Formation of Quasar Nuclei in the Hearts of Ultraluminous Infrared Galaxies

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 infraredgalaxies (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 efficientBondi-type gas accretion during the course of merger given a gas density in thecircumnuclear region of $n_H \sim 10^3 cm^{-3}$. (b) Even if there was noprogenitor galaxy with a seed SMBH, star clusters with compact remnants(neutron stars and/or black holes) produced in the circumnuclear starbursts canmerge into the merger center within a dynamical time scale of $\sim 10^9$ yearsto form a SMBH with $\gtrsim 10^8 M_\odot$. Note, however, that thecontribution of compact remnants supplied from hidden star clusters isnecessary to lead to the formation of a SMBH. In conclusion, the ultraluminousinfrared galaxies observed in the local universe can make a SMBH in theircenter during the course of merger either by gas accretion onto a seed SMBH orby dynamical relaxation of compact remnants made in the violent circumnuclearstarbursts. Therefore, it is quite likely that the ULIGs will finally evolve tooptically luminous quasars as suggested by Sanders et al.Comment: 6 pages, no figure. The Astrophysical Journal (Letters), in pres