I don't understand how astronomers estimate the redsfhit of a cluster. As far as I understand a cluster of galaxies is something really "big", so I expect that different galaxies in the cluster have different redshifts.

The redshift of the cluster is some average of the redshift of every galaxy in the cluster?


Yes, it is the average redshift of galaxies that belong to a cluster.

There is of course an uncertainty in that, but a typical velocity dispersion among galaxies in a massive galaxy cluster is 1000 km/s. So the redshift error due to the uncertainty in the mean is $$\Delta z \sim \frac{\Delta v}{c} = \frac{\sigma_v}{c\sqrt{n}}= \frac{0.0033}{\sqrt{n}},$$ where $n$ is the number of galaxies in the cluster with a measured redshift.

  • $\begingroup$ I can't see why $0.0033$ appears in your equation. $\endgroup$ – Nothing Jun 24 '20 at 22:16
  • 1
    $\begingroup$ $\Delta z\sim \Delta v/c$ $\endgroup$ – ProfRob Jun 24 '20 at 22:35

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