2008ApJ...683..114S

Extragalactic sources from the IRAS Faint Source Catalog (FSC) that have the optically faintest magnitudes (E≳18) 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_ν(25 µm) determined with the IRS is fainter than the sensitivity limit for the FSC. Sources have 0.12<z<1.0 and luminosities (ergs/s) 43.3<log[νL_ν(5.5 µm)]<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 νL_ν(5.5 µm). 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_ν(25 µm)/f_ν(1.4 GHz)], does not distinguish between AGNs and starbursts for these sources.