Astronomy 162: Professor Barbara Ryden

Monday, March 3

ACTIVE GALAXIES

``A mathematician may say anything he pleases, but a physicist must be at least partially sane.'' - J. Willard Gibbs

Key Concepts

Quasars, blazars, Seyfert galaxies, and radio galaxies are all active galaxies.
Active galaxies contain an accretion disk around a central black hole, with two perpendicular jets.
An active galaxy's appearance depends on the orientation of the accretion disk.

(1) Quasars, blazars, Seyfert galaxies, and radio galaxies are all active galaxies.

In an ordinary galaxy:

Most light comes from stars.
Intensity of the light peaks at visible wavelengths.
Light is distributed throughout the galaxy.

In an active galaxy:

Most light comes from gas.
Intensity of the light peaks at radio wavelengths.
The nucleus is highly luminous.

Quasars are a type of active galaxy. The luminosity of a quasar's ``quasistellar'' nucleus can be from 10 to 100,000 times the luminosity of our galaxy (which is an ordinary galaxy, by the way). Quasars are powered by gas falling inward toward a central supermassive black hole. Around 11 billion years ago, quasars were much more numerous than they are now. So, where have all the quasars gone? As we will see, active galaxies are still around; they're just less active because they aren't fed as much gas.

Blazars are a type of active galaxy which are still present today. Blazars, like quasars, were first discovered as ``quasistellar'' points of light. Blazars can vary significantly in brightness in less than a day, indicating that most of their light must come from a region less than one light-day across (about 200 AU). Long-exposure photographs of blazars reveal ``fuzz'' around the quasistellar source. The ``fuzz'' is elliptical in shape, and has the absorption line spectrum typical of ordinary galaxies. This indicates that blazars are elliptical galaxies with highly luminous central nuclei. (The nucleus itself has neither emission lines nor absorption lines in its spectrum).

Seyfert galaxies are a type of active galaxy; they are spiral galaxies with extremely bright nuclei. The luminosity of Seyfert galaxies ranges from 0.1 to 10 times the luminosity of our galaxy. About 2% of spiral galaxies are Seyferts. The spectra of Seyfert galaxies are characterized by emission lines from highly ionized gas. In some Seyferts, the emission lines are broadened by random motions up to 10,000 kilometers/second. Within the nuclei of Seyfert galaxies, then, there exists very hot gas which is swirling around very rapidly. The luminosity of the nucleus of a Seyfert galaxy can vary wildly on timescales of less than a month. This implies that the size of the nucleus is less than one light-month (= 5000 A.U. = 0.025 parsec). Thus, a very large amount of energy is emerging from a very small volume.

Aside from their bright nuclei, Seyfert galaxies look like ordinary spiral galaxies. Here, for instance, is the Seyfert galaxy NGC 1566:

[Image source: Copyright © Anglo-Australian Observatory]

Radio galaxies are a type of active galaxy; they are elliptical galaxies with jets of gas extending away from a central nucleus. The jets emit radio waves by the synchrotron process (electrons accelerated by a magnetic field). About 0.01% of all galaxies are radio galaxies. The luminosity of radio galaxies at radio wavelengths (just radio waves, not including shorter wavelengths) is from 0.1 to 10 times the luminosity of the Milky Way at all wavelengths.

Example of a WEAK radio galaxy: M87 (a giant elliptical galaxy in the Virgo cluster -- you've seen its mug shot in the Galaxy Gallery.)

Length of jet = 2000 parsecs
Speed of jet = 25,000 km/sec (1/12 the speed of light)
Luminosity of jet = 0.01 L_MW

As radio galaxies go, M87 is very weak. If it were much further away than it is (17 Mpc), its radio emission would not be very conspicuous. The jet in M87, in addition, is smaller than the galaxy itself.

Example of a STRONG radio galaxy: Cygnus A (a peculiar elliptical galaxy 240 Mpc away from Earth). In a photograph taken at visible wavelengths, Cygnus A looks like a distorted elliptical galaxy, a few thousand parsecs across. At radio wavelengths, however, we see a pair of tremendously long jets of gas, which dwarf the central galaxy of stars.

Length of jets = 50,000 pc apiece
Speed of jets = close to speed of light
Luminosity of jets = 4 L_MW

The jets of Cygnus A are very straight and narrow until they are slowed by plowing through intergalactic gas; at this point, they broaden into two lobes of turbulent, swirling gas. Shown below is a false-color radio image of the jets and turbulent lobes of Cygnus A.

[Image source = Very Large Array, National Radio Astronomical Observatory]

(2) Active galaxies contain an accretion disk around a central black hole, with two perpendicular jets.

The energy source in the nucleus of active galaxies must satisfy the following conditions:

Luminosity of up to 10 L_MW
Variable on timescales of days or weeks
Capable of producing jets of gas up to 100,000 pc long.

The most probable energy source is the accretion of gas onto a supermassive black hole.

At the heart of every active galaxy is a supermassive black hole. A `supermassive' black hole, in this context, is one with a mass of 1 million to 10 billion M_sun. Black holes with masses in this range will have Schwarzschild radii of 0.02 to 200 A.U. (that's about 10 light-seconds to 1 light-day). Blackbody radiation comes from an accretion disk of hot gas swirling around the black hole like water around a drain. disk of hot gas swirling around the black hole. The gas becomes hotter and hotter as it swirls inward, so most of the light comes from the inner parts of the accretion disk (within 10 times the Schwarzschild radius). Instabilities in the accretion disk cause the variations in luminosity that are seen in Seyfert galaxies.

Why should a supermassive black hole produce jets? We know, after all, that gravity is an attractive force, not a repulsive force. If gas is falling TOWARD the black hole in the accretion disk, why should other gas be simultaneously accelerated AWAY from the black hole in a jet? The detailed mechanism which creates jets is poorly known, much to the frustration of astronomers. The general picture, however, looks something like this:
The inner regions of the accretion disk is very hot. It is so hot, in fact, that gas starts to evaporate from the disk. As the hot ionized gas drifts away from the disk, it is accelerated and squeezed into a narrow jet by the disk's magnetic field. As the electrons of the ionized gas are accelerated by the magnetic field, they emit the synchrotron emission that we detect with our radio telescopes here on Earth.

(3) An active galaxy's appearance depends on the orientation of the accretion disk.

Imagine that the accretion disk is oriented edge-on with respect to you. In that case, you will have a good view of both jets, one stretching out to either side. The hot inner regions of the accretion disk, however, will be hidden away from you by the outer, cooler parts of the disk. You will see, in this case, a double-lobed radio galaxy like Cygnus A.

Now imagine that the accretion disk is tilted, so that you see it at an angle. In this case, you will see both blackbody radiation from the inner regions of the disk AND synchrotron emission from the jets. If the result is very luminous, you will call what you see a quasar; if it's a little less luminous, you will call it a radio galaxy.

Now imagine that you see the accretion disk face-on, so that one of the jets is pointing straight toward you, and the other one (pointing away from you) is hidden by the accretion disk. In this case, with a jet pointing directly at you, you will receive a huge quantity of synchrotron emission. The resulting radio source is what we call a blazar.

Prof. Barbara Ryden (ryden@astronomy.ohio-state.edu)

Updated: 2003 Mar 3