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 similar or equal to 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 10(46) erg s(-1) < L-bol < 2.4 x 10(47) erg s(-1). The high-quality observations and the accurate fitting of the Mg II lambda 2798 line enable us to study, systematically, the distribution of BH mass (MBH) and normalized accretion rate (L/L-Edd) at z similar or equal to 4.8. We find that 10(8) M-circle dot less than or similar to M-BH less than or similar to 6.6 x 10(9) M-circle dot with a median of similar to 8.4 x 10(8) M-circle dot. We also find that 0.2 less than or similar to L/L-Edd less than or similar to 3.9 with a median of similar to 0.6. Most of these sources had enough time to grow to their observed mass at z similar or equal to 4.8 from z = 20, assuming a range of seed BH masses, with similar to 40% that are small enough to be stellar remnants. Compared to previously studied samples at z similar or equal to 2.4 and similar or equal to 3.3, the masses of the z similar or equal to 4.8 BHs are typically lower by similar to 0.5 dex and their L/L-Edd is higher by a similar factor. The new z similar or equal to 4.8 sample can be considered as the progenitor population of the most massive BHs at z similar or equal to 2.4 and similar or equal to 3.3. Such an evolutionary interpretation requires that the growth of the BHs from z similar or equal to 4.8 to z similar or equal to 3.3 and z similar or equal to 2.4 proceeds with short duty cycles, of about 10%-20%, depending on the particular growth scenario.