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2008MNRAS.385.1621K - Mon. Not. R. Astron. Soc., 385, 1621-1627 (2008/April-2)
The evolution of black hole mass and spin in active galactic nuclei.
KING A.R., PRINGLE J.E. and HOFMANN J.A.
Abstract (from CDS):
We show that for essentially all AGN parameters, the disc produced by any accretion episode of this type has lower angular momentum than the hole, allowing stable co- and counter-alignment of the discs through the Lense-Thirring effect. A sequence of randomly oriented accretion episodes as envisaged above then produces accretion discs stably co- or counter-aligned with the black hole spin with almost equal frequency. Accretion from these discs very rapidly adjusts the hole's spin parameter to average values (the precise range depending slightly on the disc vertical viscosity coefficient α2) from any initial conditions, but with significant fluctuations (Δa ∼ ±0.2) about these. We conclude that (i) supermassive black holes (SMBH) should on average spin moderately, with the mean value decreasing slowly as the mass increases; (ii) SMBH coalescences leave little long-term effect on; (iii) SMBH coalescence products in general have modest recoil velocities, so that there is little likelihood of their being ejected from the host galaxy; (iv) black holes can grow even from stellar masses to ∼5x109Msolar at high redshift z ∼ 6; and (v) jets produced in successive accretion episodes can have similar directions, but after several episodes the jet direction deviates significantly. Rare examples of massive holes with larger spin parameters could result from prograde coalescences with SMBHs of similar mass, and are most likely to be found in giant ellipticals. We compare these results with observation.
Abstract Copyright: ©2008 The Authors. Journal compilation © 2008 RAS
Journal keyword(s): accretion, accretion discs - black hole physics - galaxies: active
Simbad objects: 2
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