Seyfert Galaxies

Seyfert galaxies were first identified by Carl Seyfert in 1943. Initially they were classified 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 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. It appears that about 1% of all spiral galaxies might 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.
Seyfert Galaxy NGC 4258

Seyfert (and LINER)
Galaxy NGC 4258, Image: Soren Larsen, UC Lick Observatories

Since Seyferts generally exhibit both permitted and forbidden lines, the emission must be produced in different regions of the galaxy
where different physical conditions prevail. Forbidden lines can only be produced in low density regions, whereas higher densities must occur where
permitted lines are produced. In terms of their spectra there are now three sub-types of Seyfert recognized.

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 5,000 or 10,000 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.

Seyfert Spectra


Example spectra of Type I and Type II Seyfert galaxies. Upper spectrum is actually more like a Type 1.5 (Note the narrow lines extending above
the core of the braod lines). Image from Bill Keel’s Slideset on AGN.

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 5,000-10,000 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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