Weak lensing magnification in SpARCS

Abstract

The growth of large scale structure is one of the fundamental predictions of any cosmological model. Galaxy clusters are the highest peaks in the cosmological matter density field and therefore of prime importance in cosmology. The calibration of the high-redshift (z > 1) galaxy cluster mass-richness relation is particularly important as it contains information about galaxy clusters in their assembly phase, when assumptions such as virial/hydrostatic equilibrium might not be valid. Measuring the mass-richness relation over a wide range in redshift will help to better understand the astrophysics of clusters over time and simultaneously provide cosmological structure growth constraints. <br /> An independent and novel method to acquire this information is the use of the weak gravitational lensing magnification effect, which is able to accurately measure the masses of large samples of high-z clusters in a statistical way (i.e. through stacking) without the need to resolve background galaxies. This magnification effect leads to a change of source counts which then can be analysed by measuring the angular cross-correlation function of optically selected Lyman-break galaxies and highredshift clusters. We apply this method to the hundreds of new high-z galaxy clusters found in the SpARCS (Spitzer Adaptation of the Red-Sequence Cluster Survey) infrared survey, observed also with the CFHT in the optical ugrz-bands. <br /> We measure the cross-correlation between the positions of galaxy cluster candidates and LBGs and detect a weak lensing magnification signal for all bins at a detection significance of 2.6-5.5 sigma. In particular, the significance of the measurement for clusters with z > 1 is 4.1 sigma; for the entire cluster sample we obtain an average M200 of 1.28 +0.23/-0.21 10^14 solar masses. <br /> Our measurements demonstrate the feasibility of using weak lensing magnification as a viable tool for determining the average halo masses for samples of high redshift galaxy clusters. The results also establish the success of using galaxy over-densities to select massive clusters at z > 1. Additional studies are necessary for further modelling of the various systematic effects we discussed.