Serendipitously Detected Galaxies in the Hubble Deep Field1

We present a catalog of 74 galaxies detected serendipitously during a campaign of spectroscopic observations of the Hubble Deep Field North (HDF) and its environs. Among the identiÐed objects are Ðve candidate Lya emitters at a galaxy cluster at z\ 0.85, and a Chandra source with a heretofore z Z 5, undetermined redshift of z\ 2.011. We report redshifts for 25 galaxies in the central HDF, 13 of which had no prior published spectroscopic redshift. Of the remaining 49 galaxies, 30 are located in the singleorbit HDF Ñanking Ðelds. We discuss the redshift distribution of the serendipitous sample, which contains galaxies in the range 0.10\ z \ 5.77 with a median redshift of z\ 0.85, and we present strong evidence for redshift clustering. By comparing our spectroscopic redshifts with optical/IR photometric studies of the HDF, we Ðnd that photometric redshifts are in most cases capable of producing reasonable predictions of galaxy redshifts. Finally, we estimate the line-of-sight velocity dispersion and the corresponding mass and expected X-ray luminosity of the galaxy cluster, we present strong arguments for interpreting the Chandra source as an obscured active galactic nucleus, and we discuss in detail the spectrum of one of the candidate Lya emitters. z Z 5