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Eruptive phenomena near the galactic centre.

Four categories of expanding features are described: (a) Features observed in the 21 cm radiation of atomic hydrogen, (b) agglomerations of dense molecular clouds, (c) an extended ionized-hydrogen feature, (d) high-velocity "clouds" of ionized gas within ~ 1/2 pc from the centre. (a) These lie between R ~ 1 and ~ 4 kpc, in a region where there is little or no normally rotating gas. The velocity components in radial direction are between 50 and 170 km s-1 and are usually, if not always, directed away from the centre. The largest have masses ranging from two to eighty million times that of the Sun. They have arm-like structures. These seem to be no expanding H I features within R ~ 750 pc. The hydrogen within this distance appears to be quiescent, and rotates with the circular velocity. It permits derivation of the gravitational field. (b) Dense molecular clouds exist within R ~ 250 pc. Their distribution is extremely asymmetrical. Velocities are much lower than the circular velocities; they must move along highly excentric paths, and have probably been expelled from the centre. Direct evidence for such expulsion is given by a dense molecular ring at R = 190 pc which expands at an average velocity of 150 km s-1. (c) Ionized hydrogen shows an arc-like structure at R ~ 40 pc, presumably likewise ejected from the centre. (d) Observations of a Ne II line at 12.8 μm show highly turbulent gas with random velocities up to at least 200 km s-1 in a region extending to about 1/2 pc from the centre. These clouds are reminiscent of the conditions in type II Seyfert nuclei, though on a very much smaller linear scale. In the same region there is a strong concentration of radiation at wavelengths of 10 μm and longer, due to dust, probably heated by supergiants or star clusters. A noteworthy property of the central region is that the expanding H I- and molecular features, and probably also the rotating H I disk, lie in a plane which is inclined to the general galactic plane; the inclination is about 10°. The radio source at the galactic centre (Sagittarius A) contains an extremely compact component. The diameter of this nucleus is about 0 Prime-.001, corresponding to 10 astronomical units, or 1.5 × 109 km. It may contain the energy source for the observed expulsion phenomena. These require initial kinetic energies between 1054 and 1055 erg, to be imparted at intervals of a few million years.