Nebular excitation of low-luminosity emission nuclei

Low-luminosity nebular emission is found in the centers of a large fraction of galaxies, and in many cases the energy source powering these systems is ambiguous. This is particularly true for H II/LINER "transition" objects, although a reasonable explanation for these sources is that their emission is the composite result of a weak accretion-powered system surrounded by star-forming regions. This contribution describes results from a Hubble Space Telescope spectroscopic survey that probes the structure of nearby transition objects and other emission nuclei. The results provide only limited support for the composite picture for these sources; the emission nuclei do not show a strong tendency for the nebular classification to depend on aperture size. The strongest variations in forbidden line ratios appear to be mediated by a gradient of decreasing nebular density with increasing radius.