The ionizing radiation field in the galactic centre and the nature of LINERs

The Galactic centre contains a low-ionization nebula that has been previously interpreted as plasma photoionized by relatively cool O stars. We consider the possibility that this material is instead ionized by more energetic continua and described by a relatively low ionization parameter (ratio of ionizing photon to nucleon density). We find that the predicted spectrum is more sensitive to ionization parameter and cloud density than to details of the ionizing continuum shape. The Galactic centre spectrum can be generated by stars with a range of temperatures irradiating clouds with multiple components of differing densities. The present calculations also show that optical emission from many LINERs (low-ionization nuclear emission-line regions) can be generated by relatively hot, yet normal O stars embedded in gas clouds similarly characterized by a range of densities.