The Coma Cluster

The Coma cluster is a rich cluster of galaxies; the systemic redshift of the cluster is about 7000 km/s.

The image below covers the inner 20 arcminutes of the cluster. It was taken with the 0.9m telescope at the Kitt Peak National Observatory using an I band filter (effective wavelength about 8600 Angstroms).

The image below covers the inner one degree of the cluster.

The diagram to the right plots the measured recessional velocity against the distance from the cluster center over the inner two degrees. Notice that the dispersion in velocities is greater in the central regions of the cluster.

Using the velocity information, we can calculate the virial mass of the Coma cluster. From observations of the number density of galaxies with radial distance from the center, we can calculate the luminous mass. The difference between the two would give an estimate of the dark matter content.

In doing the problem more exactly, we might take into account the individual masses of the galaxies and the fact that we only observe a certain fraction of the total galaxy population.

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