The extended ROSAT-ESO Flux-Limited X-ray Galaxy Cluster Survey (REFLEX II) V. Exploring a local underdensity in the Southern Sky

TL;DR

This study uses X-ray galaxy cluster data from the REFLEX II survey to identify a significant local underdensity in the southern sky up to 170 Mpc, which is not large enough to explain cosmic acceleration without dark energy.

Contribution

It provides the first detailed analysis of local density variations using X-ray clusters, revealing a significant southern underdensity inconsistent with large void models.

Findings

Detected a 30-40% underdensity in the southern sky out to 170 Mpc.

Found the density distribution to be homogeneous beyond 300 Mpc.

Observed the underdensity is confined mainly to the South Galactic Cap.

Abstract

Several claims have been made that we are located in a locally underdense region of the Universe based on observations of supernovae and galaxy density distributions. Two recent studies of K-band galaxy surveys have provided new support for a local underdensity in the galaxy distribution out to distances of 200 - 300 Mpc. If confirmed, such large local underdensities would have important implications on the interpretation of local measurements of cosmological parameters. Galaxy clusters have been shown to be ideal probes to trace the large-scale structure of the Universe. In this paper we study the local density distribution in the southern sky with the X-ray detected galaxy clusters from the REFLEX II cluster survey. From the normalized comoving number density of clusters we find an average underdensity of ~30 - 40% in the redshift range out to z ~0.04 (~170 Mpc) in the southern extragalactic sky with a significance larger than 3.4sigma. On larger scales from 300 Mpc to over 1 Gpc the density distribution appears remarkably homogeneous. The local underdensity seems to be dominated by the South Galactic Cap region. A comparison of the cluster distribution with that of galaxies in the K-band from a recent study shows that galaxies and clusters trace each other very closely in density. In the South Galactic Cap region both surveys find a local underdensity in the redshift range z= 0 to 0.05 and no significant underdensity in the North Galactic Cap at southern latitudes. Our results to not support cosmological models that attempt to interpret the cosmic acceleration by a large local void, since the local underdensity we find is not isotropic and limited to a size significantly smaller than 300 Mpc radius.