THE GROWTH OF THE STELLAR SEEDS OF SUPERMASSIVE BLACK HOLES

The collapse of baryons into extremely massive stars with masses similar to 10(4) M-circle dot in a small fraction of protogalaxies at z greater than or similar to 10 is a promising candidate for the origin of supermassive black holes (SMBHs), some of which grow to a billion solar masses by z similar to 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-* similar to 10(3) M-circle dot ((M) over dot(acc)/10(-3) M-circle dot yr(-1))(8/7), where (M) over dot(acc) 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-circle dot yr(-1)), the lifetime of the star instead limits its final mass to similar to 10(6) M-circle dot. 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. Ly 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, strong He II lambda 1640, and continuum emission are likely to be the key observational signatures of the progenitors of SMBHs at high redshift.