Decoding the stellar fossils of the dusty Milky Way progenitors

TL;DR

This study models the formation and chemical evolution of the Milky Way's halo, revealing the roles of faint supernovae, dust cooling, and the transition from Population III to Population II stars in shaping the metallicity distribution.

Contribution

It introduces an improved semi-analytical model that links hierarchical galaxy formation with detailed chemical evolution, providing new insights into the origins of metal-poor stars and dust effects.

Findings

Faint supernovae dominate metal yields at very low metallicities.

Pop III stars likely had a mass cutoff below 140 solar masses.

Dust cooling triggers the formation of the first low-mass stars.

Abstract

We investigate the metallicity distribution function (MDF) in the Galactic halo and the relative fraction of Carbon-normal and Carbon-rich stars. To this aim, we use an improved version of the semi-analytical code GAlaxy MErger Tree and Evolution (GAMETE), that reconstructs the hierarchical merger tree of the MW, following the star formation history and the metal and dust evolution in individual progenitors. The predicted scaling relations between the dust, metal and gas masses for MW progenitors show a good agreement with observational data of local galaxies and of Gamma Ray Burst (GRB) host galaxies at 0.1 < z < 6.3. We find that in order to reproduce the observed tail of the MDF at [Fe/H] < -4, faint SN explosions have to dominate the metal yields produced by Pop III stars, disfavoring a Pop III IMF that extends to stellar masses > 140 M_{sun}, into the Pair-Instability SN progenitor mass range. The relative contribution of C-normal and C-enhanced stars to the MDF and its dependence on [Fe/H] points to a scenario where the Pop III/II transition is driven by dust-cooling and the first low-mass stars form when the dust-to-gas ratio in their parent clouds exceeds a critical value of D_crit = 4.4 x 10^{-9}.