The Structure and Stellar Content of the Central Region of M33
  Using Gemini QuickStart infrared observations of the central 22" of M33, we analyze the stellar populations in this controversial region. Based on the slope of the giant branch we estimate the mean metallicity to be -0.26+/-0.27, and from the luminosities of the most luminous stars, we estimate that there were two bursts of star formation ~2 and ~0.5 Gyr ago. We show that the stellar luminosity function not only has a different bright end cutoff, but also a significantly different slope than that of the Galactic bulge, and suggest that this difference is due to the young stellar component in M33. We combine our infrared Gemini data with optical HST-WFPC2 measurements revealing a CMD populated with young, intermediate, and old age stellar populations. Using surface brightness profiles from 0.1" to 18', we perform simple decompositions and show that the data are best fit by a three-component, core + bulge + disk model. Finally, we find no evidence for radial variations of the stellar populations in the inner 3"-10" of M33 based on a spatial analysis of the color-magnitude diagrams and luminosity functions. 
Andrew W. Stephens (Princeton / P. Universidad Catolica) and Jay A.   Frogel (NASA Headquarters)