THE EFFECTS OF RESONANCE SCATTERING AND DUST ON THE UV LINE SPECTRUM OF RADIO GALAXIES

In the powerful, high redshift radio galaxies, it is believed that the dominant source of ionization for the interstellar gas is the hard ra diation 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 distr ibution of dust must be properly accounted for in the diagnostic proce ss. In this paper, we examine the formation of the three strong lines: CIV lambda 1549, Ly alpha and CIII]lambda 1909 which are often observ ed in the nuclear and extended emission from these sources. We find th at the observed trends, in particular the high CIV lambda 1549/Ly alph a ratio, are often better explained by geometrical (viewing angle) eff ects 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 Ly alp ha, can increase the observed CIV/Ly alpha ratio. The existence of HI absorption clouds (i.e., mirrors) external to the emission region lead s also to the presence of large, diffuse haloes of what appears to be pure, narrow Ly alpha emission.