Detecting the Gravitational Redshift of Cluster Gas.

We examine the gravitational redshift of radiation emitted from within the potential of a cluster. Spectral lines from the intracluster medium (ICM) are redshifted in proportion to the emission-weighted mean potential along the line of sight, amounting to approximately 50 km s-1 at a radius of 100 kpc h-1, for a cluster dispersion of 1200 km s-1. We show that the relative redshifts of different ionization states of metals in the ICM provide a unique probe of the three-dimensional matter distribution. An examination of the reported peculiar velocities of cD galaxies in well-studied Abell clusters reveals that they are typically redshifted by an average of approximately 200 km s-1. This can be achieved by gravity with the addition of a steep central potential associated with the cD galaxy. Note that, in general, gravitational redshifts cause a small overestimate of the recessional velocities of clusters by an average of approximately 20 km s-1.