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S. Corder (University of Kansas), K. J. Mighell (NOAO/KPNO)
We have studied the stellar populations near the nucleus of the nearby spiral galaxy M33 using {\sl{Hubble Space Telescope}} Wide Field Planetary Camera 2 (WFPC2) observations in the F555W (~V) and F814W (~I) filters. The color-magnitude diagrams (CMDs) reveal a young stellar population indicated by a bright blue main-sequence (MV\!\gtrsim\!-6.2 mag), a red supergiant plume (MI\!\gtrsim\!-6.3 mag), very red [\,(V\!-\!I)\!>\!3.5 mag] asymptotic giant branch (AGB) stars, and a wide red giant branch (RGB) with a limiting magnitude of I\!\approx\!24 mag and a signal-to-noise ratio SNR\,\geq\,10 in both filters. Davidge (2000, AJ, 119, 748) recently reported on near-infrared adaptive-optics observations of the nuclear region of M33 in the J, H, and K bands obtained with the KIR imager and the Canada-France-Hawaii Telescope (CFHT) AO system. Comparing our optical space-based photometry with Davidge's near-infrared ground-based photometry, we have identified 51 of Davidge's stars on our {\sl{HST}} images. While most of the infrared-bright stars seen near the nucleus of M33 are old AGB stars, some of these stars are young red supergiants (RSGs); we find that the upper ~0.9 mag tip of Davidge's near-infrared CMDs of the circumnuclear region of M33 samples a mixed stellar population of old AGB stars and young RSGs. Many of our brightest V-band dropouts are clearly seen on Davidge's 1200-s K-band image of the nuclear region of M33. We demonstrate that the intermediate-age infrared-bright AGB stars are more centrally concentrated than the ancient low-mass RGB stars; we associate this stellar population with the M33 bulge identified in the H band by Minniti, Olszewski, & Rieke (1993, ApJ, 410, L79).
This work is carried out through the NOAO/KPNO REU Program, supported by the National Science Foundation. Support was also provided by a grant from the National Aeronautics and Space Administration (NASA), Order No.\ S-67046-F, which was awarded by the Long-Term Space Astrophysics Program (NRA 95-OSS-16).
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