Physical conditions of the coronal line region in Seyfert galaxies

The launch of the Infrared Space Observatory and new atomic data have opened a window for the study of high-ionization gas in active galactic nuclei (AGNs). We present the results of a large number of photoionization simulations of the "coronal line" region in AGNs, employing new atomic data from the Opacity and Iron Projects. Our grid of line emission spans 8 orders of magnitude in gas density and 14 orders of magnitude in ionizing flux in an effort to identify the optimal conditions in which these lines form. We show that coronal lines form at distances from just outside the broad-line region to ∼400L^1/2_34.5 pc, in gas with ionization parameter -2.0 ≲ log U(H) ≲ 0.75, corresponding to gas densities of 10²-10^8.5 cm^-3, with electron temperatures ∼12,000-150,000 K. A large range of distances from the central source implies significant line width variation among the coronal lines. We identify several line ratios that could be used to measure relative abundances, and we use these to show that the coronal line gas is likely to be dust free.