Seyfert Galaxies

Seyfert galaxies are usually spirals with very bright but very small nuclei. They are named for American astronomer Carl Seyfert, who first studied their properties in the early 1940s (see the right panel). Seyfert's are the most commonly observed active galaxies, with about 1 in 100 spirals observed falling in the Seyfert classification.

Example: NGC 7742
The top right image shows a Hubble Space Telescope view of the Seyfert galaxy NGC 7742, which lies about 22 Mpc away in the constellation Pegasus. As is true for most Seyferts, the central region is very bright at visible wavelengths. It is likely that this region contains a huge black hole. Faint spiral arms surround the central region, and contain many blue star-forming regions.

A Small Primary Powerhouse

There is strong reason to believe that the central powerhouse of a Seyfert galaxy is even smaller than the timescale for intensity variation at optical wavelengths would suggest. Much of the emission is probably from larger clouds of dust surrounding the smaller main power source that absorb the primary radiation and re-emit it as infrared (recall that spiral galaxies are rich in dust). Some X-ray observations, which peer partially through the dust, indicate even more rapid variations with a period of days (see the figure below right). This suggests directly that the energy source of a Seyfert galaxy is light days or smaller in extent.

Spectral Characteristics
Seyfert galaxies exhibit a strong continuum from IR through X-ray regions of the spectrum, with emission lines of highly ionized atoms that are sometimes variable. Emission lines suggest a low-density gas source, while high ionization implies a hot source responsible for the ionization. Some Seyferts appear to have jets leading to continuum RF emission, but these are modest on the scale of the jets and RF emission observed for active elliptical galaxies.
Rapid Brightness Changes
Seyfert galaxies can fluctuate rapidly in brightness, exhibiting 50-percent changes in optical brightness in a matter of weeks and even larger changes over periods of months. The brightness variation at X-ray wavelengths for a Seyfert galaxy can be even faster, as shown in the figure below right for the Seyfert galaxy IRAS 13224-3809 (the designation "Seyfert 1" for this galaxy will be defined below). By the arguments given previously for brightness variation in quasars, this rapid fluctuation implies a very compact energy source, no larger than light days in diameter (comparable in size with the Solar System, which is about half a light day in diameter).

Energetic Cores
The cores of Seyfert galaxies are small, appearing almost star-like in many images. Yet the energy output of these cores can be staggering. The brightest Seyfert galaxies emit ten times the power of an average spiral galaxy from a central region not much larger than the Solar System. Emission lines from Seyfert galaxies can be very broad (see the further discussion below). This broadening presumably is caused by high velocities for the emitting clouds, which leads to different Doppler shifts for clouds moving in various directions relative to our line of sight. The broadest lines indicate velocities for the emitting sources as high as 10,000 km/s, which is very large compared with velocities found in the cores of normal galaxies.
Two Kinds of Seyfert Galaxies
The thousand or so Seyfert galaxies that are now known can be subclassified into two groups, Seyfert 1 galaxies and Seyfert 2 galaxies (these are also termed type 1 and type 2 Seyferts, respectively). These classifications have the following characteristics.

  • Seyfert 1 galaxies have both broad and narrow emission lines and are very luminous at UV and X-ray wavelengths. The broad emission lines are associated with allowed transitions in elements like hydrogen, helium, and iron (see the box below). Their width, as noted above, is caused by Doppler broadening that suggests the lines are produced in clouds with average velocities of about 10,000 km/s. The narrow lines are associated with forbidden transitions, for example in twice-ionized oxygen (O III), and their widths imply that the source velocities are generally less than 1000 km/s. The different widths of these two kinds of lines, and the association of broad lines with permitted transitions and narrow lines with forbidden transitions, suggests that they originate in different physical regions of a Seyfert 1 galaxy. (See the discussion of density dependence for forbidden lines in the box below.) The galaxy IRAS 13224-3809 discussed above is an example of a Seyfert 1 galaxy.
  • Seyfert 2 galaxies have relatively narrow emission lines suggesting source velocities less that 1000 km/s, and are weak at X-ray and UV wavelengths but very strong in the IR. Generally, their continuum emission is weaker than for Seyfert 1 galaxies. In Seyfert 2s both the forbidden and allowed lines are narrow, suggesting that both kinds of lines originate in the same region with relatively low source velocities. Seyfert 2 galaxies appear to be about three times more numerous than Seyfert 1 galaxies. The galaxy NGC 7742 shown above is an example of a Seyfert 2 galaxy.

    • (Astronomers also classify some Seyfert galaxies as intermediate between these two cases but for our elementary discussion it will be sufficient to consider only these extreme classifications.) In spite of these differences, we shall see shortly that a single model may be able to explain both types of Seyfert galaxies, and that this realization suggests a possible common cause for all classes of active galaxies and for quasars.

    Allowed and Forbidden Transitions

    As we have noted earlier in conjunction with the color of emission nebulae (the discussion of the interstellar medium in Chapter 23), atomic electromagnetic transitions can be divided into allowed transitions and forbidden transitions. Allowed transitions occur very rapidly, but forbidden transitions are slow. Because forbidden transitions are slower, they are typically observed only in very low density gases. The reason is that an atom excited to a state that can only decay electromagnetically by a forbidden transition is very likely to lose its energy by collision with another atom rather than by emitting a photon, unless the gas density is very low. If the gas density is low enough, the probability of collision is small and the state may live long enough to decay by the forbidden electromagnetic transition. Thus, the primary significance of forbidden transitions for our discussion is that their presence typically signals a gas that has very low density.