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Re: This Week's Finds in Mathematical Physics (Week 238)

  1. Aug 18, 2006 #1
    Some updates on the Bullet Cluster / dark matter saga.
    You can now see a picture of the Bullet Cluster here:


    I've also updated a bunch of information and fixed some mistakes
    in the initial version of "week238", thanks in part to Matt Owers.

    >Markevitch and company have been studying the "Bullet Cluster", a
    >a bunch of galaxies that has a small bullet-shaped subcluster zipping
    >away from the center at 4,500 kilometers per second.

    >It seems that one of the rapidly moving galaxies in this subcluster
    >has hit a bystander galaxy - I'm not sure, but a high-speed collision
    >of galaxies occurred.

    In fact, the whole subcluster hit another subcluster!

    So, the picture is a picture of colliding bunches of galaxies.
    The individual galaxies in these bunches mainly shoot right past
    each other - but the intergalactic gas in one bunch is hitting
    the gas in the other, and getting so hot it emits X-rays.

    >So, dark matter is seeming more and more real. In fact, last year
    >folks found evidence for "ghost galaxies" made mainly of dark matter
    >and cold hydrogen, with very few stars:
    >6) PPARC, New evidence for a dark matter galaxy,

    Apparently the consensus is now that this ghost, VIRGOHI 21,
    is hydrogen stripped off from a galaxy by the "wind" it felt
    as it fell into the Virgo Cluster. This effect is called
    "ram pressure stripping" - the gas of a galaxy can be stripped
    off if the galaxy is moving rapidly through a cluster, due
    to interaction with the gas in the cluster.
  2. jcsd
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