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, zk1 candidates in the Extragalactic First Look Survey. The sample selection criteria are f24 mk 0:9 mJy, f (24 m)/ f (8 m)k 3:16, and f (24 m)/ f (0:7 m)k10. Of the 52 spectra, 47 (90%) produced measurable redshifts based solely on the mid-IR spectral features, with 35/47 (74%) at 1:5P zP3:2. 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 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 m, continuum luminosities ( L ), ranging from 1010.3 to 1012.6 L . Our spectra have mid-IR slope 5Y15 m k2:1, 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 9:8 m > 1. Their L1600 8 and LIR 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. Subject headinggs: galaxies: evolution — galaxies: high-redshift — galaxies: starburst — infrared: galaxies