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Recession of M81 and the M81 Group?

  1. Dec 1, 2008 #1

    AJH

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    I have found M81 and M82 with my 10x50 binoculars. I understand that they are both part of the M81 group, separate from our own Local Group, and was under the impression that anything outside of our Local Group (including the Virgo cluster and other groups) was receding from us due to the general expansion of space overcoming gravity at greater distances.

    I have read that M81 emits light with a negative red shift whereas other galaxies in the M81 Group (including M82) emit radiation with positive red shifts and that this might be due to M81 alone happening to be temporarily approaching us as part of its rotation around a galactic-group gravitational centre. If so, should at least some of the many other galaxies in the group also be approaching us and displaying negative redshifts? Are we just deducing that M81 is ultimately receding from us due to that trend in neighbouring galaxies? I have come across a recessional velocity for M81 of 95 km/sec quoted a couple of times and wonder how this is arrived at.

    I find M81 and M82 more interesting objects to view than M33 (for instance) and would like to know when I look at them if both or either are moving away from us.
     
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  3. Dec 1, 2008 #2

    Chronos

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    Gravitational attraction between us and M81 group exceeds the force of expansion, at least for the foreseeable future.
     
  4. Dec 2, 2008 #3

    AJH

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    Thanks very much for giving me a definitive answer on this. I have encountered contradictory suggestions in several guides and websites (as there also is regarding distances).

    I would be interested to know for certain if other binocular objects such as M51 and M101 were receding from us. All the sources I have read state that the Virgo Cluster and ourselves are moving apart, albeit slowed by gravity.

    Is there a rule-of-thumb distance from us (e.g. 15 or 25 million l.y.a.) beyond which galaxies are going to be receding from us?
     
  5. Dec 2, 2008 #4

    Chronos

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    The largest scale at which mass density has been measured with any precision is a galactic supercluster. These galactic superclusters can as large as 20 mega parsecs. Our Milky way, and local group, is a member of the Virgo supercluster. Hubble expansion is slowed 10-20% by gravity in a typical supercluster.
     
  6. Dec 3, 2008 #5

    AJH

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    I would re-cap what I have learnt from this thread, seds.org and ned.caltech.edu as follows:

    1) A measure of the velocity of a galaxy would depend on the point of reference (Sun, galaxy centre, group centre etc) of the observor as well as the object's activity within its own group - a galaxy might thus have a negative red-shift and still be receding from us.

    2) Members of nearer groups beyond the Local Group such as M81 can still gravitationally interact with the Local Group so any current recession on their part may not necessarily be long-term.

    3) There is a clearer pattern of recession away from us in somewhat further groups such as M51 group, M101 group etc.

    4) The attractive effect of massive of groups such as the Virgo cluster counteracts the general trend of recession.
     
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