The complex narrow-line region in NGC 4151

Abstract

We present new narrow-band images of NGC 4151, taken in excellent seeing conditions with the 4.2-m William Herschel Telescope (WHT) at the Observatorio del Roque de los Muchachos on La Palma. These reveal a wealth of detail that can be used to test models for the anisotropy of the narrow-line region. From an [Om] (λλ4959, 5007)/Hα line ratio map we identify three important features: (i) a jet-like linear structure that extends in a series of relatively high-ionization blobs along PA~ 50° out to a radius of more than 15 arcsec to the south-west and 5 arcsec to the north-east; (ii) a lower ionization 'halo' surrounding the linear structure and (iii) a band of lower ionization or higher obscuration through the nucleus, almost perpendicular to the linear structure. We propose here that the latter can be identified with the central obscuring disc or torus responsible for collimating the nuclear radiation field which is believed to ionize the extended linear structure. The other features, however, do not fit in with a simple model in which a cone of radiation from the nucleus illuminates a uniform ISM in the host galaxy.