Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-11-03
Mon.Not.Roy.Astron.Soc.350:456,2004
Astronomy and Astrophysics
Astrophysics
19 pages, 6 figures, 1 table. v2: Corrected references. v3: Extended Section 5.1, corrected Section 3.2, various other correct
Scientific paper
10.1111/j.1365-2966.2004.07650.x
We investigate the constraints on models of supermassive black hole (SMBH) and quasar formation obtainable from two recent observational developments: the discovery of luminous quasars at z~6, and estimates of the local mass density of SMBHs. If ~90 per cent of this mass was accreted at redshifts z<3, as suggested by the observed quasar luminosity functions, these joint constraints pose a challenge for models, which must account for the observed luminous quasar population at z~6 within a very limited `mass budget'. We investigate a class of models based within the hierarchical structure formation scenario, in which major mergers lead to black hole formation and fuelling, and the resulting quasars shine at their Eddington-limited rate until their fuel is exhausted. We show that the simplest such model, in which a constant fraction of the gas within the halo is accreted in each major merger, cannot satisfy both constraints simultaneously. When this model is normalized to reproduce the number density of luminous quasars at z~6, the mass budget is grossly exceeded due to an overabundance of lower mass SMBHs. We explore a range of modifications to the simple model designed to overcome this problem. We show that both constraints can be satisfied if the gas accretion fraction scales as a function of the halo virial velocity. Similar scalings have been proposed in order to reproduce the local M-sigma relation. Successful models can also be constructed by restricting the formation of seed black holes to redshifts above z~11.5 or to haloes above a velocity threshold ~55 km/s, or assuming that only a fraction of major mergers result in formation of a seed SMBH. (abridged)
Bromley J. M.
Fabian Andrea C.
Somerville Rachel S.
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