Detection of 1720 MHz Hydroxyl Masers at the Galactic Center: Evidence for Shock-excited Gas and Milligauss Fields

Abstract

Radio observations of the Sgr A region have been carried out at the 1720 MHz transition of the OH molecule, using the Very Large Array and the Australia Telescope Compact Array. OH(1720 MHz) maser emission is detected at seven different positions within a few arcminutes of the Galactic center. Most of the masers are located to the southeast of Sgr A*, at the boundary of the Sgr A East nonthermal source with the M-0.02-0.07 molecular cloud. One maser is located within the circumnuclear disk (CND), and another may arise as the result of an expansion of Sgr A East into molecular gas to the northwest of Sgr A*. It is likely that these maser features are excited by the interaction of shocks in the Galactic center with adjacent molecular gas. Significant circular polarization is observed toward 10 distinct spectral components in the seven maser spots; if the V signal is due to Zeeman splitting, preliminary measurements of strong fields [local line-of-sight components of B (BLOS) are estimated to be between 2 and 4 mG] are inferred toward all sources. The direction of these magnetic fields is positive for all masers except the CND maser, which is negative. The potential for these maser features as a diagnostic of shocked gas in the Galactic center is discussed.