Gravitational redshifts of clusters and voids

Abstract

Gravitational redshift as a relativistic effect in cosmological objects is investigated. Possible signatures of the gravitational redshift in measurements of satellite galaxies in clusters of galaxies, intracluster gas, as well as galaxies associated with voids are investigated by developing simple theoretical models. In the analysis of the gravitational redshift of satellite galaxies, we develop a very simple analytic model for satellite galaxies virialised in halos, which enables us to evaluate the signals depending on the properties of the halo occupation distribution of galaxies. We obtain results consistent with recent previous results, though our results are restricted to the satellite galaxies inside the virial radius. In the analysis of intracluster gas, we develop a simple analytic model including the effect of random motions of gases, which are assumed to generate nonthermal pressure. We demonstrate a possible contribution of the random motions of gases to the gravitational redshift. We also investigate a possible signature of the gravitational redshift in measurements of galaxies associated with voids, for the first time as far as we know, by utilising a simple analytic model. We show that the second-order Hubble term, which appears in the expansion of the scale factor around the centre of a void, may make a significant contribution depending on the way the galaxy samples are analysed.