Normal Galaxies in the Infrared

Abstract

A normal galaxy is one that derives its luminosity primarily from nuclear burning in stars, and is at neither the high nor the low extreme of the luminosity distribution. In such galaxies, the visible and ultraviolet is the only direct window onto the photospheres of stars, whereas the rest of the spectrum reflects reprocessed light from stars (Fig. 1). Dust in the interstellar medium reprocesses part of the stellar luminosity into infrared emission from a few μm to 1 mm or longer wavelengths. The radio emission at millimeter wavelengths derives from thermal emission from ionized plasmas in HII regions, whereas the synchrotron emission from cosmic ray electrons (CR e⁻) trapped in the magnetic field of the galaxy fill in the cm-wavelength spectrum. In the X-rays, the luminosity is dominated by very hot plasmas at T ≳ 10 K⁶ created by shocks from supernova explosions. The global spectrum of normal galaxies has some very stable signatures, and other quite variable aspects. Much of this article is an attempt to understand what makes for stability in the spectrum, and what significance to attach to the variable parameters.

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