Redshifts from Spitzer spectra for optically faint radio-selected infrared sources

Spectra have been obtained with the Infrared Spectrograph on the Spitzer Space Telescope for 18 optically faint sources (R greater than or similar to 23.9mag) having f(nu)(24 mu m) > 1.0mJy and having radio detections at 20 cm to a limit of 115 mu Jy. The sources are within the Spitzer First Look Survey. Redshifts are determined for 14 sources from strong silicate absorption features ( 12 sources) or strong PAH emission features ( two sources), with median redshift of 2.1. Results confirm that optically faint sources of similar to 1mJy at 24 mu m are typically at redshifts z similar to 2, verifying the high efficiency in selecting high-redshift sources based on extreme infrared-to-optical flux ratio, and indicate that 24 mu m sources that also have radio counterparts are not systematically different than samples chosen only by their infrared-to-optical flux ratios. Using the parameter q = log [f(nu)(24 mu m)/f(nu)(20 cm)], 17 of the 18 sources observed have values of 0 < q < 1, in the range expected for starburst-powered sources, but only a few of these show strong PAH emission as expected from starbursts, with the remainder showing absorbed or power-law spectra consistent with an AGN luminosity source. This confirms previous indications that optically faint Spitzer sources with f(nu)(24 mu m) >= 1.0 mJy are predominately AGNs and represent the upper end of the luminosity function of dusty sources at z similar to 2. Based on the characteristics of the sources observed so far, we predict that the nature of sources selected at 24 mu m will change for f nu(24 mu m) less than or similar to 0.5 mJy to sources dominated primarily by starbursts.