Recent observational results on high spatial resolution images of ultraluminous infrared galaxies (ULIGs) have revealed very luminous, young, compact, and heavily obscured super-star clusters in their central regions, suggested to be formed by gas-rich major mergers. By using stellar and gaseous numerical simulations of galaxy mergers, we first demonstrate that the central regions of ULIGs are the most promising formation sites of super-star clusters owing to the rather high gaseous pressure of the interstellar medium. Based on simple analytical arguments, we then discuss the possibility that super-star clusters in a ULIG can be efficiently transferred into the nuclear region owing to dynamical friction and consequently merge with one another to form a single compact stellar nucleus with a seed massive black hole. We thus suggest that multiple merging between super-star clusters formed by nuclear starbursts in the central regions of ULIGs can result in the formation of massive black holes.
The Astrophysical Journal Letters,
Vol. 557, no. 1 (Aug 2001), pp. L19-L22