A Galactic Center 50-kms$^{-1}$ Molecular Cloud with an Expanding Shell

Abstract

We performed high-resolution observations of a Galactic Center 50-km s$^{-1}$ molecular cloud in the CS $J$$=$ 1–0 line using the Nobeyama Millimeter Array (NMA). The molecular cloud mainly has three different spatial components with large velocity widths of up to 60 kms$^{-1}$. The northwest component is located at an apparent contact point to the Sgr A east shell, and elongated along the boundary of the shell. The large velocity width of the component is responsible for interaction with the Sgr A east shell. The molecular gas distribution in CS line emission is dissimilar to that observed previously in NH$_3$ line emissions. The appearance shows presumably the area of CS line emission enhanced by shock. The central and southwest components are located just out of the Sgr A east shell and far from it, respectively. However, these components have large velocity widths. We found a well-shaped circular molecular shell with expanding motion in the 50-km s$^{-1}$ molecular cloud. This is responsible for the large velocity width. The continuum source in the expanding molecular shell has a steep spectrum in mm-wavelengths, although this was not identified in a previous 5-GHz map. This source may be an SNR with an ionized sheath. From the aspect ratio of an expanding molecular shell of 1.1, the magnetic field in/around the shell is estimated to be smaller than 100$\mu$Gauss. This weak magnetic field is consistent with on-going active star formation in the 50-km s$^{-1}$ molecular cloud. A comparison among CS line emission, low-frequency continuum, and millimeter continuum toward the 50-km s$^{-1}$ molecular cloud suggests a face-on view of the Sgr A region. The molecular cloud is located in the Sgr A halo region.