Ground-state ammonia and water in absorption towards Sgr B2

¹ Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala Sweden
e-mail: per.bergman@chalmers.se
² European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
³ Stockholm Observatory, AlbaNova, 10691 Stockholm, Sweden
⁴ GEPI, Observatoire de Paris, CNRS, 5 place Jules Janssen, 92195 Meudon, France
⁵ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁶ LERMA & UMR 8112 du CNRS, École Normale Supérieure, 24 rue Lhomond, 75005 Paris, France
⁷ Swedish Space Corporation, PO Box 4207, 17104 Solna, Sweden

Received: 28 November 2009
Accepted: 7 June 2010

Abstract

Context. Observations of transitions to the ground-state of a molecule are essential to obtain a complete picture of its excitation and chemistry in the interstellar medium, especially in diffuse and/or cold environments. For the important interstellar molecules H₂O and NH₃, these ground-state transitions are heavily absorbed by the terrestrial atmosphere, hence not observable from the ground.

Aims. We attempt to understand the chemistry of nitrogen, oxygen, and their important molecular forms, NH₃ and H₂O in the interstellar medium of the Galaxy.

Methods. We have used the Odin^* submillimetre-wave satellite telescope to observe the ground state transitions of ortho-ammonia and ortho-water, including their ¹⁵N, ¹⁸O, and ¹⁷O isotopologues, towards Sgr B2. The extensive simultaneous velocity coverage of the observations,  > 500 km s^-1, ensures that we can probe the conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the Galactic centre, and the more diffuse gas in the Galactic disk clouds along the line-of-sight.

Results. We present ground-state NH₃ absorption in seven distinct velocity features along the line-of-sight towards Sgr B2. We find a nearly linear correlation between the column densities of NH₃ and CS, and a square-root relation to N₂H⁺. The ammonia abundance in these diffuse Galactic disk clouds is estimated to be about 0.5–1 × 10^-8, similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of absorption in the 3 kpc arm, and the absence of such a feature in the spectrum, we conclude that the water abundance is around 10^-7, compared to  ~10^-8 for NH₃. The Sgr B2 molecular cloud itself is seen in absorption in NH₃, ¹⁵NH₃, H₂O, , and , with emission superimposed on the absorption in the main isotopologues. The non-LTE excitation of NH₃ in the environment of Sgr B2 can be explained without invoking an unusually hot (500 K) molecular layer. A hot layer is similarly not required to explain the line profiles of the 1_1,0 ← 1_0,1 transition from H₂O and its isotopologues. The relatively weak ¹⁵NH₃ absorption in the Sgr B2 molecular cloud indicates a high [¹⁴N/¹⁵N] isotopic ratio  > 600. The abundance ratio of and is found to be relatively low, 2.5–3. These results together indicate that the dominant nucleosynthesis process in the Galactic centre is CNO hydrogen burning.