The Serendipitous Discovery of a Group or Cluster of Young Galaxies at [ITAL]z[/ITAL] ≃ 2.40 in Deep [ITAL]Hubble Space Telescope[/ITAL] WFPC2 Images

We report the serendipitous discovery of a group or cluster of young galaxies at z ≃ 2.40 in a 24 orbit Hubble Space Telescope /WFPC2 exposure of the field around the weak radio galaxy 53W002. Potential cluster members were identified on ground-based narrowband redshifted Lyα images and confirmed via spectroscopy. In addition to the known weak radio galaxy 53W002 at z = 2.390, two other objects were found to have excess narrowband Lyα emission at z ≃ 2.40. Both have been spectroscopically confirmed, and one clearly contains a weak active galactic nucleus (AGN). They are located within 1' of 53W002 or ~0.23 h 100 −1 Mpc ( q 0 = 0.5) at z ≃ 2.40, which is the physical scale of a group or small cluster of galaxies. Profile fitting of the WFPC2 images shows that the objects are very compact, with scale lengths ≃ 0 1 (≃0.39 h 100 −1 kpc), and are rather faint (luminosities < L *), implying that they may be subgalactic-sized objects. We discuss these results in the context of galaxy and cluster evolution and the role that weak AGNs may play in the formation of young galaxies.