The Evolution of Starburst Galaxies to Active Galactic Nuclei

Abstract

Recent observations of luminous IRAS galaxies (L_( IR) > 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 with the Owens Valley Millimeter-Wave Interferometer at 2.6 and 1.3 mm for five of these galaxies demonstrate that approximately half of the interstellar matter is contained in the central kpc. Inter-ferometry on the most luminous galaxies (L _(IR) ≥ 10¹¹L⊙) reveals that approximately half of the total interstellar matter is contained in the central kpc with mean densities of several hundred H₂ cm⁻³. Such gas concentrations should result in the very rapid formation of stars, i.e. a central star burst yielding a massive central star cluster. The deep potential of the central star cluster and the high density of interstellar gas will 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 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.