A thick reflection nebula illuminated by a power law

Abstract

We present the results of modelling the energy distribution of a power law reflected by a dusty cloud. We explore in a fully consistent manner the case in which the reflecting nebula is of considerable optical depth. We apply the model to the observations of the detached emission line cloud at 8 kpc from the nucleus of Pks 2152–69 for which continuum flux measurements exist up to the far UV. Our results for the thick case qualitatively agree with the results of Fosbury et al. (1991) (who studied the optically thin case) in that we also need a population of grain sizes dominated by small grains (smaller on average than in models of the standard extinction curve) in order to reproduce the energy distribution of the detached continuum source. We model the high excitation emission line spectrum of the cloud using the code MAPPINGS. Photoionization calculations in which the ionized gas contains the reflecting dust can easily reproduce the high electronic temperature as well as the apparent case B Lyα/Hβ ratio observed in the detached cloud, confirming that dust is a workable hypothesis for explaining the Lyα emission and the blue extended light detected around many high redhift radio galaxies.