Gravitational Redshifts in Clusters and Voids

TL;DR

This paper investigates gravitational redshift effects in cosmological structures like clusters, voids, and intracluster gas using simple theoretical models, revealing potential observational signatures and contributions from gas motions and void expansion.

Contribution

It develops simple analytic models for gravitational redshift in satellite galaxies, intracluster gas, and voids, including effects of gas motions and the Hubble expansion, providing new insights into observable signatures.

Findings

Satellite galaxy redshifts depend on halo occupation properties.

Gas motions contribute to gravitational redshift signals.

Void galaxy measurements may be affected by second-order Hubble terms.

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.