EVOLUTION OF THE HIGH-MASS END OF THE STELLAR INITIAL MASS FUNCTIONS IN STARBURST GALAXIES

We investigate the time evolution and spatial variation of the stellar initial mass function (IMF) in star-forming disk galaxies by using chemodynamical simulations with an IMF model depending both on local densities and metallicities ([Fe/H]) of the interstellarmedium (ISM). We find that the slope (alpha) of a power-law IMF(N(m) proportional to m(-alpha)) for stellar masses larger than 1 M-circle dot evolves from the canonical Salpeter IMF (alpha approximate to 2.35) to be moderately top-heavy one (alpha approximate to 1.9) in the simulated disk galaxies with starbursts triggered by galaxy interaction. We also find that alpha in star-forming regions correlates with star formation rate densities (Sigma(SFR) in units of M-circle dot yr(-1) kpc(-2)). Feedback effects of Type Ia and II supernovae are found to prevent IMFs from being too top-heavy (alpha < 1.5). The simulation predicts alpha approximate to 0.23 log Sigma(SFR) + 1.7 for log Sigma(SFR) >= -2 (i.e., more top-heavy in higher Sigma(SFR)), which is reasonably consistent with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller a and thus are more top-heavy (d alpha/dR similar to 0.07 kpc(-1) for R <= 5 kpc). The predicted radial alpha gradient can be tested against future observational studies of the alpha variation in star-forming galaxies.