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Dwarf galaxies suggest dark matter theories may be wrong

  1. Sep 17, 2011 #1


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    I realize BBC science news is not a totally reliable source, but I'm wondering if any of the knowledgeable folks here have any comment on this:


    I tried to find if there had already been a thread on it, but I may have not looked hard enough. If there is one, just point me to it.
  2. jcsd
  3. Sep 18, 2011 #2


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    Well, the lack of dwarf galaxies is very well-known. However, the line connecting the lack of dwarf galaxies to dark matter isn't so easy, and for a rather simple reason: very small galaxies tend to expel most of their visible matter early-on, and so dwarf galaxies are really, really hard to detect. Most astrophysicists and cosmologists think it's more likely that we just aren't seeing the most of the dwarf galaxies. Warm dark matter is a possibility, but the evidence so far is sketchy at best.
  4. Sep 18, 2011 #3


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    Dwarf galaxies are just another poorly accounted for contributor to baryonic mass in the universe. WMAP has already ruled out baryonic mass as a significant component of the 'missing' mass of the universe.
  5. Sep 18, 2011 #4


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    But the article is not arguing against dark matter at all, just arguing for warm dark matter rather than cold dark matter. Is there other evidence that suggests these guys are off on a wild goose chase? Chalnoth suggests that this is likely (the wild goose chase) but possibly could be onto something.

    I take it that they MAY be onto something but more evidence is needed. Hm ... Chalnoth, I think I just restated what YOU said. :smile:
  6. Sep 18, 2011 #5


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    I agree, they could be on to something and it seems worth checking out. Here is a related paper:

    The Haloes of Bright Satellite Galaxies in a Warm Dark Matter Universe
    Mark Lovell, Vincent Eke, Carlos Frenk, Liang Gao, Adrian Jenkins, Tom Theuns, Jie Wang, Alexey Boyarsky, Oleg Ruchayskiy
    (Submitted on 14 Apr 2011)
    High resolution N-body simulations of galactic cold dark matter haloes indicate that we should expect to find a few satellite galaxies around the Milky Way whose haloes have a maximum circular velocity in excess of 40 kms. Yet, with the exception of the Magellanic Clouds and the Sagittarius dwarf, which likely reside in subhaloes with significantly larger velocities than this, the bright satellites of the Milky Way all appear to reside in subhaloes with maximum circular velocities below 40 kms. As recently highlighted by Boylan-Kolchin et al., this discrepancy implies that the majority of the most massive subhaloes within a cold dark matter galactic halo are much too concentrated to be consistent with the kinematic data for the bright Milky Way satellites. Here we show that no such discrepancy exists if haloes are made of warm, rather than cold dark matter because these haloes are less concentrated on account of their typically later formation epochs. Warm dark matter is one of several possible explanations for the observed kinematics of the satellites.
    6 pages, 5 figures

    The paper by Lovell et al refers to this one submitted in late February, to be published in MNRAS letters:

    Too big to fail? The puzzling darkness of massive Milky Way subhaloes
    Michael Boylan-Kolchin, James S. Bullock, Manoj Kaplinghat
    (Submitted on 28 Feb 2011)
    We show that dissipationless LCDM simulations predict that the majority of the most massive subhaloes of the Milky Way are too dense to host any of its bright satellites (LV > 105 Lsun). These dark subhaloes have circular velocities at infall of 30-70 km/s and infall masses of [0.2-4] x 1010 Msun. Unless the Milky Way is a statistical anomaly, this implies that galaxy formation becomes effectively stochastic at these masses. This is in marked contrast to the well-established monotonic relation between galaxy luminosity and halo circular velocity (or halo mass) for more massive haloes. We show that at least two (and typically four) of these massive dark subhaloes are expected to produce a larger dark matter annihilation flux than Draco. It may be possible to circumvent these conclusions if baryonic feedback in dwarf satellites or different dark matter physics can reduce the central densities of massive subhaloes by order unity on a scale of 0.3 - 1 kpc.
    6 pages, 5 figures

    I see that Simon White of the Garching Max Planck Institute for Astrophysics is in on this paper as a co-author with Carlos Frenk:
    Where will supersymmetric dark matter first be seen?
    L. Gao, C. S. Frenk, A. Jenkins, V. Springel, S. D. M. White
    (Submitted on 11 Jul 2011)
    I believe this was already published in MNRAS. The material is somewhat related to what was discussed in the other two.

    Another related paper (quite a lot of activity on this idea recently)
    The Baryons in the Milky Way Satellites
    Owen H. Parry, Vincent R. Eke, Carlos S. Frenk, Takashi Okamoto
    (Submitted on 17 May 2011 (v1), last revised 22 Jun 2011 (this version, v2))
    We investigate the formation and evolution of satellite galaxies using smoothed particle hydrodynamics (SPH) simulations of a Milky Way(MW)-like system, focussing on the best resolved examples, analogous to the classical MW satellites. Comparing with a pure dark...
    ...or the subhalos may have initially formed with lower concentrations as would be the case, for example, if the dark matter were made of warm, rather than cold particles.
    16 pages, 11 figures
    Last edited: Sep 18, 2011
  7. Sep 18, 2011 #6


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    Very cool ... thank you Marcus
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