2005ApJ...622L.105H

We have surveyed a field covering 9.0 deg² within the NOAO Deep Wide-Field Survey region in Bootes with the Multiband Imaging Photometer on the Spitzer Space Telescope (SST) to a limiting 24 µm flux density of 0.3 mJy. Thirty-one sources from this survey with F_24µm>0.75 mJy that are optically very faint (R≳24.5 mag) have been observed with the low-resolution modules of the Infrared Spectrograph on SST (IRS). Redshifts derived primarily from strong silicate absorption features are reported here for 17 of these sources; 10 of these are optically invisible (R≳26 mag), with no counterpart in B_W, R, or I. The observed redshifts for 16 sources are 1.7<z<2.8. These represent a newly discovered population of highly obscured sources at high redshift with extreme infrared-to-optical ratios. Using IRS spectra of local galaxies as templates, we find that a majority of the sources have mid-infrared spectral shapes most similar to ultraluminous infrared galaxies powered primarily by active galactic nuclei. Assuming that the same templates also apply at longer wavelengths, bolometric luminosities exceed 10¹³ L_☉.