Observations at high velocity resolution of the ionized interstellar medium in NGC 4151

We have used the Utrecht Echelle Spectrograph to obtain observations at high velocity resolution (6 km s(-1)) of the extended narrow-line region (ENLR) in the Seyfert nucleus of NGC 4151. We compare the velocity structure of the ionized gas with neutral hydrogen observations and confirm that the photoionization of quiescent ambient gas is the preferred mechanism by which to account for the extended region of ionized gas associated with this source. We observe line-splitting in the ENLR of NGC 4151 which is consistent with gas associated with an ionization front expanding outwards from the ENLR.