Nonthermal Spectra

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 righthand figure.

Normal Galaxy Spectra

A collection of billions of ordinary stars is expected to be approximately blackbody, because the spectrum of the individual stars is blackbody. Such a spectrum is dominated by a continuum that peaks in the visible spectrum. For example, the Milky Way emits radio waves, but its radio luminosity is about a million times smaller than its visible luminosity. In addition, the lines observed for normal galaxies (as for its stars) are mostly absorption lines, with few emission lines. Thus, spectra for normal galaxies, as for the stars that they contain, are typically a continuum peaking at visible-light wavelengths with absorption lines superposed on the continuum.

Spectra for Some AGNs

In contrast, active galaxies are dominated by nonthermal emission and the strongest lines are emission lines. The nonthermal emission from these galaxies is largely associated with synchrotron radiation, which is illustrated in this animation and was introduced in Chapter 5.

The lefthand figure illustrates the difference between the thermal emission spectrum of a normal galaxy and the nonthermal spectrum associated with quasars and a kind of AGN called a Seyfert galaxy that we shall discuss in a separate module of this chapter. Obviously, these spectra imply that active galaxies and quasars are something very different from normal galaxies. On the other hand, the similarity of the spectra for quasars and Seyfert galaxies suggests that there might be a relationship between quasars and active galaxies.