The [alpha/Fe] ratios of very metal-poor stars within the integrated galactic initial mass function theory

Abstract

The aim of this paper is to quantify the amplitude of the predicted plateau in [alpha/Fe] ratios associated with the most metal-poor stars of a galaxy. We assume that the initial mass function (IMF) in galaxies is steeper if the star formation rate (SFR) is low-as per the integrated galactic initial mass function (IGIMF) theory. A variant of the theory, in which the IGIMF depends upon the metallicity of the parent galaxy, is also considered. The IGIMF theory predicts low [alpha/Fe] plateaus in dwarf galaxies, characterized by small SFRs. The [alpha/Fe] plateau is up to 0.7 dex lower than the corresponding plateau of the Milky Way. For a universal IMF one should expect instead that the [alpha/Fe] plateau is the same for all the galaxies, irrespective of their masses or SFRs. Assuming a strong dependence of the IMF on the metallicity of the parent galaxy, dwarf galaxies can show values of the [alpha/Fe] plateau similar to those of the Milky Way, and almost independent of the SFR. The [Mg/Fe] ratios of the most metal-poor stars in dwarf galaxies satellites of the Milky Way can be reproduced either if we consider metallicity-dependent IMFs or if the early SFRs of these galaxies were larger than we presently think. Present and future observations of dwarf galaxies can help disentangle between these different IGIMF formulations.