Perhaps the cow is very large after all

The saga of galaxies without dark matter continues.

At first, I thought the thing sounded pretty cool. Lots of accusations were made that they'd got the distance wrong, which means they'd underestimated the total mass , but most of them didn't seem very credible to me, (granting that I'm no expert in this). But then another claim said that there were two groups in this part of the sky, one close and one more distant. That sounded a lot more believable and would give a good excuse for everyone getting the distances confused.

This latest measurement says nope, these galaxies are far away and don't have any dark matter. They use deeper Hubble data and show that prospect of the galaxies being much closer just don't fit. They say that previous data was too shallow, so that the red giant stars needed for distance measurements (red giants act as a sort of standard candle of known brightness) weren't visible, biasing the result in favour of a lower distance measurement. They don't really comment as to why different authors found different distances from the same data, except perhaps a hint that the calibration of the magnitudes may have been wrong.

Given the recent discovery of a whole population of gas-rich objects without dark matter, for which distance concerns aren't so important as they're quite a bit further away, these original claims should probably be given rather more credibility. There hasn't been much response to those latest detections yet, though it's still early days and the publication was only a letter, not a full article. Of course it's possible the original objects are actually normal objects but the new ones are indeed dark matter deficient, although that would be a bit weird.

The authors say that even deeper Hubble observations are on their way. Will this finally settle the matter ? I somehow doubt it. To my mind, the focus should switch to those gas-rich galaxies without dark matter, which don't have such distance or velocity ambiguities. Perhaps when there's a full paper published people will start to realise that there's a whole other bunch of really interesting objects to work on instead of these two usual suspects.

A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy NGC 1052-DF4 from Deep Hubble Space Telescope Data

Previous studies have shown that the large, diffuse galaxies NGC1052-DF2 and NGC1052-DF4 both have populations of unusually luminous globular clusters as well as a very low dark matter content. Here we present newly-obtained deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of one of these galaxies, NGC1052-DF4.