Spitzer mid-infrared spectroscopy of infrared luminous galaxies at z ∼ 2.: I.: The spectra

We present the mid-infrared spectra obtained with the Spitzer IRS for 52 sources, selected as infrared luminous, z greater than or similar to 1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f(24 mu m) greater than or similar to 0.9mJy, v(fv)(24 mu m)/ vf(v) (8 mu m) greater than or similar to 3.16, and vf(v) (24 mu m)/vfv (0.7 mu m) greater than or similar to 10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1: 5 less than or similar to z less than or similar to 3.2. Keck spectroscopy of a subsample (17/47) agrees with the mid-IR redshift measurements. The observed spectra fall into three categories: (1) 33% (17/52) have strong PAH emission and are probably powered by star formation with total IR luminosity roughly a factor of 5 higher than the local starburst ULIRGs; (2) 33% (17/52) have only deep silicate absorption at 9.8 mu m, indicatiing deeply embedded dusty systems-these data alone cannot determine the energetic nature of the heating sources in these systems; and (3) the remaining 34% are mid-IR continuum-dominated systems with weak PAH emission and/or silicate absorption. This subsample is probably AGNs. We derived monochromatic, rest-frame 5.8 mu m, continuum luminosities (vL(v)), ranging from 10(10.3) to 10(12.6) L-circle dot. Our spectra have mid-IR slope alpha(5-15 mu m) greater than or similar to 2.1, much redder than the median value of 1.3 for the optically selected PG quasars. From the silicate absorption feature, we estimate that roughly two-thirds of the sample have optical depth tau 9.8 mu m > 1. Their L-1600 (angstrom) and L-IR suggest that our sample is among the most luminous and most dust-enshrouded systems of its epoch. Our study has revealed a significant population of dust-enshrouded galaxies at z similar to 2, whose enormous energy output, comparable to that of quasars, is generated by AGNs, as well as starbursts. This IR-luminous population has very little overlap with submillimeter and UV-selected populations.