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Session 79 - Star Clusters in Other Galaxies.
Display session, Friday, January 09
The formation of stellar clusters is poorly understood. In spite of vigorous star formation in the Galaxy, there are no young, massive clusters. In contrast, the LMC and some colliding galaxies produce large numbers of populous young clusters. To understand what parameters influence cluster formation, we need to study cluster populations in different environments. We have identified 50 well resolved star clusters covering a morphological sequence, in HST WFPC2 U, B and V images of M33. Thirteen of these clusters are also detected in far UV (1700 Angstrom) images of the same fields. These fields sample different environments in M33, such as the nuclear regions, some spiral arm regions, and outer parts of the galaxy, allowing us to explore how the star formation history varies across the galaxy and how it correlates with galactic structure and metallicity. The excellent resolution afforded by HST has allowed us to search crowded nuclear and spiral arm regions, resulting in the identification of 40 previously unidentified star clusters. The majority of these clusters are young, having B-V < 0.3, thus the blue and UV filter combination provides important information.
Luminosities, sizes, and B-V and U-B colors have been compared with stellar evolution models to obtain age estimates. Massive star clusters are found to have a range of ages from 0.001 to 10 Gyrs, where formation of these clusters has been fairly continuous during this period. We have already obtained spectroscopy for 12 clusters in our sample, giving more information about ages, metallicities, and kinematics.
Program listing for Friday