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We present evidence from ground-based images and spectroscopy for the existence of a young group or cluster of galaxies at z=2.40. These objects were found in a 24-orbit HST/WFPC2 exposure of the high-redshift weak radio galaxy 53W002 at z=2.390 and its surroundings (see parallel poster by Windhorst and Keel). Possible cluster members were identified from a deep Steward Observatory 90-inch exposure taken through a specially designed narrow-band (150\AA\ FWHM, $\lambda_{cent}=4130$\AA) filter to detect objects emitting significant redshifted Ly$\alpha$ at z=2.40. We also determine their Ly$\alpha$--B and UBVRIgri colors from deep Steward 90-inch and Palomar 200-inch images.
In addition to the known weak radio galaxy 53W002 at z=2.390, three other galaxies were found to have excess Ly$\alpha$ emission on the (Ly$\alpha$--B) Steward images. One of these was also spectroscopically confirmed recently with the MMT Blue Spectrograph. All three are very blue and compact objects located within one arcminute of 53W002 on the WFPC2 images, or $\sim$0.5 Mpc for $H_{o}$=50 and $q_{o}$=0.5 -- the physical scale of a group or small cluster of galaxies. Profile fitting of the WFPC2 images shows that two of the z=2.40 objects have a weak AGN embedded in an exponential disk. 53W002 itself has a weak AGN (containing $\sim$20\% of the total light) and a rather regular $r^{1/4}$-law profile. The other non-radio emitting object at z=2.40 is a possible merger. We discuss these results in the context of galaxy and cluster evolution, and the role that weak AGN may play in the formation of young galaxies.
This work was supported by NASA/HST grant GO-5308-01-94A from STScI, which is operated by AURA, Inc., under NASA contract NAS5-26555.
