An overview of active galaxies
An overview of active galaxies

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An overview of active galaxies

7 Continuum emission processes

7.1 Blackbody radiation

Interpretation of the light astronomers collect from AGN depends on understanding the physical processes leading to the emission of that light. Because the conditions in the emitting regions of AGN are very different from those on the surface of the Earth, some of these processes may be entirely unfamiliar to you. This subsection begins with a discussion of blackbody radiation, which should be familiar, and covers material which you will need to appreciate Peterson's discussion of AGN.

Blackbody radiation is in thermal equilibrium with matter at a fixed temperature. Often the emission from astronomical objects is a close approximation to this thermal radiation. Many thermal sources of radiation, for instance stars, have spectra which resemble the blackbody spectrum, which is mathematically described by the Planck function. The spectrum of an accretion disc can be modelled as the sum of blackbody spectra of varying temperature, and this work can be applied to accretion discs in AGN as well as those in binary star systems.

The blackbody spectra shown in Figure 9 illustrate the way that the spectra peak at wavelengths which depend on temperature. Away from the peak, the shape of the Planck function at substantially shorter wavelengths (high energies) is known as the Wien tail, which is mathematically described by

Figure 9
Figure 9 The blackbody spectrum for various temperatures. The peak emission occurs at a wavelength described by the Wien displacement law. The shape at substantially longer wavelengths is known the Rayleigh–Jeans tail, at substantially shorter wavelengths it is the Wien tail

The shape of the Planck function at substantially longer wavelengths than the peak (low energies) is known as the Rayleigh–Jeans tail, which is mathematically described by

These ‘tails’ at both extremes of wavelength are sometimes referred to as the long-wavelength (or low-energy) cutoff and the short-wavelength (or high-energy) cutoff.


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