TheHubble Space TelescopeACS Grism Parallel Survey. II. First Results and a Catalog of Faint Emission-Line Galaxies atz ≤ 1.6

We present the first results from the HST/ACS Grism Parallel Survey, a largeprogram obtaining deep, slitless ACS grism spectroscopy of high-latitude HSTparallel fields. We report on 11 high Galactic latitude fields here, each withgrism integration times >12 ks. We identify 601 compact emission line galaxiesat z < 1.6, reaching emission lines to a flux limit of > 5 E-18 ergs/cm^2/s (3sigma). We determine redshifts by cross correlation of the target spectra withtemplate spectra, followed by visual inspection. We measure star formationrates from the observed [OII] 3727, [OIII] 5007 and Halpha line fluxes.Follow-up observations with the Keck telescope of one of the survey fieldsconfirms our classification and redshifts with sigma(z)~0.02. This is one ofthe deepest emission line surveys to date, covering a total area of 121arcmin^2. The rough estimate of the co-moving number density of emission-linegalaxies in our survey at 0.3 < z < 1.3 is ~4.5 E-3 h^{-3}_70 Mpc^{-3}. Wereach deeper into the emission-line luminosity function than either the STIS orNICMOS grism parallel surveys, finding an apparent space density of emissionline galaxies several times higher than those surveys. Because of the ACS highspatial resolution, our survey is very sensitive to faint, compact galaxieswith strong emission lines and weak continua. The ACS grism survey provides theco-moving star formation density at z < 1.6 at a high level of completeness.