The Chemical Evolution of the Milky Way: the Three Infall Model

TL;DR

This paper introduces a three-infall chemical evolution model for the Milky Way, highlighting distinct formation timescales and star formation efficiencies for the halo, thick disk, and thin disk, emphasizing the importance of extragalactic gas accretion.

Contribution

The model uniquely incorporates three separate gas infall episodes and emphasizes the role of extragalactic gas accretion in thick disk formation, improving understanding of Galactic evolution.

Findings

Short formation timescales for halo (~0.2 Gyr) and thick disk (~1.25 Gyr).

Maximum star formation efficiency during the thick disk phase.

Different gas density thresholds for star formation in each Galactic component.

Abstract

We present a new chemical evolution model for the Galaxy that assumes three main infall episodes of primordial gas for the formation of halo, thick and thin disk, respectively. We compare our results with selected data taking into account NLTE effects. The most important parameters of the model are (i) the timescale for gas accretion, (ii) the efficiency of star formation and (iii) a threshold in the gas density for the star formation process, for each Galactic component. We find that, in order to best fit the features of the solar neighbourhood, the halo and thick disk must form on short timescales (~0.2 and ~1.25 Gyr, respectively), while a longer timescale is required for the thin-disk formation. The efficiency of star formation must be maximum (10 Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk formation. Also the threshold gas density for star formation is suggested to be different in the three Galactic components. Our main conclusion is that in the framework of our model an independent episode of accretion of extragalactic gas, which gives rise to a burst of star formation, is fundamental to explain the formation of the thick disk. We discuss our results in comparison to previous studies and in the framework of modern galaxy formation theories.