Challenges to the Two-Infall Scenario by Large Stellar Age Catalogs

TL;DR

This paper tests the two-infall galaxy formation model against recent stellar age and abundance data, finding it explains some features but struggles with others, especially regarding metallicity constancy and age distribution of stars.

Contribution

It critically evaluates the two-infall scenario using new stellar age measurements, revealing limitations and constraints on the model's parameters and assumptions.

Findings

Two-infall model explains alpha-bimodality but faces challenges with metallicity constancy.

Model predicts old or very young stars where data shows intermediate ages.

Constraints on merger events' role in disk evolution.

Abstract

Stars in the Milky Way disk exhibit a clear separation into two chemically distinct populations based on their [$\alpha$/Fe] ratios. This $\alpha$-bimodality is not a universal feature of simulated disk galaxies and may point to a unique evolutionary history. A popular and well-studied explanation is the two-infall scenario, which postulates that two periods of substantial accretion rates dominate the assembly history of the Galaxy. Thanks to recent advances in stellar age measurements, we can now compare this model to more direct measurements of the Galaxy's evolutionary timescales across the disk. We run multi-zone galactic chemical evolution models with a two-infall-driven star formation history and compare the results against abundance patterns from APOGEE DR17, supplemented with stellar ages estimated through multiple methods. Although the two-infall scenario offers a natural explanation for the $\alpha$-bimodality, it struggles to explain several features of the age--abundance structure in the disk. First, our models generically require a massive and long-lasting dilution event, but the data show that stellar metallicity is remarkably constant across much of the lifetime of the disk. This apparent age-independence places considerable restrictions upon the two-infall parameter space. Second, most local metal-rich stars in APOGEE have intermediate ages, yet our models predict these stars should either be very old or very young. Some of these issues can be mitigated, but not completely resolved, by pre-enriching the accreted gas to low metallicity. These restrictions also place limits on the role of merger events in shaping the chemical evolution of the thin disk.