Spitzer mid-infrared spectroscopy of infrared luminous galaxies at z ∼ 2.: II.: Diagnostics

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z similar to 1-3 and likely L-IR similar to 10(12)-10(13) L-circle dot. Our study aims at quantifying the star formation and active galactic nucleus (AGN) processes in these sources that recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum (i.e., AGN) dominated sources, but similar to 60% of these show PAH emission, suggesting the presence of star formation activity. These sources have redder mid-IR colors than typical optically selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5 mu m hot dust continua. However, their steep 30-14 mu m slopes suggest that star formation might dominate the total infrared luminosity. Six of our z similar to 2 sources have EW6.2 greater than or similar to 0.3 mu m and L-14 mu m greater than or similar to 10(12) L-circle dot (implying L-IR greater than or similar to 10(13) L-circle dot). At these luminosities, such high EW6.2 ULIRGs do not exist in the local universe. We find a median optical depth at 9.7 mu m of = 1.4. This is consistent with local IRAS-selected ULIRGs but differs from early results on SCUBA-selected z similar to 2 ULIRGs. Similar to local ULIRGs, about 25% of our sample shows extreme obscuration (tau(9.7 mu m) greater than or similar to 3), suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs but are an order of magnitude more luminous. It is not clear whether our z similar to 2 ULIRGs are simply scaled-up versions of local ULIRGs or subject to fundamentally different physical processes.