Formation of Massive Black Holes in Early Mergers ?

We review theoretical and observational arguments favoring a scenario in which a typical massive black hole (MBH) is formed in the merger core of colliding disk systems at high z during the build-up of a spheroid. Low-mass (~ 10^{5-6} M_sun) seed black holes are assumed to have been formed earlier. The most massive black holes giving rise to the most luminous active galactic nuclei (AGN) in active phases are expected to grow in violent mergers of large disk-plus-bulge systems that lead to giant elliptical galaxies, the typical hosts of radio galaxies and quasars. We consider current ultraluminous infrared galaxies (ULIRG) as systems closely resembling the predecessors of early formed massive ellipticals. As an example, we discuss the evidence for an AGN in the merging system NGC 6240 which we advertize as a prototypical active ULIRG, obscured in X-rays by a high column density absorber.