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W. H. Waller (Tufts University), M. G. Lee (Seoul National University), H. S. Park (Seoul National University), S. C. Kim (Seoul National University), E. M. Malumuth (NASA/GSFC, Raytheon), J. Wm. Parker (Southwest Research Institute)
HST/WFPC2 photometry of the ionizing stellar populations in 6 giant H II regions of M33 reveal a significant steepening of the IMF slope (\Gamma) with increasing galactocentric radius and concurrently decreasing O/H abundance, amounting to d(\Gamma)/d[O/H] ~ 1/dex (Park et al. 2001, in preparation). This IMF steepening represents the first strong evidence for a systematic environmental effect on stellar population at the high-mass end. The photoevaporative process, pioneered by J. J. Hester et al. (1996) to explain their HST/WFPC2 observations of the Eagle Nebula (M16), provides a viable mechanism for ablating massive protostellar cores and thus steepening the IMF slope. The ablation is expected to increase with decreasing O/H abundance and associated dust that would otherwise shield the protostellar core from the UV photon field. By contrast, considerations of metallicity-dependent accretion and cooling/fragmentation processes predict a flattening of the IMF slope at lower O/H --- which is not seen.
Predictions include the most top-heavy (flattest) IMFs occurring near the metal-rich centers of star-forming galaxies, and the most bottom-heavy (steepest) IMFs occurring in the metal-poor disks of dwarf irregular galaxies at the current epoch and in the disks of larger primeval galaxies at high-z ... as in the Hubble Deep Fields. Such metallicity-dependent IMFs would require adjustment of the total computed star formation rates in these systems, the SFRs decreasing in galaxy centers and increasing in dwarf irregular and primeval disk galaxies.
W.H.W., J.W.P., and E.M.M. acknowledge prior support from NASA through its HST GO program.
The author(s) of this abstract have provided an email address for comments about the abstract: waller@cove.com