Hydroxyl, water, ammonia, carbon monoxide, and neutral carbon towards the Sagittarius A complex

VLA, Odin, and SEST observations^⋆

¹ Stockholm Observatory, Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
e-mail: rolandk@astro.su.se
² Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden
³ Omnisys Instruments AB, Solna strandväg 78, 171 54 Solna, Sweden

Received: 1 October 2012
Accepted: 11 March 2013

Abstract

Aims. The Sagittarius A complex in the Galactic centre comprises an ensemble of molecular clouds of different species with a variety of geometrical and kinematic properties. This work aims to study molecular abundances, morphology, and kinematics by comparing hydroxyl, water, carbon monoxide, ammonia, and atomic carbon and some of their isotopologues, in the +50 km s^-1 cloud, the circumnuclear disk (CND), the +20 km s^-1 cloud, the expanding molecular ring and the line-of-sight spiral arm features, including the Local/Sgr arm, the −30 km s^-1 arm, and the 3-kpc arm.

Methods. We observed the +50 km s^-1 cloud, the CND and the +20 km s^-1 cloud, and other selected positions at the Galactic centre with the VLA, and the Odin satellite. The VLA was used to map the 1665 and 1667 MHz OH lambda doublet main lines of the (²Π_3/2) state, and the Odin satellite was used to map the 557 GHz H₂O (1₁₀ − 1₀₁) line as well as to observe the 548 GHz H₂¹⁸O (1₁₀-1₀₁) line, the 572 GHz NH₃ (1₀ − 0₀) line, the 576 GHz CO J = 5 − 4 line and the 492 GHz C i (³P₁–³P₀) line. Furthermore, the SEST was used to map a region of the Sgr A complex in the 220 GHz C¹⁸O J = 2−1 line.

Results. Strong OH absorption, H₂O emission and absorption lines were seen at all observed positions, and the H₂¹⁸O line was detected in absorption towards the +50 km s^-1 cloud, the CND, the +20 km s^-1cloud, the expanding molecular ring, and the 3-kpc arm. Strong CO J = 5−4, C¹⁸O J = 2−1, and neutral carbon C i emissions were seen towards the +50 and +20 km s^-1 clouds. NH₃ was only detected in weak absorption originating in the line-of-sight spiral arm features. The abundances of OH and H₂O in the +50 and +20 km s^-1 clouds reflect the different physical environments in the clouds, where shocks and star formation prevail in the +50 km s^-1 cloud and giving rise to a higher rate of H₂O production there than in the +20 km s^-1 cloud. In the CND, cloud collisions and shocks are frequent, and the CND is also subject to intense UV-radiation emanating from the supermassive black hole and the central star cluster. The CND is rich in H₂O and OH, and these abundances are considerably higher than in the +50 and +20 km s^-1 clouds. We compare our estimated abundances of OH, H₂O, and NH₃ with similar and differing results for some other sources available in the literature. As compared to the quiescent cloud values of a few × 10^-9, or lower, the H₂O abundance is markedly enhanced in the front sides of the Sgr A molecular cloud cores, (2−7) × 10^-8, as observed in absorption, and highest in the CND. A similar abundance enhancement is seen in OH. The likely explanation is PDR chemistry including grain surface reactions, and perhaps also the influence of shocks. In the redward high-velocity line wings of the +50 and +20 km s^-1 clouds and the CND, the H₂O abundances are estimated to be (1−6) × 10^-6 or higher, i.e., similar to the water abundances in outflows of the Orion KL and DR21 molecular clouds, which are said to be caused by the combined action of shock desorption from icy grain mantles and high-temperature, gas-phase shock chemistry.