Gravitational redshifting of galaxies in the SPIDERS cluster catalogue

Abstract

ABSTRACT Data from the SPectroscopic IDentification of ERosita Sources (SPIDERS) are searched for a detection of the gravitational redshifting of light from ${\sim}20\, 000$ galaxies in ∼2500 galaxy clusters using three definitions of the cluster centre: its Brightest Cluster Galaxy (BCG), the redMaPPer identified Central Galaxy (CG), or the peak of X-ray emission. Distributions of velocity offsets between galaxies and their host cluster’s centre, found using observed redshifts, are created. The quantity $\hat{\Delta }$, the average of the radial velocity difference between the cluster members and the cluster systemic velocity, reveals information on the size of a combination of effects on the observed redshift, dominated by gravitational redshifting. The change of $\hat{\Delta }$ with radial distance is predicted for SPIDERS galaxies in General Relativity (GR), and f(R) gravity, and compared to the observations. The values of $\hat{\Delta }=-13.5\pm 4.7$ km s−1, $\hat{\Delta }=-12.5\pm 5.1$ km s−1, and $\hat{\Delta }=-18.6\pm 4.8$ km s−1 for the BCG, X-ray, and CG cases, respectively, broadly agree with the literature. There is no significant preference of one gravity theory over another, but all cases give a clear detection (>2.5σ) of $\hat{\Delta }$. The BCG centroid is deemed to be the most robust method in this analysis, due to no well-defined central redshift when using an X-ray centroid, and CGs identified by redMaPPer with no associated spectroscopic redshift. For future gravitational redshift studies, an order-of-magnitude more galaxies, ${\sim}500\, 000$, will be required – a possible feat with the forthcoming Vera C. Rubin Observatory, Euclid and eROSITA.