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

TL;DR

This study investigates whether disk fragmentation in primordial protogalaxies prevents the formation of supermassive stars, concluding that fragmentation does not hinder SMS formation under typical conditions.

Contribution

It demonstrates that in metal-poor protogalactic disks, fragmentation does not stop the inward migration and merging of clumps, allowing supermassive stars to form.

Findings

Clumps grow to ~30 Msun and migrate inward within ~10^4 years.

Dust-induced fragmentation does not significantly alter the migration process below Z>3*10^{-4} Zsun.

Fragmentation does not prevent supermassive star formation in primordial, metal-poor disks.

Abstract

Supermassive stars (SMSs; >10^5 Msun) formed in the first protogalaxies with virial temperature T_vir>10^4 K are expected to collapse into seeds of supermassive black hole (SMBHs) 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 disk, and the fate of the clumps formed in the disk by gravitational instability. Interior to a characteristic radius R_f=few*10^{-2} pc, the disk fragments into massive clumps with M_c~30 Msun. The clumps grow via accretion and migrate inward rapidly on a timescale of ~10^4 yr, which is comparable or shorter than the Kelvin-Helmholz time >10^4 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} Zsun, because clump migration below this metallicity remains as rapid as in the primordial case. Our results suggest that fragmentation of a compact, metal--poor disk can not prevent the formation of a SMS.