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

H. Boehringer, Gayoung Chon+2 more|arXiv (Cornell University)|Oct 8, 2014

Galaxies: Formation, Evolution, Phenomena47 references22 citations

TL;DR

This study uses the REFLEX II X-ray galaxy cluster survey to map large-scale structure in the southern sky, finding a 30–40% underdensity in cluster density at redshifts z = 0 to 0.04, with significance >3.4σ. The underdensity is localized in the South Galactic Cap and not isotropic, challenging models requiring a large, spherical void to explain cosmic acceleration without dark energy.

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.

Motivation & Objective

• To assess the large-scale density distribution of galaxy clusters in the southern extragalactic sky using the REFLEX II survey.
• To test whether a local underdensity in the cluster distribution supports alternative cosmological models that explain cosmic acceleration via a large void instead of dark energy.
• To compare cluster density with galaxy density from K-band surveys to determine if clusters and galaxies trace the same large-scale structure.
• To quantify the size, significance, and spatial extent of any local underdensity and assess its implications for cosmological parameter estimation.
• To evaluate whether the observed underdensity is consistent with the requirements of minimum void models proposed to explain supernova data without dark energy.

Proposed method

• Utilized the flux-limited X-ray galaxy cluster sample from the extended ROSAT-ESO Flux-Limited X-ray Galaxy Cluster Survey (REFLEX II), covering the southern extragalactic sky.
• Measured the normalized comoving number density of clusters in redshift bins up to z ≈ 0.3 to map large-scale structure and identify deviations from homogeneity.
• Calculated significance levels for underdensities using statistical tests, with a significance threshold of >3.4σ for robust detection.
• Applied cluster biasing models (with bias factors of 2.5–3) to infer the underlying matter density deficit from the observed cluster underdensity.
• Compared the cluster density distribution with K-band galaxy data from Whitbourn & Shanks (2014) and other surveys to assess cross-tracing of large-scale structure.
• Analyzed spatial distribution across Galactic Caps (North vs. South) to determine anisotropy of the underdensity and assess directional dependence.

Experimental results

Research questions

• RQ1 Is there a significant local underdensity in the distribution of X-ray-detected galaxy clusters in the southern sky?
• RQ2 Does the observed underdensity in the cluster distribution imply a corresponding underdensity in the underlying matter distribution, and how large is it?
• RQ3 Is the underdensity isotropic or confined to a specific region, such as the South Galactic Cap?
• RQ4 Do galaxy clusters and K-band galaxies trace the same large-scale density fluctuations in the southern sky?
• RQ5 Can the observed underdensity be reconciled with cosmological void models that aim to explain cosmic acceleration without dark energy?

Key findings

• A significant underdensity of 30–40% in the comoving number density of X-ray-detected galaxy clusters is found in the redshift range z = 0 to 0.04 in the southern extragalactic sky, with significance >3.4σ.
• The underdensity is strongly localized in the South Galactic Cap and not observed in the North Galactic Cap, indicating anisotropy in the large-scale structure.
• The cluster underdensity implies a corresponding underdensity of ~15% in the underlying matter distribution, after accounting for cluster bias (b ≈ 2.5–3).
• The local Hubble constant is inferred to be ~3% higher than the cosmic mean in this region, assuming ΛCDM cosmology.
• The observed underdensity is too small and anisotropic to support minimum void models that require a spherical void of radius ~300 Mpc to explain cosmic acceleration.
• Galaxy and cluster distributions in the southern sky trace the same density fluctuations, with both surveys showing a ~40% underdensity in the South Galactic Cap and no significant deficit in the North Galactic Cap.