# Galaxy discovered lacking Dark Matter

Recently a Galaxy was discovered that contained no Dark Matter. I was wondering what explanation could be offered for this ? This Galaxy also contains only 1% as many Stars as our Milky Way does . Is there be a connection between these two things, lack of many Stars and lack of Dark Matter ?

The paper describing this finding is van Dokkum et al. (2018). In a galaxy, the ratio of the stellar mass $M_\star$ to dark matter mass $M_\mathrm{DM}$ is normally very small, increasing with mass until Milky Way-sized galaxies where it reaches $\sim1/30$, then decreasing again. But the dwarf galaxy NGC1052–DF2, seems to contain little or no dark matter. Galaxies of that mass ($M_\star\sim2\times10^8\,M_\odot$) typically has several hundred times more DM than stars.

How the galaxy was formed is not known, but the authors speculate on a few different mechanisms that all have to do with the fact that gas, in contrast to DM, may cool and condense and thus form clouds of very high $M_\mathrm{gas}/M_\mathrm{DM}$ ratios:

• NGC1052–DF2 is located neat an large elliptical galaxy (NGC 1052) which could have gone through a merging event, tidally stripping a chunk of gas from one of the mergers. This is consistent with NGC1052–DF2 having a large velocity wrt. the elliptical.

• NGC1052–DF2 could have formed from low-metallicity gas that was swept up in quasar winds (as described in Natarajan et al. 1998).

• Lastly, NGC1052–DF2 could have formed from the fragmentation of gas accreting onto the elliptical, possibly aided by shocks.

EDIT (thanks to @WayfaringStranger): A fourth possibility is that the authors have misinterpreted the data. Shortly after the paper was put out, several other papers criticized the statistical methods used by van Dokkum et al. to infer their result (Martin et al. 2018; Famaey et al. 2018; Laporte et al. 2018). van Dokkum wrote in a very long reply on his blog how at least the first of these papers actually confirm, rather than refute, his results. I am not enough of a statistician to comment on who's right and who's wrong, but note that there is currently an ongoing debate on the Facebook group astrostatistics.

You also ask whether the lack of DM is related to its "lack of stars". I wouldn't say that NGC1052–DF2 "lacks stars", anymore than the Milky Way "has too many stars" — it's just a small galaxy. But in general, the smaller a galaxy is, the larger the scatter in the $M_\star/M_\mathrm{DM}$ ratio is. Small galaxies, or small clumps of gas, have shallow gravitational potentials, so a small galaxy can more easily get stripped of gas, and a small chunk of gas can more easily escape a galaxy, or accretion stream, without attracting dark matter. In contrast, if would be very hard to conceive a very large galaxy having a considerably different $M_\star/M_\mathrm{DM}$ ratio, and indeed the scatter for massive galaxies is less than a factor of two (e.g. More et al. 2010).

• That Freaky Discovery of Galaxies With No Dark Matter Seems to Be Debunked. "According to new distance calculations, the galaxy in question is much closer than initial measurements suggested. This changes both the mass of the galaxy as a whole, and the proportion of that mass that could be normal matter." – Keith McClary Jul 17 '19 at 22:16
• @KeithMcClary Ooh exciting! I think van Dokkum begs to differ, though :D – pela Jul 18 '19 at 10:29

Bullet cluster We've already seen an example of a large galaxy stripping dark matter from a smaller one.

• No, this is different. The Bullet Cluster is stripped of gas, because it's collisional. The stars and DM, being collisionless, stays together. – pela Apr 7 '18 at 17:37
• @pela Quite possibly, but 14 billion years seems a long time to go without at least a major gravitational interaction. (Bside – Wayfaring Stranger Apr 7 '18 at 18:48
• That's true, but I don't think I understand what that has to to do with stars being separated from dark matter. – pela Apr 8 '18 at 0:22
• Not missing dark matter after all. Just perhaps a little low: arxiv.org/abs/1804.04136 arxiv.org/abs/1804.04167 arxiv.org/abs/1804.04139 Much ado about not very much. Doesn't take much to strip DM from what's already an ultra-diffuse galaxy. – Wayfaring Stranger Apr 17 '18 at 0:00
• @pela even "collisionless" stars carry a lot more momentum the dark matter particles. Try vacuuming up a mixture of fine sawdust and marbles some time. Easy to get all the sawdust without disturbing the marbles. Stars vs dark matter case is millions of times more extreme, – Wayfaring Stranger Apr 17 '18 at 0:04