The Central Starburst and Ionization Mechanism in the LINER/H II Region Transition Nucleus in NGC 4569

Abstract

We present a comprehensive study to determine if the LINER/H II region transition spectrum in NGC 4569 can be generated solely by photoionization by the nuclear starburst. A review of the multiwavelength data from the literature reveals no additional sources that contribute to the ionization. We find that the young starburst dominating the UV emission is distinct from the nuclear population of A supergiants identified in the optical spectrum by Keel (1996). Spectral synthesis analysis provides constraints on the physical nature of the starburst, revealing a 5-6 Myr, approximately instantaneous starburst with subsolar metallicity. These results are used to model the spectral energy distribution of the ionizing continuum. Luminosity constraints place limits on the steepness of the extinction curve for the young starburst. The Savage & Mathis (1979) curve satisfies all luminosity constraints and the derived reddening is similar to the emission line reddening. These results imply extreme conditions in the nuclear starburst, with ~5x10^4 O and B stars compacted in the inner 9" x 13" region of the nucleus. Using photoionization analysis and employing all observational constraints on the emission line gas, we find very specific conditions are required if the spectrum is generated solely by stellar photoionization. At least two spatially distinct components are required - a compact region with strong O III emission and an extended, low density component emitting most of the S II flux. A high density component is also needed to generate the O I flux. Additionally, a limited contribution from Wolf-Rayet stars to the ionizing SED is necessary, consistent with the results of Barth & Shields (2000). We present a physical interpretation for the multi-component emission line gas.