ORIGIN OF A BOTTOM-HEAVY STELLAR INITIAL MASS FUNCTION IN ELLIPTICAL GALAXIES

We investigate the origin of a bottom-heavy stellar initialmass function (IMF) recently observed in elliptical galaxies by using chemical evolution models with a non-universal IMF. We adopt the variable Kroupa IMF with the three slopes (alpha(1), alpha(2), and alpha(3)) dependent on metallicities ([Fe/H]) and densities (rho(g)) of star-forming gas clouds and thereby search for the best IMF model that can reproduce (1) the observed steep IMF slope (alpha(2) similar to 3, i.e., bottom-heavy) for low stellar masses (m <= 1M(circle dot)) and (2) the correlation of alpha(2) with chemical properties of elliptical galaxies in a self-consistent manner. We find that if the IMF slope alpha(2) depends on both [Fe/H] and rho(g), then elliptical galaxies with higher [Mg/Fe] can have steeper alpha(2) (similar to 3) in our models. We also find that the observed positive correlation of stellar mass-to-light ratios (M/L) with [Mg/Fe] in elliptical galaxies can be quantitatively reproduced in our models with alpha(2) proportional to beta[Fe/H] + gamma log rho(g), where beta similar to 0.5 and gamma similar to 2. We discuss whether the IMF slopes for low-mass (alpha(2)) and high-mass stars (alpha(3)) need to vary independently from each other to explain a number of IMF-related observational results self-consistently. We also briefly discuss why alpha(2) depends differently on [Fe/H] in dwarf and giant elliptical galaxies.