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posted by janrinok on Tuesday July 14 2015, @11:49PM   Printer-friendly
from the they're-dark-Jim,-but-not-as-we-know-it dept.

About 321 million light-years away from us is the Coma Cluster, a massive grouping of more than 1,000 galaxies. Some of its galaxies are a little unusual, however: they're incredibly dim. So dim, in fact, that they have earned the title of "Ultra-Dark Galaxies" (UDGs). (The term is actually "Ultra-Diffuse Galaxies", as their visible matter is thinly spread, though "ultra-dark" has been used by some sources and, let's face it, sounds a lot better). This was discovered earlier this year in a study that identified 47 such galaxies.

Dimness isn't necessarily unusual in a galaxy. Most of a galaxy's light comes from its stars, so the smaller a galaxy is (and thus the fewer stars it has), the dimmer it will be. We've found many dwarf galaxies that are significantly dimmer than their larger cousins.

What was so unusual about these 47 is that they're not small enough to account for their dimness. In fact, many of them are roughly the size of our own Milky Way (ranging in diameter from 1.5 to 4.6 kiloparsecs, compared with the Milky Way's roughly 3.6) but have only roughly one thousandth of the Milky Way's stars. The authors of the recent study interpret this to mean that these galaxies must be even more dominated by dark matter than are ordinary galaxies.


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  • (Score: 1, Interesting) by Anonymous Coward on Wednesday July 15 2015, @11:58AM

    by Anonymous Coward on Wednesday July 15 2015, @11:58AM (#209317)

    The cosmic microwave background, the afterglow of the Big Bang as it were, is very nearly completely uniform, but it has very slight deviations (anisotropies) in it that are neatly explained by hypothesising dark matter.

    According to figure 1 of Angus and Diaferio (2011) MOND + sterile neutrinos can fit this data as well:

    We present a new particle mesh cosmological N-body code for accurately solving the modified Poisson equation of the quasi-linear formulation of modified Newtonian dynamics (MOND). We generate initial conditions for the Angus cosmological model, which is identical to Λ cold dark matter (ΛCDM) except that the CDM is switched for a single species of thermal sterile neutrinos. We set the initial conditions at z= 250 for a (512 Mpc h−1)3 box with 2563 particles, and we evolve them down to z= 0. We clearly demonstrate the ability of MOND to develop the large-scale structure in a hot dark matter cosmology and contradict the naive expectation that MOND cannot form galaxy clusters. We find that the correct order of magnitude of X-ray clusters (with TX > 4.5 keV) can be formed, but that we overpredict the number of very rich clusters and seriously underpredict the number of lower mass clusters. We present evidence that suggests the density profiles of our simulated clusters are compatible with those of the observed X-ray clusters in MOND. As a last test, we computed the relative velocity between pairs of haloes within 10 Mpc and find that pairs with velocities larger than 3000 km s−1, like the bullet cluster, can form without difficulty.

    http://mnras.oxfordjournals.org/content/417/2/941.full [oxfordjournals.org]

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  • (Score: 0) by Anonymous Coward on Wednesday July 15 2015, @04:12PM

    by Anonymous Coward on Wednesday July 15 2015, @04:12PM (#209428)
    Sterile neutrinos are a form of dark matter!
  • (Score: 1, Interesting) by Anonymous Coward on Wednesday July 15 2015, @04:47PM

    by Anonymous Coward on Wednesday July 15 2015, @04:47PM (#209448)

    According to figure 1 of Angus and Diaferio (2011) MOND + sterile neutrinos can fit this data as well:

    So you don't need dark matter because you can describe the observations with a theory including dark matter in the form of sterile neutrinos?

    So now the competing theories are "MOND and dark matter" vs. "just dark matter"? Well, Occam's razor tells me quite clearly which of those two options I should prefer.

    • (Score: 0) by Anonymous Coward on Wednesday July 15 2015, @05:27PM

      by Anonymous Coward on Wednesday July 15 2015, @05:27PM (#209464)

      It is not *cold* though.