This cow is probably small, not far away

I've written quite a bit about that famous galaxy apparently devoid of dark matter, of which you can read a pretty complete summary here. Some of the criticism levelled against the central claim, such as the statistical evidence from the globular cluster velocity dispersion, hasn't stood up to scrutiny. The main challenge has been to establish the distance : if the galaxy is far away, then it's large and weird, but if it's closer, then it's small and quite normal.

Thus far I've been more persuaded by the "large and weird" camp. But after an excellent conference last week (of which more in PotC soon !), and this latest paper, I'm slowly switching my allegiance. I'm no expert on the distance measurement techniques, but it looks to me like the authors here directly address the problems van Dokkum previously listed.

There are two here arguments that I, as a non-expert in distance measurements, find compelling. First they say that there's evidence for two different galaxy groups at different distances - one at 13 Mpc, which the Trujillo camp have espoused and would rectify all the weird anomalies of the galaxy, and one at 19 Mpc, which is close to the van Dokkum estimate and would make everything weird. That their estimates do find evidence of galaxies at the higher distances in this region makes it far more plausible that it's not as simple as someone having just made a mistake - it's much easier to believe that people have simply been confused because this little patch of the Universe is in fact quite confusing. And it establishes that there's nothing funny going on that makes it impossible for one method to find a larger distance value here (for whatever reason).

Secondly, they point out that the velocities of the galaxies don't tell you much about their distance information, especially as things in groups move around quite a bit. That is, at 13 Mpc we expect galaxies to be redshifted to an equivalent of 900 km/s, whereas at 19 Mpc it would be more like 1300 km/s. But for galaxies to have peculiar velocities (that is, deviations from the expected value assuming uniform Hubble expansion) of 200-300 km/s in groups is nothing extraordinary at all. So the overall picture of the anomalous galaxies actually being closer, which also makes them rather typical objects in every other sense, looks completely self-consistent.

I would point out though that when the authors say :

In fact, Blakeslee & Cantiello (2018) warn about the use of such a calibration in a range where it has not been explored...

Then this is very misleading. They are trying to refute a problem with the larger distance estimate, but the citation they give actually supports the larger value ! To omit this is not really very fair. And of course we should hear van Dokkum's inevitable response - I doubt we can quite call the matter settled just yet. But if I had to bet, I'd be switching my money to the "small and close" interpretation.

The TRGB distance to the second galaxy "missing dark matter". Evidence for two groups of galaxies at 13.5 and 19 Mpc in the line of sight of NGC1052

A second galaxy ``missing dark matter" (NGC1052-DF4) has been recently reported. Here we show, using the location of the Tip of the Red Giant Branch (TRGB), that the distance to this galaxy is 14.2+-0.7 Mpc. This locates the galaxy 6 Mpc closer than previously determined.