Formation of Quasar Nuclei in the Heart of Ultraluminous Infrared Galaxies

Abstract

We investigate whether or not a supermassive black hole (SMBH) with mass $\gtrsim 10^8 M_\odot$ can be made in the heart of ultraluminous infrared galaxies (ULIGs) during the course of mergers between/among gas rich galaxies. (a) If one progenitor galaxy had a seed SMBH with mass of $\sim 10^7 M_\odot$, this seed SMBH can grow up to $\gtrsim 10^8 M_\odot$ due to efficient Bondi-type gas accretion during the course of merger given a gas density in the circumnuclear region of $n_H \sim 10^3 cm^{-3}$. (b) Even if there was no progenitor galaxy with a seed SMBH, star clusters with compact remnants (neutron stars and/or black holes) produced in the circumnuclear starbursts can merge into the merger center within a dynamical time scale of $\sim 10^9$ years to form a SMBH with $\gtrsim 10^8 M_\odot$. Note, however, that the contribution of compact remnants supplied from hidden star clusters is necessary to lead to the formation of a SMBH. In conclusion, the ultraluminous infrared galaxies observed in the local universe can make a SMBH in their center during the course of merger either by gas accretion onto a seed SMBH or by dynamical relaxation of compact remnants made in the violent circumnuclear starbursts. Therefore, it is quite likely that the ULIGs will finally evolve to optically luminous quasars as suggested by Sanders et al.