The galaxy-wide initial mass function of dwarf late-type to massive early-type galaxies

Abstract

Observational studies are showing that the galaxy-wide stellar initial mass\nfunction are top-heavy in galaxies with high star-formation rates (SFRs).\nCalculating the integrated galactic stellar initial mass function (IGIMF) as a\nfunction of the SFR of a galaxy, it follows that galaxies which have or which\nformed with SFRs > 10 Msol yr^-1 would have a top-heavy IGIMF in excellent\nconsistency with the observations. Consequently and in agreement with\nobservations, elliptical galaxies would have higher M/L ratios as a result of\nthe overabundance of stellar remnants compared to a stellar population that\nformed with an invariant canonical stellar initial mass function (IMF). For the\nMilky Way, the IGIMF yields very good agreement with the disk- and the\nbulge-IMF determinations. Our conclusions are that purely stochastic\ndescriptions of star formation on the scales of a pc and above are falsified.\nInstead, star formation follows the laws, stated here as axioms, which define\nthe IGIMF theory. We also find evidence that the power-law index beta of the\nembedded cluster mass function decreases with increasing SFR. We propose\nfurther tests of the IGIMF theory through counting massive stars in dwarf\ngalaxies.\n