2011ApJ...740...99D

We present observations of a sample of 28 luminous infrared galaxies (LIRGs) from the IRAS Bright Galaxy Catalog. These galaxies were previously observed by Goldader et al. using 2 µm spectroscopy on UKIRT. They found that stellar population synthesis models constrained by the spectroscopic diagnostics implied star formation rates that accounted for their infrared luminosities, and there was also no spectroscopic evidence for "buried" active galactic nuclei (AGNs) at 2 µm. To search for evidence of AGNs buried deeply in extinction we have supplemented the UKIRT spectra with Spitzer mid-infrared (5.2-38.0 µm) spectra. Using a variety of spectroscopic diagnostics we find that ∼50% of the sample shows some evidence for an AGN. We also find that the luminosity of about ~(17±4)% of our sample is probably dominated by emission from AGNs, and the remaining ~(80±4)% have luminosities dominated by starbursts. Since ∼50% of the sample shows some evidence of concurrent AGN and starburst activities this suggests that both AGNs and starbursts commonly coexist in the LIRGs' phase of evolution. The sample consists of galaxies that show no AGN signatures at wavelengths less than 2 µm, so it appears that at wavelengths below 2 µm extinction in these galaxies masks the detection of AGNs in these and other LIRGs.