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 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 IIλ2798 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 108 M ⊙ ≲ M BH ≲ 6.6 × 109 M ⊙ with a median of 8.4 × 108 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. © 2011. The American Astronomical Society. All rights reserved.