The Growth of the Stellar Seeds of Supermassive Black Holes

The collapse of baryons into extremely massive stars with masses exceeding 10^4 M_Sun in a small fraction of protogalaxies at z > 10 is a promising candidate for the origin of supermassive black holes, some of which grow to a billion solar masses by z ~ 7. We determine the maximum masses such stars can attain by accreting primordial gas. We find that at relatively low accretion rates the strong ionizing radiation of these stars limits their masses to M_* ~ 10^3 M_Sun (dM_acc/dt / 10^-3 M_Sun yr^-1)^8/7, where dM_acc/dt is the rate at which the star gains mass. However, at the higher central infall rates usually found in numerical simulations of protogalactic collapse (>~ 0.1 M_Sun yr^-1), the lifetime of the star instead limits its final mass to >~ 10^6 M_Sun. Furthermore, for the spherical accretion rates at which the star can grow, its ionizing radiation is confined deep within the protogalaxy, so the evolution of the star is decoupled from that of its host galaxy. Lyman alpha emission from the surrounding H II region is trapped in these heavy accretion flows and likely reprocessed into strong Balmer series emission, which may be observable by the James Webb Space Telescope. This, along with strong He II 1640 Angstrom and continuum emission, are likely to be the key observational signatures of the progenitors of supermassive black holes at high redshift.