Lensing by Distant Clusters: HST Observations of Weak Shear in the Field of 3C 324

Abstract

We present the detection of weak gravitational lensing in the field of the radio galaxy 3C 324 (z = 1.206) using deep HST imaging. From an analysis of the shapes of faint R = 24.5--27.5 galaxies in the field we measure a weak, coherent distortion centered close to the radio source. This shear field most likely arises from gravitational lensing of distant field galaxies by a foreground mass concentration. In the light of previous observations of this region, which indicate the presence of a rich cluster around the radio source, we suggest that the most likely candidate for the lens is the cluster associated with the radio galaxy at z = 1.2. If so, this is the most distant cluster to have been detected by weak shear observations. Such a statement has two important consequences. Firstly, it shows that massive, collapsed structures exist in the high-redshift universe, and secondly that a significant fraction of the R = 24.5--27.5 field galaxy population lies beyond z ~ 1.2.