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 interstellar medium (ISM). We find that the slope (alpha) of a power-law IMF (N(m) ~ m^-alpha) for stellar masses larger than 1M_sun evolves from the canonical Salpeter IMF (alpha ~ 2.35) to be moderately top-heavy one (alpha ~ 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_sun 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 ~ 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 well with corresponding recent observational results. The present study also predicts that inner regions of starburst disk galaxies have smaller alpha thus are more top-heavy (d alpha/d R ~ 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.