Galaxy discovered lacking Dark Matter

Question

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 ?

Answer 1

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.

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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).

Answer 2

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