Introduction to active galaxies
Introduction to active galaxies

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Introduction to active galaxies

7 course Summary

7.1 The spectra of galaxies

  • The spectrum of a galaxy is the composite spectrum of the objects of which it is composed.

  • The optical spectrum of a normal galaxy contains contributions from stars and HII regions. An elliptical galaxy has no HII regions and has an optical spectrum that looks somewhat like a stellar spectrum but with rather fainter absorption lines. A spiral galaxy has both stars and star-forming regions, and its optical spectrum is the composite of its stars and its HII regions (which show rather weak emission lines).

  • The widths of spectral lines from a galaxy may be affected by Doppler broadening due either to thermal motion or to bulk motion of the emitting material.

  • An active galaxy has an optical spectrum that is the composite of the spectrum of a normal galaxy and powerful additional radiation characterised by strong emission lines. The broadening comes from bulk motion of the emitting gas.

  • A broadband spectrum comprises radiation from a galaxy over all wavelength ranges. To judge a broadband spectrum fairly, it is necessary to use a λFλ plot on logarithmic axes which is called a spectral energy distribution (SED).

  • The SEDs of normal galaxies peak at optical wavelengths while the SEDs of active galaxies show emission of substantial amounts of energy across a wide range of wavelengths that cannot be attributed to emission from stars alone.


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