Absorption and Emission in the Four Ground-State OH Lines Observed at 18 cm with the VLA Towards the Galactic Centre

Abstract

The OH distribution in the Sgr A Complex has been observed in the 1612, 1665, 1667, and 1720 MHz OH transitions with the Very Large Array (VLA), in the BnA and DnC configurations. Spectral line maps have been produced with a channel velocity resolution of about 9 km s–1 and with angular resolutions of 4″ × 3″, and 24″ × 22″, respectively. Some clear results are highlighted here: i) the existence of an OH streamer inside the Circumnuclear Disk (CND) near Sgr A*, ii) absorption from the CND, iii) strong absorption towards the eastern and most of the western parts of the Sgr A East shell, iv) lack of absorption towards both Sgr A West and the compact H II-regions to the east of Sgr A East, v) a double-lobed structure of the High Negative Velocity Gas (HNVG) oriented to the northeast and southwest of Sgr A*, and vi) compact point-like maser emission in all four transitions. In particular, a 1720 MHz maser at –132 km s–1 in the CND as counterpart to a 1720 MHz maser at +132 km s–1 in the CND, was observed.