Astronomy and Astrophysics, volume 574A, 26-26 (2015/2-1)
The extended ROSAT-ESO Flux-Limited X-ray Galaxy Cluster Survey (REFLEX II). V. Exploring a local underdensity in the southern sky.
BOEHRINGER H., CHON G., BRISTOW M. and COLLINS C.A.
Abstract (from CDS):
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 (∼170Mpc) in the southern extragalactic sky with a significance greater than 3.4σ. On larger scales from 300Mpc to over 1Gpc, 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 ∼300Mpc. In contrast our results show that the local underdensity is not isotropic and limited to a size significantly smaller than 300Mpc radius.
galaxies: clusters: general - X-rays: galaxies: clusters - cosmology: observations - large-scale structure of Universe