Active Galaxies

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Not all active galaxies blaze with the strength of a quasar. They do exhibit a non-thermal spectrum that has no peak and does not depend on the temperature. Also, their energy is generated in the nuclei of galaxies. The active galaxies are less energetic cousins of the quasars. Their luminosity is between the luminosities of typical galaxies and the powerful quasars. Whatever is going on in quasars, is going on in active galaxies to a lesser extent.

One type of active galaxy is the Seyfert galaxy, named after Carl Seyfert who was the first to discover the peculiar spectra of these types of galaxies. A Seyfert galaxy is a spiral galaxy with a compact, very bright nucleus that produces a non-thermal continuous spectrum with broad (fat) emission lines on top. Some of the emission lines are produced by atoms that have several electrons removed from them. Such highly ionized atoms are found only in regions of intense energy. Many Seyfert nuclei are in disks with distorted spiral arms and a companion galaxy nearby that is probably gravitationally interacting with the galaxy.

NGC 1566
The Seyfert galaxy NGC 1566.

The energy of Seyfert galaxy nuclei fluctuates quickly like the quasar fluctuations, so the energy generator must be quite small. The broad emission lines are produced by gas clouds moving at about 10,000 kilometers/second. The doppler shifts of the gas moving around the core widens the emission lines. Some Seyfert galaxies have narrow emission lines instead of broad emission lines and are bright at infrared wavelengths. These are called ``Type 2'' Seyferts to distinguish them from the classical ``Type 1'' Seyferts with the broad emission lines.

Another type of active galaxy is the radio galaxy, which emits huge amounts of radio energy. The radio emission comes from the core AND from very large regions on either side of the optical part of the galaxy called ``radio lobes''. The radio lobes can extend for millions of light years from the center of the galaxy. The radio emission from normal galaxies is thousands to millions of times less intense and is from the gas between the stars. Most radio galaxies are elliptical galaxies. The spectrum of the radio emission has the same non-thermal (synchrotron) shape as the quasars and Seyferts. The radio lobes are produced from electrons shot out from the nucleus in narrow beams called jets. When the electrons in the beam hit the gas surrounding the galaxy, the beam spreads out to form the lobes.

3C219 3C334
Quasars can have huge radio lobes too.

Some lobes are swept out into arcs behind the galaxy. This is probably caused by the galaxy moving through the gas around the galaxy. A third type of active galaxy called BL Lacertae objects (BL Lac objects for short) are probably radio galaxies with their jets pointed right at us. The energy from BL Lac objects varies very quickly and erratically.

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last updated: 28 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel