The Survey of Nearby Nuclei with STIS (SUNNS):Emission-Line Nuclei at Hubble Space Telescope Resolution

We present results from a program of optical spectroscopy for 23 nearby galaxy emission-line nuclei that takes advantage of the spatial resolution of the Hubble Space Telescope to study the structure and energetics of the central 10 - 20 pc. The data have value for quantifying central black hole masses, star formation histories, and nebular properties. This paper provides a description of the experimental design, and new findings from the study of emission lines. The sample targets span a range of nebular spectroscopic class, from HII to Seyfert nuclei. From comparison with ground-based results, we find that the central emission regions are resolved and show modest gradients in line-intensity ratios reflecting in part an inward increase in electron density on small scales. The nuclei show little tendency to change their spectroscopic classification as a function of aperture size; for the HII/LINER ``transition''objects this result may be in conflict with the customary interpretation of these sources as composite systems, although larger samples are needed to verify this finding. The measurements presented here provide a strong basis for assessing the degree of similarity between the center of the Milky Way and other galaxy nuclei. We find that the Galactic Center is representative across a wide span of properties as a low-luminosity emission-line nucleus