Spitzer mid-infrared spectroscopy of 70 μm -: Selected distant luminous infrared galaxies

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 mu m imaging survey of the NDWFS Bootes field. These are the first Spitzer IRS spectra presented of distant 70 mu m-selected sources. All the galaxies lie at redshifts 0.3 < z < 1.3 and have very large infrared luminosities of L-IR similar to (0.1-17) x 10(12) L-circle dot. 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 nu L-nu(7.7 mu m) similar to (0.4 7) x 10(11) L-circle dot imply star formation rates of similar to 40-720 M-circle dot yr(-1). Four of the galaxies show deep 9.7 mu m silicate absorption features and no significant PAH emission features (6.2 mu m equivalent widths < 0.03 mu m). The large infrared luminosities and low nu f(nu)(70 mu m)/nu f(nu)(24 mu m) 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.