, the stars are just buzzing around randomly. That's not at all unusual for dwarf galaxies. What is unusual is that the stars only appear to be moving at around ~6 km/s, whereas for an object this size, ~20 km/s might be expected. Taken at face value, this would mean that Antila 2's dark matter halo has the lowest density of any such halo. So how could the stars end up being so dang lazy ? Is it a super-extreme object or did it start life as something more normal and have lethargy thrust upon it ?

There are several possibilities. One is that maybe the shape of the dark matter isn't typical. The usual assumption, based on models, is that dark matter halos have a central "cusp" (a horrible term we just have to live with), meaning a rapid increase in density in the centre. Antila 2 might instead have a "core" - a flatter density distribution in the centre. This could happen in two ways : 1) Early feedback (explosions and winds) by young stars could have removed so much gas that the sheer mass of the moving material could have disrupted the dark matter by its gravitational influence; 2) A tidal encounter with another galaxy (i.e. the Milky Way) could have stripped away much of its dark matter. In either case the end result is that there wouldn't be so much extra mass to accelerate the stars. Any stars which were moving too quickly would have been removed, and the pathetic remnant of the dark matter halo would only have been massive enough to hold on to the most sluggish.

The authors test these scenarios. Neither seems to work by itself, but together they might be able to do it. Thanks to the proper motion data of Gaia, they're able to work out the orbit of the galaxy so they can find out how close it's come to the Milky Way and thus they can estimate the tidal forces. Their initial conditions are necessarily a bit speculative but based on more typical dwarf galaxies. What seems to work is an initially cored dwarf (presumably formed via feedback) that then has a few disruptive orbits around the Milky Way.

There's some observational evidence to support this. Antila 2 appears to be stretched in its direction of motion, its chemical content appears unusual for its brightness (suggesting much of its original stellar content has been lost). On the other hand, the disruption ought to make the object more spherical than observed, but it's not certain if this is a crippling problem or not. Such an object would be able to survive for a few gigayears - long, but it probably implies it fell into the Milky Way's orbit much later than other satellites.

Overall, the conclusions are starkly different to the final sentence in the abstract saying this object may challenge the cold dark mater model, but that was the only inconsistency I spotted. They deserve a press release for this, I just hope it's as good as the paper. :)
http://adsabs.harvard.edu/abs/2018arXiv181104082T