Active Galaxies

Some galaxies exhibit evidence of extremely violent processes taking place within them. The most common signatures are non-thermal emission from the rf to X-ray region of the spectrum, and/or jets and unusual structure associated with the visual appearance of the galaxy. As we have seen, in some cases this activity can be attributed an interaction among galaxies. However, in many other cases the source of the activity appears to be internal to the galaxy in question. We refer to these as active galaxies.

Generally, active galaxies have spectra that look rather different from that to be expected from a collection of billions of stars, as illustrated in the top right figure. The nonthermal emission from these galaxies is largely associated with synchrotron radiation, which is illustrated in the above figure and in this animation. As we have noted earlier, the emission of light by accelerated electrons spiraling in a strong magnetic field is nonthermal and commonly produces polarized light.

Active galaxies exhibit some combination of the following characteristics:

• Unusual appearance, particularly of the nucleus
• Jets emanating from the nucleus
• High luminosity
• Non-thermal continuum emission, often polarized, with excess radiation at RF, IR, UV, and X-Ray wavelengths
• Rapid variability from compact energy source in the nucleus

Some References:

• Violence in the Cosmos Active Galaxies
• Gallery of Quasars and Active Galaxies