Abstract
Features of the 21-cm line radiation that have often been attributed to the galactic center or to activity there include, near the equator, the ‘3-kpc arm’, the ‘expanding arms’ at +70 and +135 km s-1 and the ‘nuclear disk’, and, away from the equator, features observed by Cugnon, by Shane, by van der Kruit, and by Sanders, Wrixon, and Penzias, among others. Models of the features in the equatorial plane are discussed on the basis of the form of the T b(l, V) contours that they generate. Both explosions in the nucleus and resonance with a spiral wave have been suggested as the cause of the 3-kpc arm and related features. Some of the models put limits on the mass distribution, particularly the amount of condensed mass at the center. Higher-latitude features have been less thoroughly modeled, but explosions in the nucleus and resonant orbits have also been suggested for their origins.
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