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We present results from a program of optical spectroscopy for 23 nearbygalaxy emission-line nuclei. This investigation takes advantage of the spatialresolution of the Hubble Space Telescope to study the structure and energeticsof the central 10 - 20 pc, and the resulting data have value for quantifyingcentral black hole masses, star formation histories, and nebular properties.This paper provides a description of the experimental design, and new findingsfrom the study of emission lines. The sample targets span a range of nebularspectroscopic class, from HII to Seyfert nuclei. The line ratios indicative ofnebular ionization show only modest variations over order-of-magnitudedifferences in radius, and demonstrate in a systematic way that geometricaldilution of the radiation field from a central source cannot be assumed as aprimary driver of ionization structure. Comparisons between large- andsmall-aperture measurements for the HII/LINER transition objects provide a newtest that challenges conventional wisdom concerning the composite nature ofthese systems. We also list a number of other quantitative results that are ofinterest for understanding galaxy nuclei, including (1) the spatialdistribution/degree of concentration of H-alpha emission as a function ofnebular type; (2) the radial variation in electron density as a function ofnebular type; and (3) quantitative broad H-alpha estimates obtained at a secondepoch for these low-luminosity nuclei. The resulting measurements provide a newbasis for comparing the nuclei of other galaxies with that of the Milky Way. Wefind that the Galactic Center is representative across a wide span ofproperties as a low-luminosity emission-line nucleus.Comment: 28 pages, 7 figures, revised version accepted by Ap

The Survey of Nearby Nuclei with the Space Telescope Imaging Spectrograph: Emission‐Line Nuclei atHubble Space TelescopeResolution