8.1. Fuelling black holes
According to the ideas of the previous sections, an AGN requires a black
hole plus
fuel delivered at it. Some galaxies, such as M 31, M 32 and our Galaxy,
have small
black holes but are not active. There may be inactive galaxies with more
massive
black holes; they would be difficult to detect at larger distances. It
seems likely that
the black hole initially formed in an AGN episode, but this is not
necessary. Once
a black hole exhausts its fuel, it becomes quiet. However the
n-body calculations of Byrd et al
(1986,
1987)
suggest that this may occur in an episodic manner, with
`latency times' of a few times 108 y before the fuelling
begins, and between episodes
of fuelling. This behaviour is related to the free-fall time and
gravitational instability
in the disk. An existing black hole can be reactivated by further
interactions which
refuel it. The evolution of the mass of an individual black hole must be
to increase (if
it is fuelled) from Seyfert galaxy to QSO, according to the derived
masses of section 2.
This complex type of luminosity evolution of an individual AGN is quite
different from
the smoothly decreasing (with cosmic time) luminosity evolution
generally assumed
in fitting QSO magnitude and redshift counts, as for instance by
Schmidt and Green
(1983).
From the scanty observational evidence, it is certainly
conceivable that a large
fraction of `normal' galaxies contain black holes ready and available
for refuelling.
From the interaction picture, the rate of formation of AGNs increases
rapidly with the
number density of galaxies. Thus the observed increase in number density
of luminous
QSOs with redshift z can be qualitatively understood. Simplified
calculations on this basis by
De Robertis (1985)
show that the density-evolution rates required by the
counts can be approximately matched, with a maximum number density (in comoving
coordinates) around z = 2-3. Further, as the expansion continues,
if the more recently
formed black holes have smaller mean masses, the mean luminosity of the
QSOs will
also decrease. This can mimic the luminosity evolution required by the
counts, by a
proper choice of assumed conditions. However, there is such a wide range
of possible
parameters, and the details of the interaction process are so
simplified, that it is difficult to evaluate the significance of this result.
In the IRAS survey, a significant number of galaxies were discovered
with the bulk of
their radiation in the infrared spectral region. A significant number of
them, 10 of the
324 galaxies in the IRAS bright galaxy survey, have luminosity L >
1012 L
,
that is in
the QSO range. Their mean redshift is z = 0.055 and the maximum
is z = 0.08. Two
of them are previously known Seyfert galaxies, Mrk 231, a Seyfert 1, and
Mrk 273, a
Seyfert 2. Optical spectra show that one other is a Seyfert 1.5, another
a Seyfert 1.8,
and the rest, except for one with an H II region spectrum, are Seyfert
2s. All but one of
them have extended optical images, and seven of the 10 appear to be
`peculiar', being
either mergers (as Mrk 231 was previously described) or distorted
systems. Seven of
them have been observed in the mm-wavelength region for the CO (1 -> 0)
emission
line, and all of them rank among the most luminous CO sources observed.
Sanders et al
(1988b)
identify these ultraluminous infrared galaxies as QSOs in the process of
formation. They are still highly distorted, and shrouded with dust,
which is apparent
not only from the CO but from the strong reddening of their
emission-line spectra. As
the dust is blown away by the AGN radiation, supernovae, and stellar winds,
Sanders et al
(1988b)
hypothesize, and the nucleus becomes more directly apparent, there
will be a marked shift in the spectral energy distribution to shorter
wavelengths and
the characteristic AGN features will appear. The number of bright QSOs
in the same
redshift range with L > 1012
L is five,
comparable with the number of ultraluminous
infrared galaxies. Thus it seems quite likely that these are indeed the
first stage the QSO process.