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 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, 2.7 x 1046 erg s-1 < Lbol < 2.4 x 1047 erg s-1. The high-quality observations and the accurate fitting of the Mg II 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 108 Msun ∼< MBH ∼< 6.6 x 109 Msun with a median of ∼8.4 x 108 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.