BLACK HOLE MASS AND GROWTH RATE ATz≃ 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 × 10 46 erg s −1 < L bol < 2.4 × 10 47 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 ( M BH ) and normalized accretion rate ( L / L Edd ) at z ≃ 4.8. We find that 10 8 M ☉ ≲ M BH ≲ 6.6 × 10 9 M ☉ with a median of ∼8.4 × 10 8 M ☉ . 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.