A nuclear spectroscopic survey of disk galaxies. II - Galaxies with emission lines not excited by stellar photoionization

Survey spectra of relatively low signal-to-noise ratio obtained with the Lick 1 m Anna Nickel telescope have been used to identify a sample of galaxies whose nuclei exhibit emission lines with line ratios not characteristic of stellar photoionization. High S/N spectra of most of those galaxies were subsequently obtained with the Lick 3 m telescope. This paper presents an analysis of those spectra according to the methodology of Baldwin et al. (1981) to determine the excitation mechanism for those emission-line regions. The lists of galaxies identified as having shock-excited, nuclear emission-line sources are then compared to those with power-law photoionized nuclear gas. For these relatively low luminosity active galaxy nuclei, the principal differences between those two lists are that the shock-excited sources appear to have wider emission lines and they occur in more luminous galaxies than the power-law photoionized sources. A comparison of a list of AGNs relative to a list of galaxies without detected nuclear emission lines shows that the galaxies with active nuclei preferentially occur in groups, while isolated galaxies seldom have strong nuclear emission lines. That result supports a model where gas infall from tidal encounters provides the fuel for the observed optical activity