GTN GLOBAL TELESCOPE NETWORK |
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The radiation from ordinary galaxies is dominated by thermal radiation from stars and dust. Active galaxies are dominated by non-thermal processes like synchrotron radiation and thyermal processes with exceedingly high temperatures (i.e. large energies). Activity is often indicated by the presence of emission lines in the spectrum of the object. The temperatures involved vary from about 100°K for the dust in the interstellar medium to several thousand degrees for stellar photospheres. Stars generally fall in the range between 3000° and 30000° K. Stars emit most of their energy near the visual wavelength region. The hottest stars actually have their peak emission in the UV while the coolest stars have their peak emission in the near-IR to mid-IR. The Wien displacement rule describes the relationship between the temperature of a thermal source and the wavelength of the peak emission. lmax = 2898 / T (in mm) Here T is the temperature in °K.
At wavelengths longer than the peak the radiation emitted decreases slowly with increasing wavelength. (At wavelengths shorter than the peak there is a rapid decrease.) At radio wavelengths (or frequencies) we are observing on the long wavelength tail of the energy distribution for ordinary stellar and galactic sources. Thus, in the radio region, ordinary astronomical sources which are thermal emitters are brighter at higher frequencies (shorter wavelengths). Synchrotron radiation is produced when energetic electrons move through a region of space containing a magnetic field. The motions of the electrons are deflected by the magnetic field and the electrons spiral around the magnetic field lines. Since circularily moving electrons are being accelerated, these electrons emit radiation. The spectral distribution of this radiation may be described by the following relation. F(n) = F0 n-a = F0 / na Here a is termed the spectral index of the radiation. This index may be related to the energy distribution of the electrons producing the radiation, and to the strength and uniformity of the magnetic field. Radio galaxies and quasars have similar radio properties and have spectral indices between 0.7 and 1.2. Compact radio sources have a flatter spectrum and tend to have a spectral index near a = 0.4. Note that in this system thermal sources have negative spectral indices. Synchrotron sources decrease in brightness at higher frequencies, while thermal sources increase in brightness. |
