Detection of a galaxy cluster at z=2.4

Abstract

We have detected a group of galaxies at z=2.38. The group lies near a triple system of damped Lyman-alpha absorption-lines in a background QSO, and is part of a large (> 3Mpc) cluster of damped systems (Francis & Hewett 1993, AJ 105, 1633). Two of the galaxies in the group were detected using Fabry-Perot narrow band imaging and broad band imaging at the CTIO 4-m telescope. Both have been spectroscopically confirmed. An additional 10 IR-bright objects cluster around these two; from their IR/optical colours most have to be galaxies at the same redshift. The group has a diameter of ~ 80kpc, a velocity dispersion of ~ 500 kms(-1) and a mass of ~ 10(12) Solar masses; it may represent a giant elliptical galaxy in the act of formation. One galaxy in the group shows an extended tail of Lyman-alpha emission.