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 (λ_c = 1.5 μm, ΔλA = 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] λλ16583, 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).