Volume 574, February 2015
|Number of page(s)||8|
|Section||Cosmology (including clusters of galaxies)|
|Published online||20 January 2015|
The extended ROSAT-ESO Flux-Limited X-ray Galaxy Cluster Survey (REFLEX II)
V. Exploring a local underdensity in the southern sky⋆
Max-Planck-Institut für extraterrestrische Physik,
2 Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
Received: 15 August 2014
Accepted: 8 October 2014
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.
Key words: galaxies: clusters: general / X-rays: galaxies: clusters / cosmology: observations / large-scale structure of Universe
© ESO, 2015
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