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 galaxiesat $z\simeq$2.40 in a 24-orbit HST/WFPC2 exposure of the field around the weakradio galaxy 53W002. Potential cluster members were identified on ground-basednarrow-band redshifted Ly$\alpha$ images and confirmed via spectroscopy. In addition to the known weak radio galaxy 53W002 at z=2.390, two otherobjects were found to have excess narrow-band Ly$\alpha$ emission at$z\simeq$2.40. Both have been spectroscopically confirmed, and one clearlycontains a weak AGN. They are located within one arcminute of 53W002, or$\sim0.23h_{100}^{-1}$Mpc ($q_o$=0.5) at $z\simeq$2.40, which is the physicalscale of a group or small cluster of galaxies. Profile fitting of the WFPC2images shows that the objects are very compact, with scale lengths$\simeq$0\farcs 1 ($\simeq0.39h_{100}^{-1}$kpc), and are rather faint(luminosities < L*), implying that they may be sub-galactic sized objects. Wediscuss these results in the context of galaxy and cluster evolution and therole that weak AGN may play in the formation of young galaxies.