NH3in the Central 10 Parsecs of the Galaxy. II. Determination of Opacity for Gas with Large Line Widths

Abstract

The 23 GHz emission lines from the NH3 rotation inversion transitions are widely used to investigate the kinematics and physical conditions in dense molecular clouds. The line profile is composed of hyperfine components that can be used to calculate the opacity of the gas. For intrinsic line widths of a few km s-1, the 18 magnetic hyperfine components blend together to form a line profile composed of five quadrupole hyperfine lines. If the intrinsic line width exceeds one-half of the separation of these quadrupole hyperfine components (~5-10 km s-1), these five lines blend together and the observed line widths greatly overestimate the intrinsic line widths. If uncorrected, these artificially broad line widths will lead to artificially high opacities. We have observed this effect in our NH3 data from the central 10 pc of the Galaxy where uncorrected NH3(1, 1) line widths of ~30 km exaggerate the intrinsic line widths by more than a factor of 2. Models of the effect of blending on the line profile enable us to solve for the intrinsic line width and opacity of NH3 using the observed line width and intensity of two NH3 rotation inversion transitions. By using the observed line width instead of the entire line profile, our method may also be used to correct line widths in historical data where detailed information on the shape of the line profile is no longer available. We present the result of the application of this method to our Galactic center data. We successfully recover the intrinsic line width (Δvint ≈ 15 km s-1) and opacity of the gas. Clouds close to the nucleus in projected distance as well as those that are being impacted by Sgr A East show the highest intrinsic line widths. The cores of the southern streamer and the 50 km s-1 giant molecular cloud have the highest opacities.