Molecular gas near the galactic center

The interstellar matter within 400 pc of the center of our Galaxy was studied using millimeter-wave spectroscopy. The region -20.0less than or equal to + 20.0, -00.467 less than or equal to b less than or equal to + 00.467 was surveyed in the lambda = 2.72 mm transition of 13CO at spacings of 00.067 and 00.133 in l and b respectively. Fourteen distinct kinematic features were identified in the maps of these data; four of the features had not been described before in molecular surveys. Most of the high-velocity features and the nuclear disk are tilted to the galactic plane by 70 and inclined to the line of sight by 720, but the sheet of high-density gas which surrounds Sgr A and B is not measurably tilted. Spectra of the 12CO and C18O emission lines near lambda = 2.60 and 2.73 mm were taken along eight lines of sight within the survey region to estimate the excitation temperatures and column densities: two 16' x 16' densely sampled maps were made in 13CO to estimate cloud sizes. Isocyanic acid (HNCO) was fortuitously found at four of these positions in emission with large velocity widths. Two forms of molecular gas were seen in the central region: (1) clouds with radii of 20 pc, excitation temperatures approximately 8 K, and mass of approximately 105 mass of sun and (2) diffuse regions with T/sub x/ of 25K, densities of > 5 x 104 cm-3, and linewidths > 30 km s-1. A total mass of roughly 1.2 x 108 Mass of sun was derived for the interstellar matter in the inner 400 pc of the Galaxy. A model of closed orbits in a barlike bulge potential was developed to simulate the kinematic motions of the gas in the inner 400 pc of the Galaxy. The modelaccounts for most but not all of the high-velocity features, the + 135 km s-1 expanding arm being the most difficult to explain