The Ultraviolet-Optical Albedo of Broad Emission Line Clouds

We explore the e†ective UV-optical albedos of a variety of types of broad emission line clouds, as well as their possible e†ects on the observed spectra of active galactic nuclei (AGNs). An important albedo source in moderately ionized ionization-bounded clouds is that which is due to neutral hydrogen : Rayleigh scattering of continuum photons o† the extreme damping wings of Lya. The photons resulting from this scattering mechanism may contribute signiÐcantly to the Lya emission line, especially in the very broad wings. In addition, line photons emitted near 1200 (e.g., N V j1240) that stream toward the Ó neutral portion of the cloud may be reÑected o† this Rayleigh scattering mirror, so that they preferentially escape from the illuminated face. Inclusion of this e†ect can alter predicted emission-line strengths and proÐles. In more highly ionized ionization-bounded clouds, Thomson scattering dominates the UV-optical albedo, but this albedo is lessened by the hydrogen gas opacity ; these clouds are most reÑective 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, e.g., the Balmer jump. We illustrate the e†ects of gas density and line broadening on the e†ective albedo. We also discuss the reÑection e†ects of the accretion disk and the ““ dusty torus.ÏÏ The accretion disk is an e†ective reÑector of UV-optical photons, whether by electron or Rayleigh scattering, and it is possible that we observe a signiÐcant fraction of this light from the AGNs in reÑection. This e†ect can alter the emission-line proÐles and even destroy emission at the Lyman jump emitted by broad-line clouds. Finally, we discuss the possibility that continuum reÑection from broad-line clouds is at least in part responsible for the polarized broad absorption line troughs. Subject headings : galaxies : Seyfert È line : formation È quasars : emission lines È quasars : general È scattering