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We present new Gemini-North/NIRI and VLT/SINFONI H-band spectroscopy for aflux limited sample of 40 z~4.8 active galactic nuclei, selected from the SloanDigital Sky Survey. The sample probably contains the most massive active blackholes (BHs) at this redshift and spans a broad range in bolometric luminosity,2.7x10^46< L_bol < 2.4x10^47 erg/sec. The high-quality observations and theaccurate fitting of the MgII(2800A) line, enable us to study, systematically,the distribution of BH mass (M_BH) and normalized accretion rate (L/L_Edd) atz~4.8. We find that 10^8 < M_BH < 6.6x10^9 M_sun, with a median of ~8.4x10^8M_sun. We also find that 0.2 < L/L_Edd < 3.9 with a median of ~0.6. Most ofthese sources had enough time to grow to their observed mass at z~4.8 fromz=20, assuming a range of seed BH masses, with ~40% that are small enough to bestellar remnants. Compared to previously studied samples at z~2.4 and 3.3, themasses of the z~4.8 BHs are typically lower by ~0.5 dex. and their L/L_Edd ishigher by a similar factor. The new z~4.8 sample can be considered as theprogenitor population of the most massive BHs at z~2.4 and 3.3. Such anevolutionary interpretation requires that the growth of the BHs from z~4.8 toz~3.3 and z~2.4 proceeds with short duty cycles, of about 10-20%, depending onthe particular growth scenario.

Black-Hole Mass and Growth Rate at z~4.8: A Short Episode of Fast Growth Followed by Short Duty Cycle Activity