The pendulum swings and swings again

The saga of NGC 1052-DF2 and DF4 continues.

I'm not going to try and keep up with every development on this one any more because there are just too many. In brief, two galaxies were found that apparently had no dark matter at all. Given their apparent distance of about 20 Mpc, their size and velocity dispersion appear to indicate that their stars are moving so slowly that they could be gravitationally bound all by themselves, with no need of the usual extra mass.

This discovery has been controversial. First there were claims that the velocity dispersions had been measured incorrectly. This never looked terribly convincing, and indeed that idea has gone away. Then there were claims that the distance was wrong. If the galaxy is actually at about 13 Mpc, then the velocity measurements actually do require quite a lot of dark matter, as well as explaining other anomalies like the brightness of the galaxy's globular clusters. This has turned into a game of galactic ping-pong, with each team periodically refuting the other's distance claims with new data or analysis methods.

The present paper is the latest in a long series of attempts to decisively knock out the claim for a 13 Mpc distance. They use the "tip of the red giant branch" method to estimate the distance. Basically, red giant stars ought to be reasonably good standard candles (at least statistically if not individually), meaning that you know their true brightness so can get the corresponding distance. Previous claims using this method, they say, were not deep enough to detect stars at 20 Mpc distances, leading to all sorts of complications in the analysis. So they've gone and got Hubble data which should be more than sufficient to sort this out. If nothing else, their main image is certainly very pretty.

They say the distance to DF2 is actually even slightly higher than their initial claim, at 22 Mpc. DF4 they've previously said is at 20 Mpc, and though there are certainly some errors in all this, they say this is sufficient to say that at least one of them isn't part of the NGC 1052 group. That's significant, because it makes a tidal formation scenario all the less likely. In addition, they mention that they have wide-field deep imaging (not presented here) which does not show the previously-reported tidal tails, so those features are likely spurious.  

Finally, they'd previously made a boo-boo in claiming that these objects refute MOND, because MOND has an external field effect that they didn't account for. But now it looks like both DF2 and DF4 are too far away from their companion galaxies for this to be significant after all. Well, we'll see. Expect lots of angry responses from both standard model and MOND enthusiasts on this one.

A Tip of the Red Giant Branch Distance of 22.1 Mpc to the Dark Matter Deficient Galaxy NGC1052-DF2 from 40 Orbits of Hubble Space Telescope Imaging

The inferred distance is DTRGB=22.1 Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC1052. The new HST distance rules out the idea that some of NGC1052-DF2's unusual properties can be explained if it were at 13 Mpc; instead, it implies that the galaxy's globular clusters are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC1052-DF2 to NGC1052-DF4 is well-determined at 2.1 Mpc, significantly larger than the virial diameter of NGC1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics.