1996A&A...312..751V

In the powerful, high redshift radio galaxies, it is believed that the dominant source of ionization for the interstellar gas is the hard radiation field associated with the active nucleus. The photon source is generally external to the clouds being ionized and so the geometrical perspective from which the gas is observed and the presence and distribution of dust must be properly accounted for in the diagnostic process. In this paper, we examine the formation of the three strong lines: CIVλ1549, Lyα and CIII]λ1909 which are often observed in the nuclear and extended emission from these sources. We find that the observed trends, in particular the high CIVλ1549/Lyα ratio, are often better explained by geometrical (viewing angle) effects than by the presence of large quantities of dust either within or outside the excited clouds. We show that neutral condensations along the line-of-sight, by reflecting photons near the wavelength of Lyα, can increase the observed CIV/Lyα ratio. The existence of HI absorption clouds (i.e., mirrors) external to the emission region leads also to the presence of large, diffuse haloes of what appears to be pure, narrow Lyα emission.