Böhringer, H, Chon, G, Bristow, M and Collins, CA (2015) The extended ROSAT-ESO flux-limited X-ray galaxy cluster survey (REFLEX II): V. Exploring a local underdensity in the southern sky. Astronomy and Astrophysics, 574. pp. 1-5. ISSN 0004-6361

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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, in particular, provided new support for a local underdensity in the galaxy distribution out to distances of 200-300 Mpc. If confirmed, such local underdensities would have important implications interpreting local measurements of cosmological parameters. Galaxy clusters have been shown to be ideal probes for tracing 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 normalised 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 greater than 3.4σ. 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. Cosmological models that attempt to interpret the cosmic acceleration, deduced from observations of type Ia supernovae, by a large local void without the need for reacceleration, require that we are located close to the centre of a roughly spherical void with a minimum size of ∼300 Mpc. In contrast our results show that the local underdensity is not isotropic and limited to a size significantly smaller than 300 Mpc radius.

Item Type:	Article
Uncontrolled Keywords:	0201 Astronomical And Space Sciences
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Publisher:	EDP Sciences
Date Deposited:	13 Feb 2015 12:45
Last Modified:	04 Sep 2021 14:39
DOI or ID number:	10.1051/0004-6361/201424817
URI:	https://researchonline.ljmu.ac.uk/id/eprint/492

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