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Dwarf Galaxies Orbiting the Andromeda Galaxy unexplained by Dark matter

  1. Jan 6, 2013 #1
    "A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy"
    Published in Nature 3 days ago.

    Rodrigo A. Ibata, Geraint F. Lewis, Anthony R. Conn, Michael J. Irwin, Alan W. McConnachie, Scott C. Chapman, Michelle L. Collins, Mark Fardal, Annette M. N. Ferguson, Neil G. Ibata, A. Dougal Mackey, Nicolas F. Martin, Julio Navarro, R. Michael Rich, David Valls-Gabaud, Lawrence M. Widrow
    (Submitted on 3 Jan 2013)
    " Dwarf satellite galaxies are thought to be the remnants of the population of primordial structures that coalesced to form giant galaxies like the Milky Way. An early analysis noted that dwarf galaxies may not be isotropically distributed around our Galaxy, as several are correlated with streams of HI emission, and possibly form co-planar groups. These suspicions are supported by recent analyses, and it has been claimed that the apparently planar distribution of satellites is not predicted within standard cosmology, and cannot simply represent a memory of past coherent accretion. However, other studies dispute this conclusion. Here we report the existence (99.998% significance) of a planar sub-group of satellites in the Andromeda galaxy, comprising approximately 50% of the population. The structure is vast: at least 400 kpc in diameter, but also extremely thin, with a perpendicular scatter <14.1 kpc (99% confidence). Radial velocity measurements reveal that the satellites in this structure have the same sense of rotation about their host. This finding shows conclusively that substantial numbers of dwarf satellite galaxies share the same dynamical orbital properties and direction of angular momentum, a new insight for our understanding of the origin of these most dark matter dominated of galaxies. Intriguingly, the plane we identify is approximately aligned with the pole of the Milky Way's disk and is co-planar with the Milky Way to Andromeda position vector. The existence of such extensive coherent kinematic structures within the halos of massive galaxies is a fact that must be explained within the framework of galaxy formation and cosmology. "

    Together with the papers by Kroupa and Pawlowski of 2012 that are cited in the article's bibliography, this foresees a deep crisis in the Dark matter models, what do you guys think?
    BTW, one of the coauthors of this paper (Neil Ibata) is only 15 years old ,wow! Not many people can say they published in Nature at 15.
  2. jcsd
  3. Jan 6, 2013 #2


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    Nice find! at the moment you don't have the arxiv link in your post, so I will contribute it

    If you have a moment, could you explain in simple terms roughly why this leads to a crisis in DM models?

    Here's a quote from the end of their "conclusions" section on page 6:

    ==quote 1301.0446==
    ... A further possibility is that we are witnessing accretion along filamentary structures that are fortuitously aligned. In-situ formation may be possible if the planar satellite galaxies formed like tidal-dwarf galaxies (TDG) in ancient gas-rich galaxy mergers7, but then the dwarf galaxies should be essentially devoid of dark matter. If the planar M31 dwarfs are dynamically-relaxed, the absence of dark matter would be greatly at odds with inferences from detailed observations25 of Milky Way satellites assuming the standard theory of gravity. An alternative possibility is that gas was accreted preferentially onto dark matter sub-halos that were already orbiting in this particular plane, but then the origin of the plane of sub-haloes would still require explanation. We conclude 6
    that it remains to be seen whether galaxy formation models within the current cosmological framework can explain the existence of this vast, thin, rotating structure of dwarf galaxies within the halo of our nearest giant galactic neighbour.

    What I'm thinking is that there may be some explanation within the domain of galaxy formation models that does not involve a crisis or disruption of our basic view of dark matter.
    they seem to be thinking along those lines, but as yet have not come up with a plausible scenario.

    You mentioned a Kroupa Pawlowski paper from 2012, in their references. I think this is it:
    7. Pawlowski, M. S., Pflamm-Altenburg, J. & Kroupa, P. The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way. Monthly Notices of the Royal Astronomical Society 423, 1109–1126 (2012).

    I will get a link.
    The VPOS: a vast polar structure of satellite galaxies, globular clusters and streams around the Milky Way
    M. S. Pawlowski, J. Pflamm-Altenburg, P. Kroupa
    (Submitted on 23 Apr 2012)
    It has been known for a long time that the satellite galaxies of the Milky Way (MW) show a significant amount of phase-space correlation, they are distributed in a highly inclined Disc of Satellites (DoS). We have extended the previous studies on the DoS by analysing for the first time the orientations of streams of stars and gas, and the distributions of globular clusters within the halo of the MW. It is shown that the spatial distribution of MW globular clusters classified as young halo clusters (YH GC) is very similar to the DoS, while 7 of the 14 analysed streams align with the DoS. The probability to find the observed clustering of streams is only 0.3 per cent when assuming isotropy. The MW thus is surrounded by a vast polar structure (VPOS) of subsystems (satellite galaxies, globular clusters and streams), spreading from Galactocentric distances as small as 10 kpc out to 250 kpc. These findings demonstrate that a near-isotropic infall of cosmological sub-structure components onto the MW is essentially ruled out because a large number of infalling objects would have had to be highly correlated, to a degree not natural for dark matter sub-structures. The majority of satellites, streams and YH GCs had to be formed as a correlated population. This is possible in tidal tails consisting of material expelled from interacting galaxies. We discuss the tidal scenario for the formation of the VPOS, including successes and possible challenges. The potential consequences of the MW satellites being tidal dwarf galaxies are severe. If all the satellite galaxies and YH GCs have been formed in an encounter between the young MW and another gas-rich galaxy about 10-11 Gyr ago, then the MW does not have any luminous dark-matter substructures and the missing satellites problem becomes a catastrophic failure of the standard cosmological model.
    Comments: 21 pages, 8 figures, 2 tables. Accepted for publication in MNRAS. An animation of Figure 5 can be found at this http URL: http://www.youtube.com/watch?v=nUwxv-WGfHM&feature=youtu.be
    Last edited: Jan 6, 2013
  4. Jan 6, 2013 #3
    Well, the assessment of empirical findings always carries a bit of subjectiveness in the choice of words, but basically DM models can't account for the observed arrangement of dwarf galaxies in a plane around the big galaxy (first it was observed in the satellite galaxies of the Milky Way, and some attributed to pure chance, now its around Andromeda, using the same argument would be a bit of a stretch) without recurring to "fortuitous alignments", that seems as rigorous as trying to explain the anomalous velocity curves of outer stars in spiral galaxies itself as the result of fourtuitous alignments of stars.
    The other alternative is to come up with such contrived models of galaxy formation that they lose all explanatory power.
    Last edited: Jan 6, 2013
  5. Jan 7, 2013 #4
    Does the following contribute to higher velocities of stars and clusters at the outer edges of spiral galaxies?

    When stars, clusters and matter first approach a galaxy, wouldnt they likely be traveling at higher velocities than the rotational velocities of the stars already in the galaxy. So as they merge, wouldnt they then tend to increase the rotational speed of the stars and clusters already there, starting with those at the rim edge, as well as slowing down themselves, all due to gravitational interaction?
    Last edited: Jan 7, 2013
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