The narrow-line regions of LINERs as resolved with the Hubble Space Telescope

Low-ionization nuclear emission-line regions (LINERs) exist in the nuclei o f a large fraction of luminous galaxies, but their connection with the acti ve galactic nucleus (AGN) phenomenon has remained elusive. We present Hubbl e Space Telescope (HST) narrowband ([O III] lambda 5007 and H alpha + [N II ]) emission-line images of the central regions of 14 galaxies with LINER nu clei. This is the first such study of a sizable sample of LINERs at HST res olution. The compact, similar to 1 " scale unresolved emission which domina tes the line flux in ground-based observations of these LINERs is mostly re solved in the HST images. The bulk of the H alpha and [O III] emission come s from regions with sizes of tens to hundreds of parsecs. The resolved emis sion comes from a combination of knots, filaments, and diffuse gas whose mo rphology differs from galaxy to galaxy. Most of the galaxies do not show cl ear linear structures or ionization cones analogous to those often seen in Seyfert galaxies. An exception is NGC 1052, the prototypical LINER, in whic h we find a 3 " long (similar to 250 pc) biconical structure that is orient ed on the sky along the galaxy's radio jet axis. M84 also shows signs of po ssible biconical gas structures. Seven of the galaxies have been shown in p reviously published HST images to have a bright compact ultraviolet (UV) nu clear source, while the other seven do not display such a central UV source . Our images show a dusty environment in the nuclear region of all 14 galax ies, with clear indications of obscuration of the nuclei in most of the "UV -dark" cases. The data thus suggest that the line-emitting gas in most LINE Rs is photoionized by a central source (which may be stellar, nonstellar, o r a combination thereof) but that this source is often hidden from direct v iew in the UV by dust in the host galaxy. We find no obvious differences be tween the morphologies of the nine "LINER 1.9s" with detected weak broad Ha wings in their spectra and the morphologies of the other five objects. Lik ewise, there is no clear distinction in morphology between objects whose UV spectra are dominated by hot stars (e.g., NGC 4569) and those that are mor e AGN-like (e.g., NGC 4579).