Compact Lyalpha-emitting Candidates at Z ~= 2.4 in Deep Medium-Band Hubble Space Telescope WFPC2 Images

Abstract

Medium-band imaging with Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) in the F410M filter has previously revealed a population of compact Lyalpha emission objects around the radio galaxy 53W002 at z ~= 2.4. We report detections of similar objects at z ~= 2.4 in random, high-latitude HST parallel observations of three additional fields, lending support to the idea that they constitute a widespread population at these redshifts. The three new fields contain 18 Lyalpha candidates, in contrast to the 17 detected in the deeper exposure of the single WFPC2 field around 53W002. We find substantial differences in the number of candidates from field to field, suggesting that significant large-scale structure is already present in the galaxy distribution at this cosmic epoch. The likely existence of z ~= 2.4 subgalactic clumps in several random fields shows that these objects may have been common in the early universe and strengthens the argument that such objects may be responsible for the formation of a fraction of the luminous present-day galaxies through hierarchical merging.