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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.
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
Although in many cases the classes of active galaxies were originally
thought to be distinct, the modern view is that the different classes of active
galaxies are related phenomena, and that they all may be powered by rotating,
supermassive black holes at their centers.
Some References:
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