Seyfert galaxies are rich in interstellar dust, and as expected from
this, also contain
large amounts of molecular gas. CO, one of the most abundant molecules
in molecular
clouds in our Galaxy, and the most readily detectable, has in fact been
observed in many Seyferts
(Bieging et al
1981,
Heckman et al
1989).
The observational data
seem to show that the Seyfert 2 galaxies have larger molecular gas
contents than
Seyfert 1s. This is one of the few remaining observational differences which a
to conflict with the idea that Seyfert 1 and 2 galaxies are essentially
similar systems
seen from different orientations. To date no broad-line CO has been
detected; the line
profiles of the observed CO are most nearly similar to those of the H I
= 21 cm
lines in the same object. This suggests that the molecular gas is
associated more with the disk of the galaxy than with the NLR
(Heckman et al
1989).
Molecular H2 has
only forbidden emission lines in the infrared, and is much more
difficult to detect.
However, three low-excitation lines of its (1, 0) vibrational band have
been measured in NGC 1068, with relatively narrow widths
(Oliva and Moorwood
1990).
There is no
reason to suppose that the abundance ratio CO / H2 is
appreciably different from that in our Galaxy.
Very recently high-resolution interferometric measurements have been made of
CO emission in two Seyfert galaxies, NGC 3227 and NGC 7469. At 6"
resolution the
latter shows a concentration of molecular gas with a mass ~ 4 x
109 M
and diameter ~ 1.5 kpc, centered on the nucleus
(Sanders et al 1988a).
The higher resolution (2-3")
observations show that in both these galaxies a large fraction of the
molecular gas is
concentrated within a few hundred parsecs of the nucleus. In NGC 3227
the central
structure is partly resolved, and suggests that a significant amount of
the CO lies in a
roughly toroidal region, centered on the nucleus. Its velocity field
includes a rotational
component, with apparently other complications as well. In NGC 7469 the
molecular
gas distribution is more complicated, but seems to have a similar
overall pattern. On
the other hand in the third Seyfert galaxy of which interferometer
measurements have
been reported to date, NGC 5033, the CO emission is more diffuse, spread
over several kiloparsecs
(Meixner et al
1990).
Clearly, high resolution maps of the CO emission in
more Seyfert galaxies, compared with atomic-line and dust continuum
maps, will be
very useful for understanding the structure of AGNs and their surroundings.
As stated in section 2.3, all the
observational evidence agrees that in AGNs photoionization
by hard photons from the central source is the dominant mechanism of energy
input to the ionized gas. These high-energy photons are generally
considered to be
generated by non-thermal processes such as relativistic synchrotron
radiation and
Compton scattering, or in the inner parts of the accretion disk itself,
as described in
section 3.3. However, an alternate view is
that in Seyfert 2 galaxies and LINERs, the
photoionizing radiation may come from stars, rather than from
non-thermal sources.
The most recent form of this idea is the suggestion by
Terlevich and Melnick
(1985)
that the ionization is produced by high-energy photons from postulated
`extremely
hot and luminous stars', which they have named `Warmers'. They are
supposed to be
`bare core Wolf-Rayet stars,' the descendents of massive progenitors
(M > 60 M),
with temperatures up to 2 x 105 K. Observational evidence
that any such stars exist
is scanty indeed, and theoretical justification that such high effective
temperatures
actually occur is very weak. However, if sufficiently many stars of
arbitrarily high
temperature are postulated, clearly any type of power-law, broken
power-law or more
complicated form of photoionizing spectrum can clearly be matched by some
population of such objects. Of course many starburst galaxies are known
to exist, with
emission-line spectra like H II regions, photoionized by OB stars. They
are the objects marked by circles and asterisks in
figures 2,
3 and
4. The AGNs, to be understood on
this basis, must have much more luminous and hotter stars, and many more
of them.
The Warmer picture of AGNs has been discussed and defended by Terlevich et al (1987). They have attempted to explain the x-ray and radio-frequency properties of AGNs, and some of the emission-line broadening, to supernovae produced from the massive, rapidly evolving stars. However, the rapid optical and especially x-ray variations in many AGNs are extremely difficult to explain by this picture. Likewise jets, polarization of the optical continuum on Seyfert 2 galaxies and the apparent dichotomies of figures 2, 3 and 4 are not, at present, understood on this basis. Neither is the apparent continuity in the physical properties of LINERs, Seyfert 2 and Seyfert 1 nuclei explained by it.