Potential formation sites of super-star clusters in ultraluminous infraredgalaxies

Recent observational results on high spatial resolution images of ultralumi nous infrared galaxies (ULIGs) have revealed very luminous, young, compact, and heavily obscured super-star clusters in their central regions, suggest ed to be formed by gas-rich major mergers. By using stellar and gaseous num erical simulations of galaxy mergers, we first demonstrate that the central regions of ULIGs are the most promising formation sites of super-star clus ters 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 nucle ar region owing to dynamical friction and consequently merge with one anoth er to form a single compact stellar nucleus with a seed massive black hole. We thus suggest that multiple merging between super-star clusters formed b y nuclear starbursts in the central regions of ULIGs can result in the form ation of massive black holes.