Faint object camera imaging and spectroscopy of NGC 4151

We describe ultraviolet and optical imaging and spectroscopy within the central few arcseconds of the Seyfert galaxy NGC 4151, obtained with the Faint Object Camera on the Hubble Space Telescope. A narrowband image including [O III] λ5007 shows a bright nucleus centered on a complex biconical structure having apparent opening angle ∼65° and axis at position angle along 65°-245°; images in bands including Lyman-α and C IV λ1550 and in the optical continuum near 5500 Å, show only the bright nucleus. In an off-nuclear optical long-slit spectrum we find a high and a low radial velocity component within the narrow emission lines. We identify the low-velocity component with the bright, extended, knotty structure within the cones, and the high-velocity component with more confined diffuse emission. Also present are strong continuum emission and broad Balmer emission line components, which we attribute to the extended point spread function arising from the intense nuclear emission. Adopting the geometry pointed out by Pedlar et al. (1993) to explain the observed misalignment of the radio jets and the main optical structure we model an ionizing radiation bicone, originating within a galactic disk, with apex at the active nucleus and axis centered on the extended radio jets. We confirm that through density bounding the gross spatial structure of the emission line region can be reproduced with a wide opening angle that includes the line of sight, consistent with the presence of a simple opaque torus allowing direct view of the nucleus. In particular, our modelling reproduces the observed decrease in position angle with distance from the nucleus, progressing initially from the direction of the extended radio jet, through our optical structure, and on to the extended narrow-line region. We explore the kinematics of the narrow-line low- and high-velocity components on the basis of our spectroscopy and adopted model structure. For the low-velocity system both Keplerian rotation and isotropic outflow (or outflow confined to the ionizing cone) give plausible correspondence with our data. If interpreted as rotation we show consistency with earlier determinations indicating a central mass concentration of about 10 9 M ⊙. The high-velocity system kinematically conforms to radial outflow within the galaxy disk, although this does not well reproduce the observed intensity structure.