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Seyfert galaxies were first identified by Carl Seyfert in 1943
as galaxies with broad emission lines. These galaxies also have
unusually bright nuclei. Seyfert initially identified 6 such
objects and there are now approximately 90 such objects known.
They all appear to be spiral galaxies, and although 5-10% might
be elliptical, the questionable objects all have extremely small
angular sizes so it is difficult to discern any morphology. Seyferts
are "discovered" totally on the basis of the strong, broad emission
lines. It appears that about 1% of all spiral galaxies may be
Seyferts. It has been hypothesized that at least some spirals
(or possibly all spirals) go through an active Seyfert phase
under the proper conditions. Since most Seyfert galaxies appear
to be in close binary galaxy systems, it may be that tidal interactions
induce or even turn-on the Seyfert phenomenon.
In terms of their spectra there are now three sub-types of
Seyfert recognized. Since Seyferts generally exhibit both permitted
and forbidden lines, the emission must be produced in different
regions of the galaxy with different physical conditions. Forbidden
lines can only be produced in low density regions, while densities
must be greater where permitted lines are produced.
Type 1 These are the "classic" Seyfert galaxies. They have
extremely broad permitted lines of hydrogen. If the broadening
is interpreted as being due to Doppler motions, the broadening
corresponds to velocities of up to 5000 to 10000 km/sec! The
forbidden lines (typically O II, O III, N II, S II, etc.) are
only "moderately" broadened by 200-400 km/sec. For comparison,
in an ordinary galaxy the escape velocity from the galactic disk
is normally less than 300 km/sec. Of course, the emission lines
in a Seyfert originate in the nucleus of the galaxy.
Type 2 These Seyferts have "narrow" lines by comparison to
the Type 1's. The broadening rarely amounts to more than 200-400
km/sec (1000 km/sec in one case) and the permitted and forbidden
lines have the same widths.
Type 1.5 Some Type 1 objects are observed which have both broad
and narrow components for the permitted hydrogen lines. A narrow
emission peak seems to extend above the broad underlying emission
line.
The standard model for a Seyfert galaxy involves three components.
First, a tiny central source of high energy ionizing photons,
and then two distinct surrounding regions with different gas
densities. Presumably, the inner region is the Broad Line Region
(BLR) with high densities appropriate for the production of permitted
lines. The velocities in this region must approach the 5000-10000
km/sec values deduced from the line widths. Because the broad
lines are observed to undergo significant variability over periods
of weeks or months, the size of this region cannot be much greater
than a light month or so. This size corresponds to about 1011
km and is not much larger than the size of our planetary system.
Gas densities in the BLR must be on the order of 1013-1015 ions/m3.
Outside the BLR must be the Narrow Line Region (NLR) where the
gas densities are low enough to allow forbidden line production.
The scale size of the NLR must be about 102-103 times larger
than the BLR. There are no observational reports of variability
in the narrow lines.
Since there is no reason why permitted lines cannot be produced
in the low density NLR, the existence of Type 1.5 Seyferts is
totally understandable. Furthermore, slight differences in the
widths of the narrow lines suggests that the density in the NLR
decreases with radial distance from the nucleus. The critical
densities for forbidden lines from different atomic species vary,
and those lines coming from the least dense regions exhibit somewhat
narrower lines.
The continuous spectra of Seyferts seems to be a combination
of stellar, nonthermal, and IR emission from dust. Seyfert galaxies
are not strong radio sources and the most sensitive radio surveys
have detected only about half of the known Seyferts. Likewise,
Seyfert galaxies are not strong X-ray sources. (????)
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