Herschel Far-IR and submm spectroscopy of the Galactic Centre

The Galactic centre (GC), in particular the interstellar material in the immediate vicinity (<5 pc) of the central black hole, represents a unique environment for our understanding of galactic nuclei and galaxy evolution. At a distance d = 8.0 ± 0.5 kpc, the nucleus of our galaxy is a few hundred times closer than the nearest active galactic nuclei (AGNs), thus allowing high spatial resolution studies. Owing to the lower dust extinction at far-IR/Submm wavelengths and because of the strong emission/absorption from the interstellar component related to AGN, star formation activity and the diffuse clouds along the line-of-sight, the relevance of far-IR/Submm spectroscopy to characterise extragalactic nuclei has notably increased thanks to Herschel. The spectrum of the Milky Way¿s nucleus is obviously a key template. In this contribution, we present observations toward the GC taken with the PACS, SPIRE and HIFI spectrometers. The achieved angular resolution allows us to separate, for the first time at these wavelengths, the emission toward the central cavity (gas in the inner central parsec of the galaxy) from that of the surrounding circumnuclear disk (the CND). The spectrum toward Sgr A* is dominated by strong [O iii], [O i], [C ii], [N iii], [N ii], and [C i] fine-structure lines (in decreasing order of luminosity) arising in gas irradiated by UV photons from the central stellar cluster. In addition, rotationally excited lines of 12CO, 13CO, H2O, OH, H3O+, HCO+, and HCN, as well as ground-state absorption lines of OH+, H2O+, H3O+, CH+, H2O, OH, HF, CH, and NH are detected. The excitation of the 12CO ladder is consistent with a hot isothermal component at Tk ¿ 10^3.1 K and n(H2)~10^4 cm¿3. It is also consistent with a distribution of temperature components at higher density with most CO at Tk~300 K. The detected molecular features suggest that, at present, neither very enhanced X-ray nor cosmic-ray fluxes play a dominant role in the heating of the hot molecular gas.