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

R Karlsson,Aa Sandqvist,K Fathi,S Martín

doi:10.1051/0004-6361/201424426

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.

Highlights

At the very core of the Milky Way Galaxy is the ∼4-millionsolar-mass supermassive black hole (SMBH) whose non-thermal radio continuum signature is called Sgr A∗
We have presented an analysis of certain features in our concatenated data base of VLA BnA and DnC observations of 1665 and 1667 MHz OH absorption towards the Sgr A Complex at the Galactic centre (GC)
Our investigation of the properties and kinematics of the OH-streamer and the +80 km s−1 cloud in OH absorption indicated that the OH-streamer is an object inside of the circumnuclear disk (CND) and interacting with the +80 km s−1 cloud, the CND, and possibly with the strong gravitational field from the SMBH and the surrounding star cluster

Summary

Introduction

At the very core of the Milky Way Galaxy is the ∼4-millionsolar-mass supermassive black hole (SMBH) whose non-thermal radio continuum signature is called Sgr A∗. In the cavity inside the CND, there is a central cluster of old stars and bright young stars clustered around the SMBH. Inside the cavity is the mini-spiral-shaped H region called Sgr A West, which represents the inner western edge of the CND, and a northern streamer extending from the CND towards Sgr A∗. The major components of this belt are the two giant molecular clouds, known as the +20 km s−1 cloud and the +50 km s−1 cloud. A giant energetic (>1052 erg) supernovaremnant-like non-thermal continuum radio shell (diameter about 8 pc), known as Sgr A East, is plowing into this molecular belt

Results

Discussion

Conclusion