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) a re redshifted in proportion to the emission-weighted mean potential along t he line of sight, amounting to approximate to 50 km s(-1) at a radius of 10 0 kpc h(-1), for a cluster dispersion of 1200 km s(-1) We show that the rel ative redshifts of different ionization states of metals in the ICM provide a unique probe of the three-dimensional matter distribution. An examinatio n of the reported peculiar velocities of cD galaxies in well-studied Abell clusters reveals that they are typically redshifted by an average of simila r to 200 km s(-1). This can be achieved by gravity with the addition of a s teep central potential associated with the cD galaxy. Note that, in general , gravitational redshifts cause a small overestimate of the recessional vel ocities of clusters by an average of similar to 20 km s(-1).