Early Growth and Efficient Accretion of Massive Black Holes at High Redshift^*

Black hole masses of the highest redshift quasars (4 ≲ z ≲ 6) are estimated using a previously presented scaling relationship, derived from reverberation mapping of nearby quasars, and compared with quasars at lower redshift. It is shown that the central black holes in luminous z ≳ 4 quasars are very massive (≳10⁹ M_☉). It is argued that the mass estimates of the high-redshift quasars are not subject to larger uncertainties than those for nearby quasars. Specifically, the large masses are not overestimates, and the lack of similarly large black hole masses in the nearby universe does not rule out their existence at high redshift. However, active galactic nucleus host galaxies do not typically appear fully formed and/or evolved at these early epochs. This supports scenarios in which black holes build up mass very fast in a radiatively inefficient (or obscured) phase relative to the stars in their galaxies. In addition, upper envelopes of M_BH ≈ 10¹⁰ M_☉ and L_bol ≈ 10⁴⁸ ergs s^-1 are observed at all redshifts.