The Nature of the Molecular Environment within 5 Parsecs of the Galactic Center

Abstract

We present a detailed study of molecular gas in the central 10 pc of the Galaxy through spectral line observations of four rotation inversion transitions of NH3 made with the Very Large Array. Updated line widths and NH3 (1, 1) opacities are presented, and temperatures, column densities, and masses are derived for the major molecular features. We examine the impact of Sgr A East on molecular material at the Galactic center and find that there is no evidence that the expansion of this shell has moved a significant amount of the 50 km s-1 giant molecular cloud. The western streamer, however, shows strong indications that it is composed of material swept up by the expansion of Sgr A East. Using the mass and kinematics of the western streamer, we calculate an energy of ESN = (2-9) ? 1051 ergs for the progenitor explosion and conclude that Sgr A East was most likely produced by a single supernova. The temperature structure of molecular gas in the central ~20 pc is also analyzed in detail. We find that molecular gas has a two-temperature structure similar to that measured on much larger scales in the Galactic center region. The largest observed line ratios, however, cannot be understood in terms of a two-temperature model and most likely result from absorption of NH3 (3, 3) emission by cool surface layers of clouds. By comparing the observed NH3 (6, 6)-to-(3, 3) line ratios, we disentangle three distinct molecular features within a projected distance of 2 pc from Sgr A*. Gas associated with the highest line ratios shows kinematic signatures of both rotation and expansion. The southern streamer shows no significant velocity gradients and does not appear to be directly associated with either the circumnuclear disk or the nucleus. The paper concludes with a discussion of the line-of-sight arrangement of the main features in the central 10 pc.