Ground-state ammonia and water in absorption towards Sgr B2

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 7 10-8, similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of H_218O absorption in the 3 kpc arm, and the absence of such a feature in the H_217O 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, H_218O, and H_217O, 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 11,0≥ts10,1 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 H_218O and H_217O 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.Odin is a Swedish-led satellite project funded jointly by the Swedish National Space Board (SNSB), the Canadian Space Agency (CSA), the National Technology Agency of Finland (Tekes), and the centre National d\u27\uc9tudes Spatiales (CNES, France). The Swedish Space Corporation (SSC) was the industrial prime contractor and is also responsible for the satellite operation