The Nature of the Molecular Environment, within 5 pc of the Galactic Center

Robeson M. Herrnstein and Paul T. P. Ho

(1) Department of Astronomy, Columbia University, 550 West 120th Street, Mail Code 5246, New York, NY 10027,
(2) Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138,

Paper: ApJ, Jan 2005, in press


EPrint Server: astro-ph/0409271


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 NH\3 made with the Very Large Array. Updated line widths and NH\3(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 E_SN=(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 by Huttemeister et al. (1993a) on larger scales. The largest observed line ratios, however, cannot be understood in terms of a two-temperature model, and most likely result from absorption of NH\3(3,3) emission by cool surface layers of clouds. By comparing the observed NH\3 (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.

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