The low-luminosity active galactic nucleus in the centre of the Galaxy

The observations of far-infrared line and continuum spectra throughout the Galactic Centre and in some regions of the disc are analysed in order to determine the physical conditions (densities, shock velocities, radiation parameters etc.) and the relative abundances of some elements (C, N and O). Consistent model calculations of the line and continuum spectra show that, although the radiation from the stars dominates, an active galactic nucleus (AGN) is clearly present, with a radiation maximum in the Sgr A* region. The models account for the coupled effect of photoionization and shocks. The flux, similar to that found in low-luminosity AGNs, is lower by a factor of similar to 100 than that of AGN. Gas densities in the downstream line emission region range between 100 and 3000 cm-3, the shock velocities between 50 and 300 km s-1. Densities of similar to 5 x 10(6) cm(-3), close to the Sgr A* black hole, lead to self-absorption of freefree radiation in the radio frequency range, while X-ray data are explained by shock velocities of similar to 3000 km s(-1). A magnetic field of similar to 10 (-4) G shows relatively small fluctuations throughout the Galactic Centre. The dust-to-gas ratios range between 3 x 10(-15) and <= 10(-13) by number. Lower values are found far from the centre, suggesting that N and O, which are depleted from the gaseous phase, are included into molecules rather than trapped into grains.