An atlas of mid-infrared spectra of high-redshift galaxies in the Spitzer First Look Survey.

We present the reduced spectra of a sample of 52 infrared luminous galaxies in the Spitzer First Look Survey obtained with the Spitzer Infrared Spectrograph. The sources have been selected to be starburst candidates in the redshift range 1-2 based on their mid-infrared colors over an area of 3.7 square degrees. Most spectra show either emission from aromatic features (PAH) or silicate absorption. Only a few galaxies show featureless strong continua and are probably dominated by nuclear emission. Based on the mid-infrared features, the inferred redshifts are in the range of 1.8 - 2.6. These data provide direct evidence that PAH molecules are present and directly observable in ultraluminous infrared galaxies at redshift 2. Since half of our sample contains sources fainter than R = 25.5 mag (Vega), our results demonstrate the potential of using mid-infrared spectroscopy to directly probe optically faint and infrared luminous populations at high redshift.