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To big to fail

  1. Aug 13, 2015 #1

    wolram

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    There is a lot to digest here, have a browse and see what you think.


    arXiv:1508.02715 [pdf, ps, other]
    Comprehensive Assessment of the Too-Big-to-Fail Problem
    Fangzhou Jiang (1), Frank C. van den Bosch (1) ((1) Yale University)
    Comments: 19 pages, 10 figures, accepted for publication in MNRAS
    Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

    We use a semi-analytical model for the substructure of dark matter haloes to assess the too-big-to-fail (TBTF) problem. The model accurately reproduces the average subhalo mass and velocity functions, as well as their halo-to-halo variance, in N-body simulations. We construct thousands of realizations of Milky Way (MW) size host haloes, allowing us to investigate the TBTF problem with unprecedented statistical power. We examine the dependence on host halo mass and cosmology, and explicitly demonstrate that a reliable assessment of TBTF requires large samples of hundreds of host haloes. We argue that previous statistics used to address TBTF suffer from the look-elsewhere effect and/or disregard certain aspects of the data on the MW satellite population. We devise a new statistic that is not hampered by these shortcomings, and, using only data on the 9 known MW satellite galaxies with $V_{\rm max}>15{\rm kms}^{-1}$, demonstrate that $1.4^{+3.3}_{-1.1}\%$ of MW-size host haloes have a subhalo population in statistical agreement with that of the MW. However, when using data on the MW satellite galaxies down to $V_{\rm max}=8{\rm kms}^{-1}$, this MW consistent fraction plummets to $<5\times10^{-4}$ (at 68% CL). Hence, if it turns out that the inventory of MW satellite galaxies is complete down to 8km/s, then the maximum circular velocities of MW satellites are utterly inconsistent with $\Lambda$CDM predictions, unless baryonic effects can drastically increase the spread in $V_{\rm max}$ values of satellite galaxies compared to that of their subhaloes.
     
  2. jcsd
  3. Aug 13, 2015 #2

    Garth

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    Personally I would have highlighted this phrase in the sentence:
    Following on from what I said in https://www.physicsforums.com/threads/is-the-lcdm-model-correct.825595/#post-5184192"Once a model derived from interpreted data has been established to be the standard one it will take "extraordinary evidence" to support the "extraordinary claim" that it might be wrong," might this paper actually provide what will be considered as such "extraordinary evidence"? Probably not...

    They say
    Has anyone tried Scalar field dark matter to solve this TBTF problem?

    From A brief Review of the Scalar Field Dark Matter model
    Garth
     
    Last edited: Aug 13, 2015
  4. Aug 13, 2015 #3
    Hi wolram:

    The discussion appears to be interesting except I have no idea what the TBTF problem is. Can you provide a defiition and perhaps a link to an article that discusses in some detail what the TBTF problem is all about rathan than solutions?

    Regards,
    Buzz
     
  5. Aug 13, 2015 #4

    George Jones

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    Did you read the first paragraph of Introduction section of the arXiv paper to which wolram linked?
     
  6. Aug 13, 2015 #5

    Chronos

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    The inventory of MW satellite galaxies is not complete as evidenced by this paper http://arxiv.org/abs/1508.02381, Digging deeper into the Southern skies: a compact Milky-Way companion discovered in first-year Dark Energy Survey data, which also appeared yesterday

    "The Dark Energy Survey (DES) is a 5000 sq. degree survey in the southern hemisphere, which is rapidly reducing the existing north-south asymmetry in the census of MW satellites and other stellar substructure. We use the first-year DES data down to previously unprobed photometric depths to search for stellar systems in the Galactic halo, therefore complementing the previous analysis of the same data carried out by our group earlier this year. Our search is based on a matched filter algorithm that produces stellar density maps consistent with stellar population models of various ages, metallicities, and distances over the survey area. The most conspicuous density peaks in these maps have been identified automatically and ranked according to their significance and recurrence for different input models. We report the discovery of one additional stellar system besides those previously found by several authors using the same first-year DES data. The object is compact, and consistent with being dominated by an old and metal-poor population. DES J0034-4902 is found at high significance and appears in the DES images as a compact concentration of faint blue point sources at ~ 87 {kpc}. Its half-light radius of r_h = 9.88 +/- 4.31 {pc} and total luminosity of M_V ~ -3.05_{-0.42}^{+0.69} are consistent with it being a low mass halo cluster. It is also found to have a very elongated shape. In addition, our deeper probe of DES 1st year data confirms the recently reported satellite galaxy candidate Horologium II as a significant stellar overdensity. We also infer its structural properties and compare them to those reported in the literature."
     
  7. Aug 13, 2015 #6
    Hi @George:

    Here is the "definition" from the introduction:
    the overabundance of massive, dense subhaloes predicted by CDM compared to the observed number of relatively luminous galaxies of the Milky Way or the Local Group.​
    I need some help understanding what this means.
    (1) What does "relatively luminous galaxies of the Milky Way" mean?
    (2) If "of the Milky Way, or" is ignored, what is a massive subhalo?
    (3) Why is the mismach between the CDM and observation called a "too-big-to-fail" problem?

    Thanks for your post,
    Buzz
     
  8. Aug 13, 2015 #7
    Hi @Chronos:

    If I understand your post correctly, you are pointing out that the apparent TBTF mismatch between CDM and observation is not necessarily a real mismatch because the survey from which these observations were collected is incomplete. Is that correct?

    Thanks for your post,
    Buzz
     
  9. Aug 13, 2015 #8

    George Jones

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    I think that the actual quote is "the overabundance of massive, dense subhaloes predicted by CDM compared to the observed number of relatively luminous satellite galaxies of the Milky Way or the Local Group"

    This means "relatively bright galaxies that we we see orbiting our galaxy or orbiting (i.e., satellite) other large members of our Local Group of galaxies.

    A halo is something that surrounds something else. We think that dark matter halos surround the normal matter in galaxies.

    Try Sean Carroll's exposition:


    http://www.preposterousuniverse.com...ies-that-are-too-big-to-fail-but-fail-anyway/[/user]
     
  10. Aug 14, 2015 #9
    Hi George:

    Thanks very much for your prompt and excellent answers to my questions. The Sean Carrol article you cited is also an excellent explanation.

    Regards,
    Buzz
     
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