Emission-Line Galaxies from the NICMOS/Hubble Space Telescope Grism Parallel Survey

We present the first results of a survey of random fields with the slitless G141 (λc = 1.5 μm, Δλ = 0.8 μm) grism on the near-IR camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope (HST). Approximately 64 arcmin2 have been observed at intermediate and high Galactic latitudes. The 3 σ limiting line and continuum fluxes in each field vary from 7.5 × 10-17 to 1 × 10-17 ergs cm-2 s-1, and from H = 20 to 22, respectively. Our median and area-weighted 3 σ limiting line fluxes within a 4 pixel aperture are nearly identical at 4.1 × 10-17 ergs cm-2 s-1 and are 60% deeper than the deepest narrowband imaging surveys from the ground. We have identified 33 emission-line objects and derive their observed wavelengths, fluxes, and equivalent widths. We argue that the most likely line identification is Hα and that the redshift range probed is from 0.75 to 1.9. The 2 σ rest-frame equivalent width limits range from 9 to 130 Å, with an average of 40 Å. The survey probes an effective comoving volume of 105 h-350 Mpc3 for q0 = 0.5. Our derived comoving number density of emission-line galaxies in the range 0.7 < z < 1.9 is 3.3 × 10-4 h350 Mpc-3, very similar to that of the bright Lyman break objects at z ~ 3. The objects with detected emission lines have a median F160W magnitude of 20.4 (Vega scale) and a median Hα luminosity of 2.7 × 1042 ergs s-1. The implied star formation rates range from 1 to 324 M☉ yr-1, with an average [N II] λλ6583, 6548 corrected rate of 21 M☉ yr-1 for H0 = 50 km s-1 Mpc and q0 = 0.5 (34 M☉ yr-1 for q0 = 0.1).