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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 ?

Peter U
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2 Answers2

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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).

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

Wayfaring Stranger
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  • 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 07 '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 07 '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 08 '18 at 00:22
  • Not missing dark matter after all. Just perhaps a little low: https://arxiv.org/abs/1804.04136 https://arxiv.org/abs/1804.04167 https://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 00:00
  • @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 00:04
  • I'm not sure those paper refute the result, but it is an ongoing debate. You can see van Dokkum's reply here. – pela Apr 18 '18 at 06:59
  • Anyway, I think perhaps — but correct me if I'm wrong — that you misinterpret the mechanism. DM can be stripped "tidally", but that will strip gas and stars as well, since gravity affects everything. Gas can be stripped collisionally, which will separate it from stars/DM. This is what happened with the Bullet Cluster, and we see both components ("gas" and "DM+stars"). In a way, it's also what (ostensibly) happened with DF2, only we now only see the gaseous component. – pela Apr 18 '18 at 07:01
  • Perhaps I misunderstand you, but while you may compare stars to marbles, you cannot compare DM to sawdust, since DM is collisionless. You may compare gas to sawdust, though. But stars are, effectively, collisionless too, and so act more like DM. – pela Apr 18 '18 at 07:04
  • @pela No sawdust isn't collisionless, but its tiny mass compared to stars makes it far easier to yank around with slight fields. Lot's of neutral hydrogen in this sparse galaxy, is there? – Wayfaring Stranger Apr 18 '18 at 15:01
  • But a gravitational potential doesn't care about te mass of an object moving in it. A sawdust particle, a DM particle, a star, and a black hole all follow the same trajectory due to gravity. But once you introduce hydroforces, it's a different story. – pela Apr 18 '18 at 16:37
  • That said, I admit that the DF2 case is far from unambiguous. I'll edit my answer (later) to mention the papers you linked to (which I hadn't seen since I'm a week behind in checking arXiv — thanks :) ). – pela Apr 18 '18 at 16:40