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

S Recchi,F Calura,P Kroupa,B K Gibson

doi:10.1093/mnras/stt1971

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 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, characterised by small SFRs. The [alpha/Fe] plateau is up to 0.7dex 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 on 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.

Highlights

IntroductionThe relative distribution of stellar masses in a given generation (the so-called initial mass function or IMF) is a fundamental entity linking the microphysics of molecular cloud fragmentation with the global characteristics of galactic stellar populations
The relative distribution of stellar masses in a given generation is a fundamental entity linking the microphysics of molecular cloud fragmentation with the global characteristics of galactic stellar populations
The integrated galactic initial mass function (IGIMF) theory (Kroupa & Weidner 2003; Weidner & Kroupa 2005) is aimed at providing a self-consistent description of how the IMF varies as a function of another fundamental characteristic in galaxy evolution models, namely the star formation (SF) history

Summary

Introduction

The relative distribution of stellar masses in a given generation (the so-called initial mass function or IMF) is a fundamental entity linking the microphysics of molecular cloud fragmentation with the global characteristics of galactic stellar populations. The integrated galactic initial mass function (IGIMF) theory (Kroupa & Weidner 2003; Weidner & Kroupa 2005) is aimed at providing a self-consistent description of how the IMF varies as a function of another fundamental characteristic in galaxy evolution models, namely the star formation (SF) history. The stars are distributed according to a universal two-slope power-law IMF: α1 = 1.3 below 0.5 M and α2 = 2.35 above 0.5 M. The maximum stellar mass in an individual cluster is a function of the cluster mass; (ii) the stellar clusters are distributed according to a single-slope power law and (iii) the maximum possible mass of a star cluster increases with the galactic star formation rate (SFR). The main implication is that low-mass galaxies, characterized by low SF values, present a steeper (top-light) IMF than the Milky Way (MW)

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Results

Conclusion