A massive black hole may form, on a dynamical timescale, from the
runaway collapse of a dense star cluster or the instability of a
massive object. Alternatively, Hills
[27] suggests that
a black hole
of 
10
M
would be able
to grow to 
107
M within ~
109 yr. It is
premature to incorporate quasars into the general context of galactic
evolution. However, we observe that the phenomenon was more prevalent
at the epoch z 
2; and
it seems astrophysically unlikely that a
single object could maintain a quasar-level luminosity for ~ 1010 yr
(though it could "flare up" more than once if rejuvenated by an
increased infall rate from its surroundings). This raises the question
of what "switches off" the phenomenon, and various ideas suggest
themselves: typical galaxies may be "swept clean" when the gas supply
falls below a critical rate
[42]
(the explosive activity in the
nucleus may itself aid the same result); a central black hole fueled
by stars would swallow all those within the dense core, the capture
rate thereafter being limited by the rate
(
tR-1) at which dynamical
relaxation can replenish the orbits that pass close to the hole
[23,
29].
Solar-type stars are swallowed whole (because rT >
r) when Mh
exceeds a value ~ 3 x 108
M for a
Schwarzschild hole and ~ 109
M for
a Kerr hole. This might reduce the resulting luminosity, though of
course it in no way inhibits the hole's further growth. The expected
angular momentum of the hole is itself determined by the mode of
accretion onto it: if there is a stably orientated disk, the hole,
whatever its initial angular momentum, would spin up to a/m
0.99 if
it accretes enough material to at least double its mass; but if the
infalling gas or stellar debris has no preferred angular momentum,
then obviously a/m -> 0. There has been some debate
[43,
44] on whether
black holes in the range 106-108
M could exist
in the nuclei of
normal galaxies without displaying energetic activity exceeding what
is observed. The issue hinges on whether a tidally disrupted star need
necessarily all be swallowed with high
; and on whether this would
yield a brief intense flare-up (with a correspondingly short "duty
cycle") or a steady luminosity from the nucleus.
An unavoidable consequence of the quasar model outlined in this
paper is that massive and relatively quiescent black holes should lurk
in the nuclei of many large nearby galaxies. The very compact source
in the nucleus of Centaurus A could be an important and relevant
clue. At a distance ~ 5 Mpc, Cen A is the closest radio galaxy. Its
total radio power output is now only ~ 1042 erg
s-1. An energy of ~
1060 erg is, however, contained in the very extended radio
lobes, and
Cen A may have once been as powerful a radio source
as (say)
Cygnus A
or 3C 273. If this energy were generated by a black
hole (or its progenitor), the mass would be
107
M. There is a
very compact radio
source in the nucleus; and an X-ray source which varies on time-scales
that may be as short as 2 hr. Fabian and his collaborators
[34] have
attributed the X-ray and radio properties to relatively slow
quasispherical infall onto such a black hole. If this interpretation
is correct, Centaurus A has a nucleus whose violent activity is almost
defunct and is perhaps the closest massive black hole manifesting the
effects of accretion. The compact radio sources in the nuclei of some
ellipticals [42]
can be interpreted in this way too. The very small (~
10 a.u.) source in the Galactic Centre - a unique source in a unique
location could result from a very low rate of accretion onto a black
hole. The value of Mh must however be
5 x 106
M (as discussed in
Professor Oort's contribution to these proceedings), implying that our
own Galaxy could never have flared up into a quasar or radio source on
a really spectacular scale.