More on these ultra-faint galaxies, a field which continues to develop rapidly. These are galaxies of similar size to the Milky Way but with ~1000x fewer stars. Measuring their stellar masses is relatively easy, but to measure their total mass (i.e. their dark matter content) requires measurements of how fast they're rotating. Normally, the easiest way to do this is by measuring their gas, but these galaxies tend not to have any gas (with one or two oddball exceptions that are extremely gas rich, just to make life interesting).

The total mass matters because it has wide-ranging implications for cosmological models, which predict far more dwarf (low mass) galaxies than are observed. Recently I reported that the body of evidence - such few rotation measurements as were available - was swinging quite strongly in favour of these being dwarf galaxies. There still aren't anywhere near enough to solve the missing galaxy problem, but hey, at least they're not making things worse.

Except the paper link today challenges that. According to these authors, one of these ultra-diffuse galaxies has the same total mass as the Milky Way. Now, if this is true, and if most of these new discoveries are equally massive, then we've got a serious problem for galaxy formation models. There's no obvious reason why some galaxies (forming in the same environment) should accrete vastly different amounts of baryonic matter (i.e. gas which eventually forms stars) to each other. Oh, we could probably come up with a hand-waving argument to explain it, but rigorously testing these won't be easy.

On the other hand, this would probably pretty decisively rule out alternative theories of gravity like MOND, which has very successfully explained why galaxies follow a very specific trend in terms of rotation velocity and total baryonic mass. Normal theories have a tough time doing that. But these galaxies, and the possible dark galaxies I've been harping on about, seem to indicate that this neat relationship doesn't actually work after all.

Except after reading the paper it's not at all clear that the galaxy really is as massive as the authors claim :
"We emphasize that the total halo mass is an extrapolation of the measured mass by a factor of ∼ 100. A more robust and less model-dependent conclusion is that the dark matter mass within r = 4.6 kpc is similar to the dark matter mass of the Milky Way within the same radius"

Gravitational lensing studies might eventually solve this, but until then the only safe conclusion is that science is hard. Watch this space.
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