MID-INFRARED SPECTROSCOPY OF OPTICALLY FAINT EXTRAGALACTIC 70 μM SOURCES

We present mid-infrared (mid-IR) spectra of 16 optically faint sources with 70 mu m fluxes in the range 19 mJy < f(v)(70 mu m) < 38 mJy. The sample spans a redshift range of 0.35 < z < 1.9, with most lying between 0.8 < z < 1.6, and has IR luminosities of 10(12)-10(13) L-circle dot. 10 of the 16 objects show prominent polycyclic aromatic hydrocarbon (PAH) emission features; four of 16 show weak PAHs and strong silicate absorption, and two objects have no discernable spectral features. Compared to samples with f(v)(24 mu m) > 10 mJy, the 70 mu m sample has steeper IR continua and higher luminosities. The PAH-dominated sources are among the brightest starbursts seen at any redshift, and reside in a redshift range where other selection methods turn up relatively few sources. The absorbed sources are at higher redshifts and have higher luminosities than the PAH-dominated sources, and may show weaker luminosity evolution. We conclude that a 70 mu m selection extending to similar to 20 mJy, in combination with selections at mid-IR and far-IR wavelengths, is necessary to obtain a complete picture of the evolution of IR-luminous galaxies over 0 < z < 2.