Formation of quasar nuclei in the hearts of ultraluminous infrared galaxies

We investigate whether or not supermassive black holes (SMBHs) with mass ≳ 108 M⊙ can be made in the hearts of ultraluminous infrared galaxies (ULIGs) during the course of mergers between/among gas-rich galaxies. (1) If one progenitor galaxy had a seed SMBH with mass of ∼107 M⊙, this seed SMBH can grow up to ≳108 M⊙ due to efficient Bondi-type gas accretion during the course of merger, given a gas density in the circumnuclear region of nH ∼ 103 cm-3. (2) 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 timescale of ∼109 yr to form an SMBH with ≳108 M⊙. Note, however, that the contribution of compact remnants supplied from hidden star clusters is necessary to lead to the formation of an SMBH. In conclusion, the ULIGs observed in the local universe can make SMBHs in their centers during the course of merging 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