2002ApJ...578..702H

The recent discovery of bright quasars at redshift z∼6 suggests that black holes (BHs) with masses in excess of ∼10⁹ M_☉have already assembled at a very early stage in the evolution of the universe. An alternative interpretation is that these quasars are powered by less massive BHs, but their fluxes are strongly magnified through gravitational lensing by intervening galaxies. Here we analyze the flux distribution of the Lyα emission of the quasar with the highest known redshift, SDSS 1030+0524, at z=6.28. We show that this object could not have been magnified by lensing by more than a factor of ∼5. The constraint arises from the large observed size, ∼30 (comoving) Mpc, of the ionized region around this quasar and relies crucially only on the assumption that the quasar is embedded in a largely neutral intergalactic medium. Based on the line/continuum ratio of SDSS 1030+0524, we further argue that this quasar also cannot be beamed by a significant factor. We conclude that the minimum mass for its resident BH is 4x10⁸M_☉(for magnification by a factor of 5); if the mass is this low, then the quasars had to switch on prior to redshift z_f≳9. From the size of the ionized region, we are also able to place an absolute lower bound on the age of this quasar at t_Q>2x10⁷ yr.