H-ATLAS/GAMA: Magnification Bias Tomography I. Astrophysical constraints.

Abstract

This work is an update the cross-correlation measurements made by Gonzalez-Nuevo et al. (2014) with better catalogues and wider area in order to split them in different redshift bins. We implement a HOD model to extract astrophysical information about the over-densities that are producing the lensing link between the foreground (lenses) and background (sources) samples. The HOD modelisation allow us to identify a strong lensing contribution to the cross-correlation for angular scales below 30 arcsecs. We need yet to model it to discover what can we learn from it.

Introduction

Light rays coming from a distant source are deflected by the foreground gravitational field. This on the one hand stretches the area of a given sky region, thus decreasing the surface density of sources and, on the other hand, magnifies the background sources, increasing their chances of being included in a flux-limited sample. The net effect, termed magnification bias, is extensively described in the literature (see, e.g., Schneider, Ehlers & Falco 1992). It implies that an excess/decrease of background sources from a flux-limited sample is found in the vicinity of matter overdensities (Bartelmann & Schneider 1993; Moessner & Jain 1998; Scranton et al. 2005). The amplitude of the excess increases with the slope of the background source number counts. Thus gravitational lensing constitutes a direct probe of the cosmic gravitational fields. The most dramatic manifestations of lensing, called “strong lensing”, which include multiple images, arcs or “Einstein rings”, are, however, rare. These manifestations show up on angular scales of arcseconds and provide information on high density structures such as galaxies or galaxy clusters. The lower density structures, which include most of the mass in the Universe, nevertheless, can still produce observable effects via “weak lensing”. The magnification bias due to weak lensing modifies the galaxy angular correlation function because the observed images do not coincide with true source locations (Gunn 1967; Kaiser 1992; Moessner, Jain & Villumsen 1998; Loverde, Hui & Gaztañaga 2008), but the effect is generally small and difficult to single out. An unambiguous manifestation of weak lensing is the cross-correlation between two source samples with non-overlapping redshift distributions. The occurrence of such correlations has been tested and established in several contexts (see, e.g. Scranton et al. 2005; M ̵́enard et al. 2010; Hildebrandt et al. 2013; Bartelmann & Schneider 2001, and references therein).