Calibrated, cosmological hydrodynamical simulations with variable IMFs – II. Correlations between the IMF and global galaxy properties

Abstract

The manner in which the stellar initial mass function (IMF) scales with global galaxy properties is under debate. We use two hydrodynamical, cosmological simulations to predict possible trends for two self-consistent variable IMF prescriptions that respectively become locally bottom-heavy or top-heavy in high-pressure environments. Both simulations have been calibrated to reproduce the observed correlation between central stellar velocity dispersion and the excess mass-to-light ratio (MLE) relative to a Salpeter IMF by increasing the mass fraction of, respectively, dwarf stars or stellar remnants. We find trends of MLE with galaxy age, metallicity and [Mg/Fe] that agree qualitatively with observations. Predictions for correlations with luminosity, half-light radius, and black hole mass are presented. The significance of many of these correlations depends sensitively on galaxy selection criteria such as age, luminosity, and morphology. For an IMF with a varying high-mass end, some of these correlations are stronger than the correlation with the birth ISM pressure (the property that governs the form of the IMF), because in this case the MLE has a strong age dependence. Galaxies with large MLE tend to have overmassive central black holes. This indicates that the abnormally high MLE observed in the centres of some high-mass galaxies does not imply that overmassive BHs are merely the result of incorrect IMF assumptions, nor that excess M/L ratios are solely the result of overmassive BHs. Satellite galaxies tend to scatter toward high MLE due to tidal stripping, which may have significant implications for the inferred stellar masses of ultracompact dwarf galaxies.