The Milky Way Galaxy, in which we live, is a giant 20 kpc diameter disk. The center of the galaxy lies about 8 kpc away from the Sun in the constellation Sagittarius. The interstellar matter (ISM) contains a significant amount of dust that hides most of the optical objects in the galactic disk from us. The optical absorption rate in the solar vicinity is about 1 mag per 1 kpc, and the galactic extinction amounts to ∼ 30 mag for the light from the galactic center (GC), or the light is weakened by ∼ 100−(1∕5)20 ∼ 30 = 10−8 ∼ −12 times the original intensity. Namely, the GC cannot be observed in visible light, even if we used the world largest telescopes. The GC and the regions beyond are a world hidden from our eyes.
These deepest parts of the galaxy can be easily visible if we observe them through the radio window. Radio waves are transparent against the interstellar dust because they have much longer wavelengths than the dust, and avoid scattering by dust particles. This situation mimics the reception of radio and TV waves even on a foggy and rainy day. If it is seen in radio, the radio galaxy appears quite different from its optical appearance.
The change in appearance is not only because of the difference in opacity, but also because of the different locations and matter that emit radiation as well as because of the emission mechanism. Knowledge about the emission mechanism can in turn be used to investigate the origin and physics of the emitting regions.
In this chapter the fundamental physics of various emission mechanisms in interstellar space is described, starting with basic formulae for electromagnetic waves. Readers who feel this chapter is too basic may skip here and proceed to the chapters more concerned with astrophysics and astronomy, and may come back when more fundamental knowledge is necessary to understand the underlying physics of the phenomena. In this book, we use the cgs (cm, gram, second) unit system.
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