Mid-infrared spectroscopy of infrared-luminous galaxies at z similar to 0.5-3

Abstract

We present results on low-resolution mid-infrared (MIR) spectra of 70IR-luminous galaxies obtained with the infrared spectrograph (IRS)onboard Spitzer. We selected sources from the European Large AreaInfrared Survey with S-15 > 0.8 mJy and photometric or spectroscopic z> 1. About half of the samples are quasi-stellar objects (QSOs) in theoptical, while the remaining sources are galaxies, comprising bothobscured active galactic nuclei (AGN) and starbursts. Redshifts wereobtained from optical spectroscopy, photometric redshifts and the IRSspectra. The later turn out to be reliable for obscured and/orstar-forming sources, thus becoming an ideal complement to opticalspectroscopy for redshift estimation. We estimate monochromatic luminosities at several rest-framewavelengths, equivalent widths and luminosities for the polycyclicaromatic hydrocarbon (PAH) features, and strength of the silicatefeature in individual spectra. We also estimate integrated 8-1000 mu mIR luminosities via spectral energy distribution fitting to MIR andfar-IR (FIR) photometry from the Spitzer Wide-Area InfraredExtragalactic survey and the MIR spectrum. Based on these measurements,we classify the spectra using well-known IR diagnostics, as well as anew one that we propose, into three types of source: those dominated byan unobscured AGN, mostly corresponding to optical quasars (QSOs),those dominated by an obscured AGN and starburst-dominated sources.Starbursts concentrate at z similar to 0.6-1.0 favoured by the shift ofthe 7.7-mu m PAH band into the selection 15-mu m band, while AGN spreadover the 0.5 < z < 3.1 range.Star formation rates (SFR) are estimated for individual sources fromthe luminosity of the PAH features. An estimate of the average PAHluminosity in QSOs and obscured AGN is obtained from the compositespectrum of all sources with reliable redshifts. The estimated mean SFRin the QSOs is 50-100M(circle dot) yr(-1), but the implied FIRluminosity is 3-10 times lower than that obtained from stackinganalysis of the FIR photometry, suggesting destruction of the PAHcarriers by energetic photons from the AGN. The SFR estimated inobscured AGN is two to three times higher than in QSOs of similar MIRluminosity. This discrepancy might not be due to luminosity effects orselection bias alone, but could instead indicate a connection betweenobscuration and star formation. However, the observed correlationbetween silicate absorption and the slope of the NIR to MIR spectrum iscompatible with the obscuration of the AGN emission in these sourcesbeing produced in a dust torus.