ar X iv : a st ro - p h / 99 02 04 3 v 1 2 F eb 1 99 9 The Dusty Starburst Nucleus of M 33 1

We have thoroughly characterized the ultraviolet to near-infrared (0.15 2.2 μm) spectral energy distribution (SED) of the central parsec of the M33 nucleus through new infrared photometry and optical/near-infrared spectroscopy, combined with ultraviolet/optical observations from the literature and the HST archive. The SED shows evidence for a significant level of attenuation, which we model through a Monte Carlo radiative transfer code as a shell of clumpy Milky Way-type dust (τV ∼ 2± 1). The discovery of Milky Way-type dust (with a strong 2175 Å bump) internal to the M33 nucleus is different from previous work which has found SMC-like dust (no bump) near starburst regions. The amount by which dust can be processed may be related to the mass and age of the starburst as well as the extent to which the dust can shield itself. Our starburst models include the effects of this dust and can fit the SED if the nucleus was the site of a moderate (∼10 L⊙ at 10 Myrs) episode of coeval star formation about 70 Myrs ago. This result is quite different from previous studies which resorted to multiple stellar populations (between 2 and 7) attenuated by either no or very little internal dust. The M33 nuclear starburst is remarkably similar to an older version (70 Myr versus 10 Myr) of the ultra-compact starburst in the center of the Milky Way. Subject headings: galaxies: individual (M33) – galaxies: ISM – galaxies: starburst Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Department of Physics & Astronomy, Louisiana State University, Baton Rouge, LA 70803 Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract from the National Aeronautics and Space Administration Steward Observatory, University of Arizona, Tucson, AZ 85721 present address: Department of Physics, University of Cincinnati, Cincinnati, OH 45221 Hubble Fellow Visiting Astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation.