THE NUCLEUS OF OUR GALAXY

The subject of this review is the central 100 parsecs of our Galaxy, with a strong focus on the central few parsecs. Observations of the electromagnetic spectrum over 13 orders of magnitude in wavelength show a broad range of phenomena involving a number of physical processes. We discuss the stellar and interstellar components, the importance of magnetic and gravitational forces, the evidence for stellar formation and a central massive black hole, and the origin and nature of ionization, outflows and interstellar gas dynamics. The density of stars is approximately proportional to R-2 from Galactocentric radii of a few tenths to more than one hundred parsecs, the central density perhaps being more than 10(7) times as large as in the solar vicinity. On all scales one finds massive stars that must have formed within the last 10(7) years. On scales greater-than-or-equal-to 10 pc there are a number of compact x-ray sources, including a spectacular black hole candidate exhibiting time variable, hard x-ray/gamma-ray emission and a twin radio jet. In the central regions stellar collisions are probably frequent and may affect the stellar populations. The Galactic nucleus contains a remarkable concentration of dense and excited interstellar matter. The average density of gas and dust and the average thermal pressures are 10(2)-10(5) times those found in the solar neighbourhood. A component of very hot (10(7)-10(8) K) gas may be the result of a few hundred supernova explosions that occurred in the central 10(2) pc during the last 10(4)-10(5) years. The coronal gas escapes the disk as a Galactic wind. The region between 3 and 30 pc shows evidence for poloidal magnetic fields out of the plane of the disk of the Galaxy, of strength 0.1 to a few mGauss. A circum-nuclear disk or torus of dense molecular gas orbits the centre between 1.5 and 5 pc and probably is fed with gas from massive molecular clouds at R greater-than-or-equal-to 10 pc. The central parsec has a relatively low average gas density, but contains a number of predominantly atomic and ionized streamers. The luminosity of the central few parsecs appears to be dominated by a cluster of hot stars, whose ultraviolet radiation and winds excite, photoionize and perturb the gas streamers. These massive stars may be the products of a small burst of star formation that occurred a few million years ago when there may have been a much greater nuclear concentration of interstellar gas than is presently observed. Alternatively, or additionally, massive stars in the central core may form as the result of mergers of low mass stars if the central stellar density in fact significantly exceeds 10(7) M. pc-3.