Gant South Building

Jeffrey McKaig, George Mason UniversityWhy Are Optical Coronal Lines Faint in Active Galactic Nuclei? Forbidden collisionally excited optical atomic transitions from high ionization potential (IP~54eV) ions (e.g., Ne4+, Fe10+, Ar10+), are known as optical coronal lines (CLs). The spectral energy distribution (SED) of active galactic nuclei (AGN) typically extends to hundreds of electron volts and above, which should be able to produce such highly ionized gas. However, optical CLs are often not detected in AGN using large scale optical surveys such as the SDSS. In this talk I will present recent photoionization calculations with the Cloudy spectral synthesis code which determine possible reasons for the rarity of these optical CLs. I will report on the observability of optical CLs given the dust content and metallicity of the gas, as well as the ionizing slope of the incident AGN SED. Our main conclusions are (1) dust reduces the strength of most CLs by ~three orders of magnitude, primarily as a result of depletion of metals onto the dust grains. (2) In contrast to the CLs, the more widely observed lower IP optical lines such as [O III] 5007A, are less affected by depletion and some are actually enhanced in dusty gas. (3) In dustless gas many optical CLs become detectable, and are particularly strong for a hard ionizing SED. This implies that prominent CL emission likely originates in dustless gas. I will also present recent work our group has conducted which indeed suggests dust is being destroyed in objects with CL emission and may indicate an important stage in AGN host-galaxy evolution.