Emission-Line Galaxies in the Space Telescope Imaging Spectrograph Parallel Survey. I. Observations and Data Analysis

In the first 3 years of operation the Space Telescope Imaging Spectrograph (STIS) obtained slitless spectra of ~2500 fields in parallel to prime Hubble Space Telescope (HST) observations as part of the STIS parallel survey (SPS). The archive contains ~300 fields at high Galactic latitude ( > 30°) with spectroscopic exposure times greater than 3000 s. This sample contains 219 fields (excluding special regions and requiring a consistent grating angle) observed between 1997 June 6 and 2000 September 21, with a total survey area of ~160 arcmin2. At this depth, the SPS detects an average of one emission-line galaxy per three fields. We present the analysis of these data and the identification of 131 low- to intermediate-redshift galaxies detected by optical emission lines. The sample contains 78 objects with emission lines that we infer to be redshifted [O II] λ3727 emission at 0.43 < z < 1.7. The comoving number density of these objects is comparable to that of Hα-emitting galaxies in the NICMOS parallel observations. One quasar and three probable Seyfert galaxies are detected. Many of the emission-line objects show morphologies suggestive of mergers or interactions. The reduced data are available upon request from the authors.