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Title: The Cosmological Evolution of Supermassive Black Holes
Author: Babic, Ana
ISNI:       0000 0001 3434 7393
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2007
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It is now thought that all massive galaxies contain supermassive (> 106 M0 ) black holes. A model relating the history of supermassive black hole growth and the history of dark matter halo assembly is presented. It is based on the Press-Schechter formalism, and an analytic expression for the mean rate of mass accretion onto dark matter haloes. A simple assumption that the time scale' of the black hole growth is the same as the time scale for the host halo growth leads to a prediction for the mean Eddington ratio (>'Edd = LboI/LEdd) , that is a strong function of the cosmic epoch. Observational consequences of the model are investigated in the optical and X-ray bands. Good agreement between the model and the observations is found, if the relation between the dark matter halo mass and the black hole mass is allowed to evolve with redshift. The required additional evolution depends on the method for obtaining the black hole mass function, but in both investigated cases is mild, compared to some of the observational and theoretical reports in the literature. The model for the supermassive black hole gro~th is simple, yet it is able to account for a large portion of the observed active galactic nuclei (AGN) evolution. Motivated by the model's prediction that the Eddington ratio averaged over the population of haloes evolves strongly with epoch, Eddington ratios for a sample of hard X-ray selected AGN in the Chandra Deep Field South are investigated. The epoch (z < 1) and the luminosity range (L2-1O keY = 1041 - 1045 erg s-l) of the sample probe the AGN population implicated in AGN 'cosmic downsizing'. The results indicate a differen~ picture than the one previously inferred from optically-selected samples: the AGN responsible for the observed downsizing are powered by average-sized black holes (MBH rv 108 M0 ), typically radiating at low Eddington ratios (>'Edd rv 0.01). This finding, together with the fact that high-redshift, higher luminosity AGN are typically found radiating close to the Eddington limit, indicates that diminishing accretion rates play an important role in the observed AGN downsizing.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available