Does disc fragmentation prevent the formation of supermassive stars in protogalaxies?

Abstract

Supermassive stars (SMSs;≳105M⊙) formed in the first protogalaxies with virial temperature Tvir ≳ 104 K are expected to collapse into seeds of supermassive black hole in the high-redshift Universe (z ≳ 7). Fragmentation of the primordial gas is, however, a possible obstacle to SMS formation. We discuss the expected properties of a compact, metal-free, marginally unstable nuclear protogalactic disc, and the fate of the clumps formed in the disc by gravitational instability. Interior to a characteristic radius Rf = few × 10-2 pc, the disc fragments into massive clumps with Mc ~ 30M⊙. The clumps grow via accretion and migrate inward rapidly on a time-scale of ~104 yr, which is comparable or shorter than the Kelvin-Helmholtz time > 104 yr. Some clumps may evolve to zero-age main-sequence stars and halt gas accretion by radiative feedback, but most of the clumps can migrate inward and merge with the central protostar before forming massive stars. Moreover, we found that dust-induced fragmentation in metal-enriched gas does not modify these conclusions unless Z ≳ 3 × 10-4 Z⊙, because clump migration below this metallicity remains as rapid as in the primordial case. Our results suggest that fragmentation of a compact, metal-poor disc cannot prevent the formation of a SMS.