Downsizing of supermassive black holes from the SDSS quasar survey

TL;DR

This study analyzes SDSS quasars to understand how the maximum mass of active black holes evolves with redshift, revealing a gradual increase in maximum black hole mass over cosmic time.

Contribution

It introduces a model linking quasar luminosity and line width distributions to black hole mass evolution, accounting for survey flux limits and biases.

Findings

Maximum black hole mass increases with redshift as log[M_BH(max)/M_Sun]~0.3z+9

Maximum Eddington ratio (~0.45) remains constant over time

Results are robust against Malmquist bias

Abstract

Starting from the ~50000 quasars of the Sloan Digital Sky Survey for which MgII line width and 3000 A monochromatic flux are available, we aim to study the dependence of the mass of active black holes on redshift. We focus on the observed distribution in the FWHM - nuclear luminosity plane, which can be reproduced at all redshifts assuming a limiting BH mass, a maximum Eddington ratio and a minimum luminosity (due to the survey flux limit). We study the z-dependence of the best fit parameters of assumed distributions at increasing redshift and find that the maximum mass of the quasar population evolves as log[M_BH(max)/M_Sun]~0.3z+9, while the maximum Eddington ratio (~0.45) is practically independent of cosmic time. These results are unaffected by the Malmquist bias.