Obscured active galactic nuclei in luminous infrared galaxies

We examine the nature of the central power source in very luminous infrared galaxies. The infrared properties of the galaxies, including their far-infrared and 2.2 mu m fluxes, CO indices, and Brackett line fluxes are compared to models of starburst stellar populations. Among seven galaxies we found two dominated by emission from young stars, two dominated by emission from an AGN, and three transition cases. Our results are consistent with evidence for active nuclei in the same galaxies at other wavelengths. Nuclear mass measurements obtained for the galaxies indicate an initial mass function biased toward high-mass stars in two galaxies. After demonstrating our methods in well-studied galaxies, we define complete samples of high luminosity and ultraluminous galaxies. We find that the space density of embedded and unembedded quasars in the local universe is similar for objects of similar luminosity. If quasars evolve from embedded sources to optically prominent objects, it appears that the lifetime of a quasar is no more than about 10(8) yr.