Black-Hole Mass and Growth Rate at z~4.8: A Short Episode of Fast Growth Followed by Short Duty Cycle Activity
Abstract
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, 2.7x1046< Lbol < 2.4x1047 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 (MBH) and normalized accretion rate (L/LEdd) at z~4.8. We find that 108 < MBH < 6.6x109 Msun, with a median of ~8.4x108 Msun. We also find that 0.2 < L/LEdd < 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/LEdd 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.