Paper: astro-ph/0512073
Date: Fri, 2 Dec 2005 21:46:44 GMT (309kb)

Title: The Effect of Gravitational Recoil on Black Holes Forming in a
Hierarchical Universe

Authors: N. I. Libeskind, S. Cole, C. S. Frenk, J. C. Helly

Comments: 12 pages, 7 figures, Submitted to MNRAS
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Galactic bulges are known to harbour central black holes whose mass is
tightly correlated with the stellar mass and velocity dispersion of the bulge.
In a hierarchical universe, mergers of subgalactic units are accompanied by the
amalgamation of bulges and the likely coalescence of galactocentric black
holes. In these mergers, the beaming of gravitational radiation during the
plunge phase of the black hole collision can impart a linear momentum kick or
``gravitational recoil'' to the remnant. If large enough, this kick will eject
the remnant from the galaxy and populate intergalactic space with wandering
black holes. Using a semi-analytic model of galaxy formation, we investigate
the effect of black hole ejections on the scatter in the relation between black
hole and bulge mass. We find that although not the dominant source of the
measured scatter, they do make a significant contribution and may be used to
set a constraint, v_kick<500 km/s, on the typical kick velocity, in agreement
with values found from general relativistic calculations. Even for the more
modest kick velocities implied by these calculations, we find that a
substantial number of central black holes are ejected from the progenitors of
present day galaxies, giving rise to a population of wandering intrahalo and
intergalactic black holes whose distribution we investigate in high-resolution
N-body simulations of Milk-Way mass halos. We find that intergalactic black
holes make up only ~2-3% of the total galactic black hole mass but, within a
halo, wandering black holes can contribute up to about half of the total black
hole mass orbiting the central galaxy. Intrahalo black holes offer a natural
explanation for the compact X-ray sources often seen near the centres of
galaxies and for the hyperluminous non-central X-ray source in M82.

\\ ( http://arXiv.org/abs/astro-ph/0512073 , 309kb)