LINER/Hii“Transition” Nuclei and the Nature of NGC 4569

Motivated by the discovery of young, massive stars in the nuclei of someLINER/H II ``transition'' nuclei such as NGC 4569, we have computedphotoionization models to determine whether some of these objects may bepowered solely by young star clusters rather than by accretion-powered activenuclei. The models were calculated with the photoionization code CLOUDY, usingevolving starburst continua generated by the the STARBURST99 code of Leithereret al. (1999). We find that the models are able to reproduce the emission-linespectra of transition nuclei, but only for instantaneous bursts of solar orhigher metallicity, and only for ages of ~3-5 Myr, the period when theextreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars.For clusters younger than 3 Myr or older than 6 Myr, and for models with aconstant star-formation rate, the softer ionizing continuum results in anemission spectrum more typical of H II regions. This model predicts thatWolf-Rayet emission features should appear in the spectra of transition nuclei.While such features have not generally been detected to date, they could berevealed in observations having higher spatial resolution. Demographicarguments suggest that this starburst model may not apply to the majority oftransition nuclei, particularly those in early-type host galaxies, but it couldaccount for some members of the transition class in hosts of type Sa and later.The starburst models during the Wolf-Rayet-dominated phase can also reproducethe narrow-line spectra of some LINERs, but only under conditions ofabove-solar metallicity and only if high-density gas is present (n_e >~ 10^5cm^{-3}). This scenario could be applicable to some ``Type 2'' LINERs which donot show any clear signs of nonstellar activity.Comment: To appear in PASP. 22 pages, includes 9 figures, uses AASTeX v5.