Near-infrared adaptive optics imaging of the central regions of nearby Sc galaxies. I. M33

Near-infrared images obtained with the Canada-France-Hawaii Telescope (CFHT ) Adaptive Optics Bonnette (AOB) are used to investigate the stellar conten t within 18" of the center of the Local Group spiral galaxy M33. Asymptotic giant branch (AGB) stars with near-infrared spectral energy distributions similar to those of giants in the solar neighborhood and Baade's window are detected over most of the field. Neither the peak brightness nor the color of the AGE sequence on the (K, J-K) color-magnitude diagram changes with d istance from the galaxy center. The bolometric luminosity function (LF) of these stars has a discontinuity near M-bol = -5.25, and comparisons with ev olutionary tracks suggest that most of the AGE stars formed in a burst of s tar formation 1-3 Gyr in the past, indicating that the star formation rate near the center of M33 has varied significantly during the past few Gyr. Th e images are also used to investigate the integrated near-infrared photomet ric properties of the nucleus and the surrounding central light concentrati on. The nucleus is bluer than the central light concentration, in agreement with previous studies at visible wavelengths. The near-infrared photometri c properties of the nucleus are reminiscent of relatively young clusters in the Magellanic Clouds, while the photometric properties of the central lig ht concentration are similar to those of globular clusters. The CO index of the central light concentration 0".5 from the galaxy center is 0.05, which corresponds to [Fe/H] similar to -1.2 for simple stellar systems. Hence, t he central light concentration could not have formed from the chemically en riched material that dominates the present-day inner disk of M33.