Black-Hole Mass and Growth Rate at z~4.8: A Short Episode of Fast Growth
  Followed by Short Duty Cycle Activity
  We present new Gemini-North/NIRI and VLT/SINFONI H-band spectroscopy for a
flux limited sample of 40 z~4.8 active galactic nuclei, selected from the Sloan
Digital Sky Survey. The sample probably contains the most massive active black
holes (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 the
accurate 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) at
z~4.8. We find that 10^8 < M_BH < 6.6x10^9 M_sun, with a median of ~8.4x10^8
M_sun. We also find that 0.2 < L/L_Edd < 3.9 with a median of ~0.6. Most of
these sources had enough time to grow to their observed mass at z~4.8 from
z=20, assuming a range of seed BH masses, with ~40% that are small enough to be
stellar remnants. Compared to previously studied samples at z~2.4 and 3.3, the
masses of the z~4.8 BHs are typically lower by ~0.5 dex. and their L/L_Edd is
higher by a similar factor. The new z~4.8 sample can be considered as the
progenitor population of the most massive BHs at z~2.4 and 3.3. Such an
evolutionary interpretation requires that the growth of the BHs from z~4.8 to
z~3.3 and z~2.4 proceeds with short duty cycles, of about 10-20%, depending on
the particular growth scenario.
