HST and VLA Observations of Seyfert 2 Galaxies: The Relationship between Radio Ejecta and the Narrow Line Region

Abstract

We present HST/WFPC2 images, in narrow-band filters containing the [OIII] lambda5007 and Halpha+[NII] emission-lines and their adjacent continua, of a sample of seven Seyfert 2 galaxies selected on the basis of possessing either extended emission-line regions in ground-based observations or a hidden broad-line region in polarized light. Six of the galaxies have also been observed with the VLA to obtain radio maps with better quality and angular resolution than those in the literature. We find detailed correspondences between features in the radio and emission-line images that clearly indicate strong interactions between the radio jets and the interstellar medium. Such interactions play a major role in determining the morphology of the NLR, as the radio jets sweep up and compress ambient gas, producing ordered structures with enhanced surface brightness in line emission. In at least three galaxies, namely Mrk 573, ESO 428-G14, and Mrk 34, and perhaps also NGC 7212, off-nuclear radio lobes coincide with regions of low gaseous excitation (as measured by the [OIII]/(Halpha+[NII]) ratio). In Mrk 573 and NGC 4388, there is a clear trend for low brightness ionized gas to be of higher excitation. Those results may be understood if radio lobes and regions of high emission-line surface brightness are associated with high gas densities, reducing the ionization parameter. [OIII]/(Halpha+[NII]) excitation maps reveal bi-polar structures which can be interpreted as either the `ionization cones' expected in the unified scheme or widening, self-excited gaseous outflows. Only NGC 4388 and Mrk 573 show a clearly defined, straight-edged ionization cone.