The Mass Distribution of CL0939+4713 obtained from a `Weak' Lensing Analysis of a WFPC2 image

TL;DR

This paper uses weak gravitational lensing of high-redshift galaxies observed by HST to map the mass distribution of galaxy cluster CL0939+4713, demonstrating that mass correlates with light and providing mass estimates despite observational challenges.

Contribution

It applies an unbiased, parameter-free mass reconstruction technique to a WFPC2 image, showing that even small fields can yield reliable cluster mass maps and analyzing the mass-light relationship.

Findings

Mass distribution correlates with bright galaxies.

Estimated cluster mass is approximately 5-7 x 10^14 solar masses.

Mass-to-light ratio is around 100-140 h_{50}.

Abstract

The image distortions of high-redshift galaxies caused by gravitational light deflection of foreground clusters of galaxies can be used to reconstruct the two-dimensional surface mass density of these clusters. We apply an unbiased parameter-free reconstruction technique to the cluster CL0939+4713 (Abell 851), observed with the WFPC2 on board of the HST. We demonstrate that a single deep WFPC2 observation can be used for cluster mass reconstruction despite its small field of view and the irregular shape of the data field (especially for distant clusters). For CL0939, we find a strong correlation between the reconstructed mass distribution and the bright cluster galaxies indicating that mass follows light on average. The detected anti-correlation between the faint galaxies and the reconstructed mass is most likely an effect of the magnification (anti) bias, which was detected previously in the cluster A1689. Because of the high redshift of CL0939 (z_d=0.41), the redshift distribution of the lensed, faint galaxies has to be accounted for in the reconstruction technique. We derive an approximate global transformation for the surface mass density which leaves the mean image ellipticities invariant, resulting in an uncertainty in the normalization of the mass. From the non-negativity of the surface mass density, we derive lower limits on the mass inside the observed field. We can break the invariance transformation for the mass using the magnification effect on the observed number density of the background galaxies. Assuming a mean redshift of 0.8 and a fraction of 15 % (20 %) of cluster galaxies in the observed galaxy sample with 23<R<25.5 we obtain for the mass inside the field approximately 5 (7) x 10**{14}/ h_{50} solar masses which corresponds to M/L ~ 100 (140) h_{50}.