The Cosmological Evolution of Quasar Black-Hole Masses

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Virial black-hole mass estimates are presented for 12698 quasars in the redshift interval 0.1 ≤ z ≤ 2.1, based on modelling of spectra from the Sloan Digital Sky Survey (SDSS) first data release. The black-hole masses of the SDSS quasars are found to lie between ≃ 107 M&sun; and an upper limit of ≃ 3 × 109M&sun;, entirely consistent with the largest black-hole masses found to date in the local Universe. The estimated Eddington ratios of the broad-line quasars (FWHM ≥ 2000 km s-1) show a clear upper boundary at Lbol/LEdd ≃ 1, suggesting that the Eddington luminosity is still a relevant physical limit to the accretion rate of luminous broad-line quasars at z ≤ 2. By combining the black-hole mass distribution of the SDSS quasars with the 2dF quasar luminosity function, the number density of active black holes at z ≃ 2 is estimated as a function of mass. By comparing the estimated number density of active black holes at z ≃ 2 with the local mass density of dormant black holes, we set lower limits on the quasar lifetimes and find that the majority of black holes with mass ≥ 108.5 M&sun; are in place by ≃ 2.

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