A Galactic Center 50-km s-1 Molecular Cloud with an Expanding Shell

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 km s(-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 NH3 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.