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Published online by Cambridge University Press: 07 August 2017
Recent observations of luminous IRAS galaxies (LIR > 1011 L⊙) have revealed that virtually all are extremely rich in molecular gas with H2 abundances 5–20 times that of the Milky Way, and deep CCD imaging indicates that most are recent galactic mergers. Interferometric observations at millimeter wavelengths for five of these galaxies demonstrate that approximately half of the interstellar matter is contained in the central kpc. This gas concentration can result in the formation of a massive central star cluster. The deep potential of the central star cluster and the high density of interstellar gas ensure that virtually all of the gas lost during late stellar evolution sinks to the center of the cluster, building up a central, massive black hole. For a coeval star cluster of 4×109 M⊙, a central mass of approximately 1.5×109 M⊙ will accumulate within approximately 108 years and accretion at an average rate of 7 M⊙ yr−1 over this time will result in a mean accretion luminosity of 1013 L⊙. This luminosity, radiated at X-ray and uv wavelengths from the inner accretion disk ionizes the mass loss envelopes of the surrounding red giant stars providing an origin for the broad emission line regions of QSO's. The large linewidths would then be due to orbital motion in the massive central star cluster.