Morphology of the coronal-line region in active galactic nuclei

We present new images of the coronal-line region, as traced by [Si vii] 2.48 μm, in some of the nearest Seyfert 2 galaxies. In each of these galaxies, the coronal-line emission comprises a bright, compact central source and extended emission showing broad alignment along a particular direction, usually coinciding with that defined by the radio emission or the extended narrow-line region. The full extent of the coronal-line emission ranges from a few tens of parsecs to ∼150 pc radius from the nucleus and is a factor of ∼10 smaller than that seen in the extended, lower ionization gas. With a spatial resolution of 10 pc or better, the coronal region shows diffuse and filamentary structure in all cases, and it is difficult to see whether it breaks down into discrete blobs such as those seen in lower ionization lines or radio images of comparable resolution. The extent of the coronal-line emission is larger than would be predicted by photoionization models, which argues for additional in situ gas excitation, the most plausible energy source being shock excitation.