"Normal" (spiral) galaxies often have emission nebulae in their spiral arms, as have irregulars throughout; these regions, of course, show the corresponding emission lines in their spectra. However, those regions in them which are made up of stars, notably their central regions or nuclei, just show spectra which are made up by the composed light of their stars, thus starlike continuous spectra with usually some dark absorption lines, with details depending on the stellar population which makes up the observed region in the galaxy. From their composition, one would expect that galactic nuclei should have spectra like the old population II stars which they are made of. A small number of galactic nuclei, however, contain a large number of young stars of spectral types O and B (OB stars), and in connection with them, ionized gas; these are the so-called starburst galaxies; a prominent example is M82.
Some galaxies, notably the Seyferts, show large quantities of gas in their nuclei which is not associated with O or B stars. Their nuclei are called Active Galactic Nuclei (AGN's); the galaxies are sometimes called Active Galaxies. While making up the biggest portion, Seyferts are not the only galaxies with AGNs: Other examples are the radio galaxies and the quasars (D.E. Osterbrock subdivides the latter in radio active quasars and radio quiet quasi-stellar objects, QSO's). They all have in common that their high luminosity is not produced by stars.
The peculiarity of Seyfert galaxies was first noted before the nature of galaxies had been revealed, by Edward A. Fath at Lick Observatory in 1908. Fath was studying spectra of the nuclei of "spiral nebulae" (now known to be galaxies) with the 36-inch Crossley reflector at Lick. The very most of them showed starlike absorption-line spectra, which Fath understood easily as the sum of the light of stars composing them (he called them "star clusters") too far away to be resolved. However, the galaxy M 77 (NGC 1068) had a most remarkable spectrum: He recognized 6 prominent emission lines, all known from gaseous nebulae, which we now know to be the following spectral lines: H Beta, [O II] 3727, [N III] 3869, [O III] 4363, 4959, and 5007.
Vesto M. Slipher of Lowell Observatory obtained a much better spectrum of the same galaxy in 1917. Edwin P. Hubble, in 1926, noted these "planetary nebula type" emission-line spectra for 3 galactic nuclei, of M77, NGC 4051, and NGC 4151.
Carl K. Seyfert published his pioneering study in 1943. He found that for some galaxies, including the three mentioned above, the nuclei show peculiar emission lines in their spectra which come from highly ionized atoms, superimposed over a star-like absorption spectrum. He noted that these nuclei were very luminous and appeared almost starlike ("stellar or semistellar), while the emission lines were wider than the absorption lines in normal galaxies.
Seyfert lists 12 galaxies which he states "probably belong to this unusual class of objects": NGC 1068 (M 77), NGC 1275 (Perseus A), NGC 2782, NGC 3077 (in the M81 group), NGC 3227, NGC 3516, NGC 4051, NGC 4151, NGC 4258 (M 106), NGC 5548, NGC 6814, and NGC 7469. His closer investigation concentrated on 6 of these objects: M 77, NGC 1275, NGC 3516, NGC 4051, NGC 4151, and NGC 7469.
Meanwhile, studies have found that NGCs 2782 and 3077, though showing emission lines in spectra, are not to be classified as Seyfert galaxies, and thus to be removed from the list. As put by B.M. Peterson (see link below), the definition of Seyfert galaxies has evolved, and Seyfert galaxies are now identified spectroscopically by the presence of strong, high-ionization emission lines.
In 1965, Donald E. Osterbrook and R.A.R. Parker brought up the hypothesis that the active nuclei of Seyfert galaxies might be thought of as miniature quasars (quasi-stellar radio sources discovered first in 1963).
As was first published by Khachikian and Weedman (1974), one can distinguish two distinct subclasses of Seyfert galaxies, depending on presence or absence of broad bases on the permitted emission lines in their spectra. Seyfert 2 galaxies show only one set of emission lines which are comparatively narrow and originate from low-density ionized gas (electron density 10^3 to 10^6 electrons per cm^3) with widths corresponding to several 100 km/s as indicated from the line width, which is somewhat broader than the emission lines from non-active galactic nuclei. These lines are frequently referred to as "narrow lines" and occur for both permitted and forbidden spectral lines. Seyfert 1 galaxies, in addition, show a set of "broad lines" corresponding to velocities up to 1000 km/s, occurring only for the permitted lines, which indicates higher densities (10^9 electrons per cm^3).
Osterbrook (1981) has introduced the following subclasses of Seyfert 1 galaxies, based on spectroscopic details: 1.5, 1.8 and 1.9. In Seyfert 1.9, the broad lines are only detectable for H alpha lines, while for 1.8, weak broad H beta lines can be detected also, and for Seyfert 1.5 nuclei, the strengths of broad and narrow H beta components are comparable.
After a decade-long research of Seyfert galaxies and other Active Galactic Nuclei (AGNs), including also, e.g., radio galaxies, quasars, and BL Lacertae objects, there is now some consensus that all forms of AGNs are caused by the same sort of physical object, a central supermassive object accumulating gaseous matter from its surrounding neighborhood. The variety of phenomena is simply a consequence of different viewing angles and different rates of matter supply falling into the objects.
Seyferts original list:
Galaxy NGC/IC Type Con RA Dec. Hubble Type m_v Dim Dist M 77 NGC 1068 Sy 2 Cet 02:42.7 -00:01 Sb 8.9 8.2x7.3 60 Per A NGC 1275 Sy 2 Per 03:19.8 +41:31 Pec 11.9 3.2x2.3 230 NGC 2782 -- Lyn 09:14.1 +40:07 SAB(rs)a p 11.6 3.8x2.9 NGC 3077 -- UMa 10:03.3 +68:44 I0 pec 9.8 5.5x4.1 12 NGC 3227 Sy 1.5 Leo 10:23.5 +19:52 SAB(s)a p 10.3 6.9x5.4 NGC 3516 Sy 1.5 UMa 11:06.8 +72:34 (R)SB(s)0: 11.7 2.1x1.8 NGC 4051 Sy 1.5 UMa 12:03.2 +44:32 SAB(rs)bc 10.2 5.5x4.6 NGC 4151 Sy 1.5 CVn 12:10.5 +39:24 SAB(rs)ab: 10.8 6.4x5.5 M 106 NGC 4258 Sy 1.9 CVn 12:19.0 +47:18 Sbp 8.9 20.0x8.4 25 NGC 5548 Sy 1.5 Boo 14:18.0 +25:08 SA(s)0/a 12.6 1.6x1.4 NGC 6814 Sy 1.5 Aql 19:42.7 -10:19 SAB(s)bc 11.2 3.0x3.0 NGC 7469 Sy 1.2 Peg 23:03.3 +08:52 SAB(rs)a 12.3 1.5x1.0Some more bright Seyfert galaxies:
NGC 566 Sy 1 Psc 01:29.0 +32:21 S0 13.5 1.5x0.4 NGC 985 Sy 1 Cet 02:34.6 -08:47 Pec 13.5 1.3 NGC 1566 Sy 1 Dor 04:20.0 -54:56 S(s)bc? 9.4 7.1x4.8 NGC 7742 Peg 23:44.3 +10:46 S 11.5 2.0x2.0 Mrk 335 Sy 1 Peg 00:06.3 +20:12 S0/a 13.8 0.3x0.3 Mrk 1155 Sy 2 01:26.2 +33:24 S0 15.0 0.5x0.3 Mrk 3 Sy 2 06:15.6 +71:02 S0: 14.0 1.8x1.6 Mrk 509 Sy 1.2 20:44.2 -10:43 Compact 13.0 0.5Key: