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 newH-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, 2.7×1046 erg s−1 < Lbol < 2.4×1047 erg s−1. The high-quality observations and the accurate fitting of the Mgii λ2798 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 108M MBH 6.6×109M with a median of ∼8.4 × 108M . 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.