The OH-streamer in Sgr A revisited: analysis of hydroxyl absorption within 10 pc from the Galactic centre

TL;DR

This study maps hydroxyl absorption near the Galactic centre, revealing detailed structure and interactions of the OH-streamer, molecular clouds, and the circumnuclear disk, providing insights into the complex gas dynamics within 10 parsecs of Sgr A*.

Contribution

It offers a high-resolution OH absorption map of the central 10 pc, detailing the structure, kinematics, and interactions of molecular features around Sgr A* for the first time.

Findings

OH-streamer comprises three parts: head, middle, and tail.

The OH-streamer interacts with nearby molecular clouds and the CND.

A possible link between the EMR and the CND's southwest lobe was identified.

Abstract

We study the structure and kinematics of the OH-streamer and the +80 km/s cloud and their interactions with the circumnuclear disk (CND) and with other molecular clouds in the vicinity of the Galactic centre (GC), and we map OH absorption at about 6" resolution at R $\le$ 10 pc from the GC, with about 9 km/s velocity resolution. The VLA was used to map OH line absorption at the 1665 and 1667 MHz lambda doublet main lines towards the Sagittarius A complex. Strong OH absorption was found in the OH-streamer, the southern streamer (SS), the +20, +50, and +80 km/s molecular clouds, the molecular belt, the CND, the expanding molecular ring (EMR), and the high negative velocity gas (HNVG). The OH-streamer was found to comprise three parts, head, middle, and tail, and to interact with the SS/+20, +80 km/s clouds and the CND. Optical depths and column densities have been calculated for the OH-streamer and the +80 km/s cloud. The OH-streamer, the SS, the +20 and +80 km/s clouds, and the CND are intimately related in position and velocity space. The OH-streamer was found to be a clumpy object stretching in projection from the inner radius of the CND at about 1.8 pc from Sgr A* towards and partly engulfing Sgr A*. As a side result of our data, a possible link between the near side of the EMR and the CND's southwest lobe was found. Additionally, we found OH absorption against all four of the previously known Compact HII Regions A-D, located east of Sgr A East, indicating their close association with the +50 km/s cloud.