Emission-Line Galaxies from the NICMOS/HST GRISM Parallel Survey

Abstract

We present the first results of a survey of random fields with the slitless G141 ($\hat{I}{»}_c=1.5\hat{I}¼,\hat{I}”\hat{I}»=0.8\hat{I}¼$) grism on NICMOS. Approximately 64 square arcminutes have been observed at intermediate and high galactic latitudes. The 3$\ddot{I}ƒ$ limiting line and continuum fluxes in each field vary from $7.5\times 10^{-17}$ to $1\times 10^{-17}erg/c{m}^2/sec$ and from H = 20 to 22, respectively. Our median and area weighted $3\ddot{I}ƒ$ limiting line fluxes within a 4 pixel aperture are nearly identical at $4.1\times 10^{-17}erg/c{m}^2/sec$ and are 60% deeper than the deepest narrow-band 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 H$\hat{I}\pm$ and that the redshift range probed is from 0.75 to 1.9. The 2$\ddot{I}ƒ$ rest-frame equivalent width limits range from 9à to 130à with an average of 40à . The survey probes an effective co-moving volume of $10^5{h}_{50}^{-3}Mp{c}^3$ for $q_0=0.5$. Our derived co-moving number density of emission line galaxies in the range $0.7<z<1.9$ is $3.3\times 10^{-4}{h}_{50}^{3}Mp{c}^{-3}$, very similar to that of the bright Lyman break objects at $z\sim 3$. The objects with detected emission-lines have a median F160W magnitude of 20.4 (Vega scale) and a median H$\hat{I}\pm$ luminosity of $2.7\times 10^{42}erg/sec$. The implied star formation rates range from 1 to 324 M_{\odot}/yr, with an average [NII]6583,6548 corrected rate of 21 M_{\odot}/yr for H_0=50 km/s/Mpc and $q_0=0.5$ (34 M_{\odot}/yr for $q_0=0.1$).

32 pages, including 6 figures and 4 tables. Accepted for publication in the Astrophysical Journal