Spitzer IRS spectra of optically faint IRAS sources

Extragalactic sources from the IRAS Faint Source Catalog ( FSC) that have the optically faintest magnitudes ( Ek18) were selected by spatial coincidence with a source in the FIRST radio survey, and 28 of these sources have been observed with the Infrared Spectrograph on Spitzer ( IRS). While an infrared source is always detected with the IRS at the FIRST position, only 50% of the infrared sources are real FSC detections, as estimated from the number of sources for which the f ( 25 m) determined with the IRS is fainter than the sensitivity limit for the FSC. Sources have 0: 12 < z < 1: 0 and luminosities (ergs s 1) 43: 3 < log L ( 5: 5 m) < 46: 7, encompassing the range from local ULIRGs to the most luminous sources discovered by Spitzer at z 2. Detectable PAH features are found in 15 of the sources (54%), and measurable silicate absorption is found in 19 sources (68%); both PAH emission and silicate absorption are present in 11 sources. PAH luminosities are used to determine the starburst fraction of bolometric luminosity, and model predictions for a dusty torus are used to determine the AGN fraction of luminosity in all sources based on L ( 5.5 m). Approximately half of the sources have luminosity dominated by an AGN and approximately half by a starburst. The ratio of infrared to radio flux, defined as q log f ( 25 m)/ f ( 1: 4 GHz) , does not distinguish between AGNs and starbursts for these sources.