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Why do so many galaxies in a cluster have near zero velocity

  1. Sep 2, 2015 #1
    I'm looking at a velocity chart of the Coma Cluster:

    Coma Cluster.png

    And the question occurred to me: why are there so many galaxies that have a zero velocity (relative to the core of the Coma Cluster which is roughly 7,000 km s-1)? At a distance of a Mpc or two you would expect to find galaxies at the peak of their red or blue shift if they were in orbit. How can they be travelling at the same speed as the core at these distances and still be part of the cluster? What am I missing?
  2. jcsd
  3. Sep 2, 2015 #2


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    Only if the motion is in the direction of the line of sight. If the motion is perpendicular to the line of sight, there is no red or blue shift (apart from that induced by time dilation).
  4. Sep 2, 2015 #3
    Got it. I was thinking about and edge on galaxy, but a cluster is like a sphere: there are galaxies that will be going clockwise (or counter-clockwise) as we see it from Earth and those galaxies will appear to move at the same speed as the core. Thanks!
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