SpitzerIRS Spectra of Optically Faint Infrared Sources with Weak Spectral Features

Spectra have been obtained with the low-resolution modules of the InfraredSpectrograph (IRS) on the Spitzer Space Telescope (Spitzer) for 58 sourceshaving f$_{\nu}$(24 micron) > 0.75 mJy. Sources were chosen from a survey of8.2 deg$^{2}$ within the NOAO Deep Wide-Field Survey region in Bootes (NDWFS)using the Multiband Imaging Photometer (MIPS) on the Spitzer Space Telescope.Most sources are optically very faint (I > 24mag). Redshifts have previouslybeen determined for 34 sources, based primarily on the presence of a deep 9.7micron silicate absorption feature, with a median z of 2.2. Spectra arepresented for the remaining 24 sources for which we were previously unable todetermine a confident redshift because the IRS spectra show no strong features.Optical photometry from the NDWFS and infrared photometry with MIPS and theInfrared Array Camera on the Spitzer Space Telescope (IRAC) are given, with Kphotometry from the Keck I telescope for some objects. The sources withoutstrong spectral features have overall spectral energy distributions (SEDs) anddistributions among optical and infrared fluxes which are similar to those forthe sources with strong absorption features. Nine of the 24 sources are foundto have feasible redshift determinations based on fits of a weak silicateabsorption feature. Results confirm that the "1 mJy" population of 24 micronSpitzer sources which are optically faint is dominated by dusty sources withspectroscopic indicators of an obscured AGN rather than a starburst. Thereremain 14 of the 58 sources observed in Bootes for which no redshift could beestimated, and 5 of these sources are invisible at all optical wavelengths.Comment: Accepted by Ap