In 1943, Carl Seyfert published a list of 12 otherwise normal spiral galaxies which contain anomolously bright central nuclei. These galaxies are now known as "Seyferts." The following figure shows successively deeper images of the Seyfert galaxy NGC4151. In short exposures, only the bright nucleus is apparent, but deeper images reveal the normal spiral galaxy around it. In some Seyferts, the central nucleus outshines the whole surrounding galaxy. The central nucleus light output varies on timescales of less than a year So, the emitting region must be less than ~ a light year across, as the source cannot vary coherently on timescales shorter than this due to light travel time effects:

Spectra of the nuclei in Seyferts are non-stellar. They contain: Non-thermal continuum emission Narrow (=> low velocity), forbidden (=> low density material) lines which do not vary detectably (=> large emitting region) Broad (=> high velocity), forbidden permitted lines which vary on fairly short timescales (=> small emitting region) Also, strong emission in the radio, infrared, ultraviolet, and X-ray parts of the spectrum. Seyfert galaxies have been classified into two basic types: Seyfert 1 galaxies are bright in the optical, and have both broad and narrow lines in their spectra. Seyfert 2 galaxies are fainter in the optical (but the same as Seyfert 1s in the infrared), and they only have narrow lines in their spectra. Intermediate cases with weak broad emission lines also exist, and are classified as Seyfert 1.3, 1.6, etc.

In the broad-line region (BLR) The Keplerian orbital speeds of the clouds around the central massive body will be large => lines are Doppler broadened. Density is high => no forbidden lines are emitted In the narrow-line region (NLR) The Keplerian orbital speeds of the clouds will be much smaller => lines are narrow Density is low => forbidden lines are emitted So, if the above Seyfert were viewed from direction (1), you would see: Broad permitted lines Narrow Forbidden Lines Bright continuum from the central engine i.e. a Seyfert 1

If, on the other hand, it were viewed from direction (2), you would see: No broad permitted lines (obscured by dust torus) Narrow Forbidden Lines No bright continuum from the obscured central engine except in the infrared and X-ray region, which gets through the dust i.e. a Seyfert 2

Radio Galaxies:

As radio astronomers mapped the sky, they found several bright radio sources associated with distant galaxies. One of the brightest was the radio source Cygnus A. Cygnus A is a double-lobed radio source. Also notice the jets from the center. At the center sits a giant elliptical galaxy, at a distance of ~ 200 Mpc. At this distance, the lobes are separated by more than 100 kpc, and they have a radio luminosity of 1045 erg/s, 106 times greater than the radio luminosity of normal galaxies. Radio galaxies, unlike Seyferts, are generally hosted by elliptical galaxies, not spirals. Seyferts are also radio quiet.

M87, the central elliptical in Virgo, also is a radio galaxy. It is close enough that we can examine its center in detail, where we can actually see the jet in the optical

These observations and arguments all suggest that active galaxies are powered by material accreting onto a massive central black hole in the nuclei of galaxies. Accretion disk: hot, luminous gas falling into the black hole Jets: charged particles moving at relativistic speeds out of the nucleus Broad-line clouds: Gas clouds near the accretion disk, turbulent motions at high speed Dusty torus: a ring of denser gas and dust surrounding the nucleus. Narrow-line clouds: Gas clouds further out, moving more slowly

• To achieve the necessary luminosity, ~ 1-10 Msun/yr must be accreted onto the black hole. Under this model, Seyfert galaxies, radio galaxies, and quasars are similar objects -- accretion powered active nuclei. They may differ in total luminosity (quasars vs seyferts) or in radio power (seyferts vs quasars/radio galaxies) or in host galaxies (seyferts vs radio galaxies)

Quasars:

Quasi-Stellar Objects, QSOs, or quasars are believed to be the most distant and luminous objects in the Universe. Quasars were discovered in 1963 as radio telescopes began to be able to pinpoint sources of radio waves more precisely. Observations of an occultation of the radio source 3C273 by the moon made by Cyril Hazard and colleagues showed that the position of the radio source was coincident with a 12th magnitude stellar object (about 250 times fainter than can be seen by eye, but bright by astronomical standards). Normal stars like the sun are known not to be strong radio sources so 3C273 and other similar "radio stars" were dubbed Quasi-Stellar Radio Sources soon shortened to quasar. We now know that there are about 15 times as many quasars which are not strong sources of radio waves as there are radio quasars.