Emission-line galaxies from the NICMOS/<i>Hubble Space Telescope</i> grism parallel survey

We present the first results of a survey of random fields with the slitless G141 (lambda(c) = 1.5 mu m, Delta lambda = 0.8 mu m) grism on the near-IR camera and multiobject spectrometer (NICMOS) on board the Hubble Space Telescope (HST). Approximately 64 arcmin(2) have been observed at intermediate and high Galactic latitudes. The 3 sigma limiting line and continuum fluxes in each field vary from 7.5 x 10(-17) to 1 x 10-17 ergs cm(-2) s(-1) and from H = 20 to 22, respectively. Our median and area-weighted 3 sigma limiting line fluxes within a 4 pixel aperture are nearly identical at 4.1 x 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 Ho! and that the redshift range probed is from 0.75 to 1.9. The 2 sigma rest-frame equivalent width Limits range from 9 to 130 Angstrom, with an average of 40 Angstrom. The survey probes an effective comoving volume of 10(5) h(50)(-3) Mpc(3) for q(0) = 0.5. Our derived comoving number density of emission-line galaxies in the range 0.7 < z < 1.9 is 3.3 x 10(-4) h(50)(3) Mpc(-3), very similar to that of the bright Lyman break objects at z similar to 3. The objects with detected emission lines have a median F160W magnitude of 20.4 (Vega scale) and a median Ka luminosity of 2.7 x 10(42) ergs s(-1). The implied star formation rates range from 1 to 324 M-circle dot yr(-1), with an average [N II] lambda lambda 6583, 6548 corrected rate of 21 M-circle dot yr(-1) for H-0 = 50 km s(-1) Mpc and q(0) = 0.5 (34 M-circle dot yr(-1) for q(0) = 0.1).