The UV-Optical Albedo of Broad Emission Line Clouds

Abstract

We explore the effective UV-optical albedos of a variety of types of broad emission line clouds, as well as their possible effects on the observed spectra of AGN. An important albedo source in moderately ionized ionization-bounded clouds is that due to neutral hydrogen: Rayleigh scattering of continuum photons off the extreme damping wings of Lya. The photons resulting from this scattering mechanism may contribute significantly to the Lya emission line, especially in the very broad wings. In addition, line photons emitted near 1200 Angstroms (e.g., N V 1240) that stream toward the neutral portion of the cloud may be reflected off this Rayleigh scattering mirror, so that they preferentially escape from the illuminated face. Inclusion of this effect can alter predicted emission line strengths and profiles. In more highly-ionized ionization-bounded clouds, Thompson scattering dominates the UV-optical albedo, but this albedo is lessened by the hydrogen gas opacity. These clouds are most reflective on the long wavelength side of the hydrogen recombination edges; this feature may then alter the shapes of the spectral regions near the recombination edges. We illustrate the effects of gas density and line broadening on the effective albedo. We also discuss the reflection effects of the accretion disk and the ``dusty torus.'' The accretion disk is an effective reflector of UV-optical photons, whether by electron or Rayleigh scattering, and it is possible that we observe a significant fraction of this light from the AGN in reflection. This effect can alter the emission line profiles and even destroy emission at the Lyman jump emitted by broad line clouds. Finally, we discuss the possibility that continuum reflection from broad line clouds is at least in part responsible for the polarized broad absorption line troughs.