Extreme chemistry in an extreme galaxy

I normally steer clear of papers on astrochemistry because even plain atomic hydrogen is complicated enough. I made an exception for this one, though, because a) it's short and b) it's about an ultra diffuse galaxy.

UDGs are hard to measure because they're big and faint. Even getting their total mass is still extremely difficult, so measuring their chemical composition isn't much fun. But these guys seem to have managed it, using the giant Keck telescope and a fancy Integrated Field Unit. Traditional spectroscopy - which gives you the chemical composition and velocity information - would give you measurements only at a single point or along a slit, but IFUs give you spectral information at every pixel. 3D optical data cubes, because science.

This also lets them measure the kinematics. This particular UDG has a velocity dispersion of 56 km/s. For comparison the Milky Way has a rotation speed of ~220 km/s. Speed is a good proxy for total mass (i.e. how much dark matter is present) but it also depends on where you make that measurement. For disc galaxies, we can use the gas to probe regions far outside the stellar disc. We can't do that for UDGs - at least not this one - so we can't really get a good estimate of its total mass (there are methods of extrapolating, but the authors didn't try any). What they do show, however, is that the mass of dark matter within the measurable region is much, much higher than for more typical galaxies. It's way off the usual relation. If I were less cautious, I'd say that indicates it might be a very massive object indeed, but the authors (probably wisely) don't comment.

Chemically the galaxy is odd too. It seems to have had a prolonged duration of star formation lasting about 10 Gyr (don't ask me how they measure this). Given all the expected supernovae, that should make it enriched in iron - but it isn't. In fact it's magnesium-iron ratio is way, way off, even compared to other UDGs.

How could this be ? It may depends on the formation of the galaxy and when the different types of supernovae exploded. Early supernovae (from short-lived massive stars) may have blasted most of the galaxy's gas out into intergalactic space. This could also remove part of the dark matter through a "gravity tractor" since the mass of the gas could be initially very high. Whatever gas was left would have been rich in magnesium. Then later on, supernovae from accreting material in binary star systems (which take much longer) would have exploded, but since the mass of the galaxy was now much less, most of their iron-rich ejecta would have escaped.

That's their best guess for now, at any rate. But we're still very ignorant of even the basic properties of UDGs. Other exotic possibilities like continuous accretion of gas might be possible; it's also hard to see how the supposed loss of dark matter can be reconciled with its apparently heavily dark matter-dominated nature. Further research is very definitely needed.

https://arxiv.org/abs/1901.08068