The center of the galaxy: A summary of I.A.U. Symposium No. 136

Abstract

The first international conference on the central region of our galaxy was held at UCLA during July 25–29, 1988. Radio, infrared, X-, and γ-ray astronomers, benefitting from recent technological advances, confronted theorists, as well as each other, with impressive new imagery in each of these domains. Traditionally hidden by an impenetrable veil of dust, the galactic center is now largely transparent, particularly in the near-IR, where the extinction toward the galactic center is an order of magnitude less than at optical wavelenghts. On the large scale, one observes a large reservoir of molecular gas, some of which is exhibiting complex, noncircular motions. Some of the kinematical behavior is best explained in terms of energetic explosions occurring about 10 7 years ago. The radio continuum reveals numerous filaments of ionized gas, and strong, poloidal magnetic fields are implied quite unlike the field anywhere else in the galaxy. On small scales, much effort is going into the attempt to identify a unique object at the nucleus that may or may not be a massive black hole (a mass of 3 × 10 6 M ⊙ has been defended). Candidates exist for a compact mass concentration, and for a strong central luminosity source, but they are not necessarily one and the same, and there are problems identifying any of the candidates with a massive accreting object at the center. Surrounding the dynamical center of the galaxy is the prominent radio source Sgr A, which includes a warm, turbulent circumnuclear disk on 5 to 10 pc scales, streamers of ionized gas which may represent flows of material into the central potential well, and a background shell of nonthermal radio emission, probably a superimposed supernova remnant. This summary describes some of the current research activity on the galactic center, with particular attention to the implications of X- and γ-ray observations.