ULTRA-DEEP MID-INFRARED SPECTROSCOPY OF LUMINOUS INFRARED GALAXIES AT z∼ 1 AND z ∼ 2

We present ultra-deep mid-infrared spectra of 48 infrared-luminous galaxies in the GOODS-south field obtained with the Infrared Spectrograph on the Spitzer Space Telescope. These galaxies are selected among faint infrared sources (0.14–0.5 mJy at 24 μm) in two redshift bins (0.76–1.05 and 1.75–2.4) to sample the major contributors to the cosmic infrared background at the most active epochs. We estimate redshifts for 92% of the sample using polycyclic aromatic (PAH) and Si absorption features obtaining, in particular, eight new redshifts difficult to measure from ground-based observations. Only a few of these galaxies (5% at z ∼ 1 and 12% at z ∼ 2) have their total infrared luminosity dominated by emission from active galactic nuclei (AGNs). The averaged mid-IR spectrum of the z ∼ 1 luminous infrared galaxies (LIRGs) is a very good match to the averaged spectrum of local starbursts. The averaged spectrum of the z ∼ 2 ultra-luminous infrared galaxies (ULIRGs), because of a deeper Si absorption, is better fitted by the averaged spectrum of H ii-like local ULIRGs. Combining this sample with other published data, we find that 6.2 μm PAH equivalent widths (EW) reach a plateau of ∼ 1 μm for L24 μm ≲ 1011 L☉. At higher luminosities, EW6.2 μm anti-correlates with L24 μm. Intriguingly, high-z ULIRGs and sub-millimeter galaxies (SMGs) lie above the local EW6.2 μm–L24 μm relationship suggesting that, at a given luminosity, high-z ULIRGs have AGN contributions to their dust emission lower than those of local counterparts. A quantitative analysis of their morphology shows that most of the luminous IR galaxies have morphologies similar to those of IR-quiet galaxies at the same redshift. All z ∼ 2 ULIRGs of our sample are IR-excess BzK galaxies and most of them have LFIR/L1600 Å ratios higher than those of starburst galaxies at a given UV slope. The “IR excess” is mostly due to strong 7.7 μm PAH emission and underestimation of UV dust extinction. On the basis of the AGN-powered L6 μm continuum measured directly from the mid-IR spectra, we estimate an average intrinsic X-ray AGN luminosity of L2−10 keV = (0.1± 0.6)× 1043 erg s−1, a value substantially lower than the prediction by Daddi et al.