The dusty starburst nucleus of M33

We have thoroughly characterized the ultraviolet to near-infrared (0.15-2.2 mu m) 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 Hubble Space Telescope 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 (tau(v) - 2 +/- 1). The discovery of Milky Way-type dust (with a strong 2175 Angstrom bump) internal to the M33 nucleus is different from previous work, which has found SMC-Iike 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 (similar to 10(8) L-circle dot, at 10 Myr) episode of coeval star formation about 70 Myr ago. This result is quite different from previous studies, which resorted to multiple stellar populations (between two and seven) attenuated by either no or very little internal dust. The M33 nuclear starburst is remarkably similar to an older version (70 versus 10 Myr) of the ultracompact starburst in the center of the Milky Way.