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

¹ Stockholm Observatory, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
e-mail: rolandk@astro.su.se
² Oskar Klein Centre for Cosmoparticle Physics, Stockholm University, 106 91 Stockholm, Sweden
³ Institut de RadioAstronomie Millimétrique 300 rue de la Piscine, Domaine Universitaire 38406 Saint-Martin d’Hères, France

Received: 18 June 2014
Accepted: 23 June 2015

Abstract

Aims. We study the structure and kinematics of the OH-streamer and the +80  km s^-1 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 ≤ 10 pc from the GC, with about 9  km s^-1 of velocity resolution.

Methods. The VLA was used to map OH line absorption at the 1665 and 1667 MHz lambda doublet main lines of the ²Π_3/2 state towards the Sagittarius A complex.

Results. Strong OH absorption was found in the OH-streamer, the southern streamer (SS), the +20, +50, and +80  km s^-1 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, mid, and tail, and to interact with the SS/+20, +80  km s^-1 clouds and the CND. Optical depths and column densities divided by excitation temperatures have been calculated for the OH-streamer and the +80  km s^-1 cloud.

Conclusions. The OH-streamer, the SS, the +20 and +80  km s^-1 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 H ii regions A−D, located east of Sgr A East, indicating their close association with the +50  km s^-1 cloud.