First I'll apologize for my pun; I just couldn't resist.

But seriously, last year astronomers observed the dim galaxy, Dragonfly 44, and a recent article in Science reports that the content of dark matter in the group of stars observed is about 99.9% which I believe is a first observation of dark matter of concentration well beyond the more typical 23% observed in the rest of the Universe.

I know that physicists don't really understand quite yet what dark matter is, and I admit I'm not fully briefed on the details of any theories, but does this new discovery discount any present theories or put more emphasis on others as to what comprises dark matter?

  • 2
    $\begingroup$ Note that dark matter $\neq$ dark energy. Dark energy is the thing that takes up $\sim 70\%$ of the energy content of the universe, and is (at least on large scales?) homogeneously distributed. $\endgroup$
    – Danu
    Aug 28, 2016 at 21:31
  • $\begingroup$ If these objects turn out to be plentiful would it change the estimate of the percentage of DM? $\endgroup$ Aug 29, 2016 at 1:14
  • $\begingroup$ @Keith Ask the question. $\endgroup$
    – Bob Bee
    Aug 29, 2016 at 2:16
  • $\begingroup$ I supposed they have already ruled out a supermassive black hole at the center keeping the stars together. Reading the paper I see " There is also no evidence for radial variation in the velocity dispersion.." which is I assume very unusual and thus indicative of the properties of dark matter (maybe)? $\endgroup$ Aug 29, 2016 at 3:21
  • $\begingroup$ @ja72 "very unusual and thus indicative of the properties of dark matter (maybe)" Actually it's typical of large galaxies that outside a certain distance from the center the orbital velocity distribution remains approximately constant, but that is attributed to the presence of a dark matter halo as the visible mass certainly can't explain it. Nor should this be surprising because even in normal galaxies there is much more dark matter than baryonic matter, so the effect of losing most of the normal stuff can be expected to be small. $\endgroup$ Aug 29, 2016 at 17:01

1 Answer 1


Dragonfly 44 and similar objects are being called 'ultra diffuse galaxies', or UDGs. The main difference with respect to other galaxies of similar stellar mass does not seem to be in terms of dark matter content, but rather of size of the stellar distribution.

Part of a galaxy, at least in the definition I'm used to, is its dark matter halo. While the halo is on average quite massive, about $\frac{5}{6}$ of the total mass of the galaxy (the other $\frac{1}{6}$ being made up of stars and gas; this comes from the cosmic average abundances, being careful to note that dark energy doesn't come into the mass budget for a galaxy), it is also quite extended, so the density doesn't get all that high. There seems to be a relationship between the mass of the halo and the (stellar) mass of the galaxy that ends up forming inside it, though with a fair bit of scatter. There is also a correlation between the (stellar) mass of a galaxy and its size, again with quite a bit of scatter.

This point is still debated in the literature, but it seems plausible that UDGs are galaxies which are perhaps a bit under-massive and over-sized (i.e. in the tails of the distributions) given the mass of their dark matter halo. The size in particular seems to be key. The dark matter content is measured dynamically - for instance it can be estimated from the velocity dispersion of the stars once the stellar mass distribution is subtracted (itself estimated from the luminosity). Such mass estimators typically yield the dynamical mass within some characteristic radius of the tracer particles. In this example the tracers are stars, so if the stellar distribution is more extended, the end result is that the total mass (including dark matter) is measured within a more extended aperture. The dark matter to ordinary matter mass ratio within this aperture, for a UDG, ends up being quite high, because the aperture is large and encompasses a lot of dark matter. An example on the other end of the spectrum would be a very compact galaxy - here the dynamical mass is measured within some small aperture, so the dark matter contribution is quite small. Another way to think about it is that a very compact stellar system has a high mean density, which can dominate over the dark matter density within some central region, even though the dark matter halo is much more massive overall. And that central region is precisely where the dark matter mass can be measured "directly".

Some authors argue that UDGs represent some new, separate class of galaxies which constrains dark matter theories is some new way. I have yet to be convinced by any of these arguments. They are still interesting objects, but I think they are more interesting as a constraint on galaxy formation models than on dark matter theories.

As to the claim that this is the first time such a high dark matter abundance has been observed, whether Dragonfly 44 is a record holder of some kind depends on how you define the dark matter to ordinary matter mass ratio. Clearly it is an outlier, but as far as I know it's not particularly special beyond that. Now that people know that these object exist they're finding a whole bunch of them in archival data (previously they would have been dismissed as noise, but since the convincing detection of a few of them and some effort to dig through old data there are now perhaps hundreds known, depending whose definition of UDG you use).


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