The hosts and environments of local ULIRGs and QSOs

This paper reviews the recent results from a comprehensive investigation of the most luminous mergers in the local universe, the ultraluminous infrared galaxies (ULIRGs) and the quasars. First, the frequency of occurrence and importance of black hole driven nuclear activity in ULIRGs are discussed using the latest sets of optical, near-infrared, mid-infrared, and X-ray spectra on these objects. Obvious trends with luminosity, infrared color, and morphology are pointed out. Next, the host galaxy properties of ULIRGs are described in detail and then compared with local quasar hosts and inactive spheroids. A similar comparison is carried out for the environments of ULIRGs and local QSOs. By and large, the data are consistent with the scenario where ULIRGs are intermediate-mass elliptical galaxies in formation and in the process of becoming moderate-luminosity optical quasars. The powerful galactic winds detected in many ULIRGs may help shed any excess gas during this transformation. However, this evolutionary scenario does not seem to apply to all ULIRGs and quasars: Ultraluminous infrared mergers do not always result in a quasar, and low-luminosity quasars near the boundary with Seyferts do not all show signs of a recent major merger.