Active Galaxies

Some galaxies exhibit evidence of extremely violent processes taking place within them. The most common signatures are nonthermal emission from the radio frequency (RF) to X-ray region of the spectrum, and/or jets and unusual structure associated with the visual appearance of the galaxy. We refer to these as active galaxies. Since the source of the activity (though not necessarily all of its consequences) is normally concentrated in the nucleus of the galaxy, it is also common to refer to them as active galactic nuclei or AGNs.

The top right figure illustrates one type of active galaxy, a radio galaxy. Superposed on an optical image (the small blue and white object in the center) is a map of radio frequency emission (yellow and red) obtained by the Very Large Array of the National Radio Astronomy Observatory. The enormous lobes of RF emission coupled to the center by jets of RF emission are one possible signal of an AGN. More generally, active galaxies exhibit some combination of the following characteristics:

Unusual appearance, particularly of the nucleus

Jets emanating from the nucleus

High luminosities relative to normal galaxies, but generally smaller than for quasars

Excess radiation at RF, IR, UV, and X-ray wavelengths

Nonthermal continuum emission, often polarized, perhaps with broad and/or narrow emission lines

Rapid variability from a compact energy source in the nucleus

Although many AGNs are relatively nearby, generally they are more likely to be found at larger distances (longer look-back times), implying that they were more common earlier in the Universe's history. We note that many of these characteristics are also exhibited by quasars, and we shall see that quasars may be closely related to the nuclei of active galaxies. In fact, we shall eventually conclude that quasars essentially are a particularly luminous form of active galactic nucleus.