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The obscured growth of massive black holes
A.C. Fabian (IoA)
The mass density of massive black holes observed locally is consistent
with the hard X-ray Background provided that most of the radiation
produced during their growth was absorbed by surrounding gas. A simple
model is proposed here for the formation of galaxy bulges and central
black holes in which young spheroidal galaxies have a significant
distributed component of cold dusty clouds which accounts for the
absorption. The central accreting black hole is assumed to emit both a
quasar-like spectrum, which is absorbed by the surrounding gas, and a
slow wind. The power in both is less than the Eddington limit for the
black hole. The wind however exerts the most force on the gas and, as
earlier suggested by Silk & Rees, when the black hole reaches a
critical mass, it is powerful enough to eject the cold gas from the
galaxy, so terminating the growth of both black hole and galaxy. In
the present model this point occurs when the Thomson depth in the
surrounding gas has dropped to about unity and results in the mass of
the black hole being proportional to the mass of the spheroid, with
the normalization agreeing with that found for local galaxies by
Magorrian et al. for reasonable wind parameters. The model predicts a
new population of hard X-ray and sub-mm sources at redshifts above one
which are powered by black holes in their main growth phase.
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