A weak lensing survey in the fields of z 1 luminous radio sources

Abstract

In this paper we present weak lensing observations of the fields around eight z ∼ 1 luminous radio sources. These data are used to search for the lensing signatures of mass structures that either are physically associated with the radio objects, or are foreground systems projected along the line of sight. The radio sources were all the subjects of deep imaging with WFPC-2 on the Hubble Space Telescope, providing high-quality shape information on large numbers of faint galaxies around these sources. Statistical analysis of the coherent shear field visible in the shapes of the faint galaxies indicates that we have detected a weak lensing signal close to one of the targets, 3C 336 at z = 0.927, with a high level of confidence. A second, independent WFPC-2 observation of this target reinforces this detection. Our results support the earlier suggestion of weak lensing in this field by Fort et al. using ground-based data. Using limits on the likely redshift distribution of the faint galaxies used m our analysis, we convert our observed lensing signal into an estimate of the mass of the cluster around 3C 336. We also combined the shear distributions in the remaining seven fields to improve our sensitivity to weak shear signals from any structure typically associated with these sources. We find no detectable signal, and estimate an upper limit on the maximum shear allowed by our observations. We conclude that while some z ∼ 1 luminous radio sources are found in the central regions of rich clusters, the majority do not lie within the central regions of massive collapsed structures at these epochs. The upper limit on the mass of any structure around a typical radio source is similar to that expected of a group or poor cluster. Our data also limit the lensing contribution of foreground structures, although this is sensitive to the assumed offset between the dominant source of shear and the line of sight to the radio source. We suggest that further lensing observations of distant radio sources and their host environments may allow the cluster Lx-mass relationship to be mapped at high z. This is crucial for interpreting the results of the next generation of deep X-ray surveys, and thus for providing a strong observational constraint on the redshift evolution of the cluster mass function out to z = 1.