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 ≳ 1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f_{24 μm} ≳ 0.9 mJy, νf_ν(24 μm)/νf_ν(8 μm) ≳ 3.16, and νf_ν(24 μm)/νf_ν(0.7 μm) ≳ 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 ≲ z ≲ 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 μm, 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 μm, continuum luminosities (νL_ν), ranging from 10^10.3 to 10^12.6 L_☉. Our spectra have mid-IR slope α_{5-15 μm} ≳ 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 τ_{9.8 μm} > 1. Their L_{1600 Å} 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 ~ 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.