Ultraluminous Infrared Galaxies: QSOs in Formation?

We present new near-infrared Keck and VLT spectroscopic data on the stellardynamics in late stage, ultra-luminous infrared galaxy (ULIRG) mergers . We nowhave information on the structural and kinematic properties of 18 ULIRGs, 8 ofwhich contain QSO-like active galactic nuclei. The host properties (velocitydispersion, effective radius, effective surface brightness, M_K) ofAGN-dominated and star formation dominated ULIRGs are similar. ULIRGs fallremarkably close to the fundamental plane of early type galaxies. They populatea wide range of the plane, are on average similar to L*-rotating ellipticals,but are well offset from giant ellipticals and optically/UV bright, low-zQSOs/radio galaxies. ULIRGs and local QSOs/radio galaxies are very similar intheir distributions of bolometric and extinction corrected near-IRluminosities, but ULIRGs have smaller effective radii and velocity dispersionsthan the local QSO/radio galaxy population. Hence, their host masses andinferred black hole masses are correspondingly smaller. The latter are moreakin to those of local Seyfert galaxies. ULIRGs thus resemble local QSOs intheir near-IR and bolometric luminosities because they are (much more)efficiently forming stars and/or feeding their black holes, and not becausethey have QSO-like, very massive black holes. We conclude that ULIRGs as aclass cannot evolve into optically bright QSOs. They will more likely becomequiescent, moderate mass field ellipticals or, when active, might resemble theX-ray bright, early type galaxies that have recently been found by the ChandraObservatory.Comment: accepted to be published in ApJ, 7 figure