2008ApJ...673..119B

We present mid-infrared spectroscopy obtained with the Spitzer Space Telescope of a sample of 11 optically faint, infrared luminous galaxies selected from a Spitzer MIPS 70 µm imaging survey of the NDWFS Boötes field. These are the first Spitzer IRS spectra presented of distant 70 µm-selected sources. All the galaxies lie at redshifts 0.3<z<1.3 and have very large infrared luminosities of L_IR∼0.1-17x10¹² L_☉. Seven of the galaxies exhibit strong emission features attributed to polycyclic aromatic hydrocarbons (PAHs). The average IRS spectrum of these sources is characteristic of classical starburst galaxies but with much larger infrared luminosities. The PAH luminosities of νL_ν(7.7 µm)∼0.4-7x10¹¹ L_☉ imply star formation rates of ∼40-720 M_☉/yr. Four of the galaxies show deep 9.7 µm silicate absorption features and no significant PAH emission features (6.2 µm equivalent widths <0.03 µm). The large infrared luminosities and low νf_ν(70 µm)/νf_ν(24 µm) flux density ratios suggests that these sources have AGNs as the dominant origin of their large mid-infrared luminosities, although deeply embedded but luminous starbursts cannot be ruled out. If the absorbed sources are AGN-dominated, a significant fraction of all far-infrared bright, optically faint sources may be dominated by AGNs.