On the $\alpha$/Fe bimodality of the M31 disks

TL;DR

This study uses advanced chemical evolution models to demonstrate the presence of an $ m oldsymbol{ extalpha}$/Fe bimodality in the M31 galaxy's disks, revealing complex star formation and merger histories across different regions.

Contribution

It provides the first detailed explanation of $ m oldsymbol{ extalpha}$/Fe bimodality in M31 disks, linking it to star formation episodes and merger events at various radii.

Findings

Inner disk shows clear $ m oldsymbol{ extalpha}$/Fe bimodality.

Outer disk exhibits a slight $ m oldsymbol{ extalpha}$/Fe offset in young populations.

Bimodality depends on M31's merging history and star formation episodes.

Abstract

An outstanding question is whether the $\alpha$/Fe bimodality exists in disk galaxies other than in the Milky Way. Here we present a bimodality using our state-of-the-art galactic chemical evolution models that can explain various observations in the Andromeda Galaxy (M31) disks, namely, elemental abundances both of planetary nebulae, and of red-giant branch stars recently observed with the James Webb Space Telescope. We find that in M31 a high-$\alpha$ thicker-disk population out to 30 kpc formed by more intense initial star burst than in the Milky Way. We also find a young low-$\alpha$ thin disk within 14 kpc, which is formed by a secondary star formation M31 underwent about 2-4.5 Gyr ago, probably triggered by a wet merger. In the outer disk, however, the planetary nebula observations indicate a slightly higher-$\alpha$ young ($\sim$2.5 Gyr) population at a given metallicity, possibly formed by secondary star formation from almost pristine gas. Therefore, an $\alpha$/Fe bimodality is seen in the inner disk ($<$14 kpc), while only a slight $\alpha$/Fe offset of the young population is seen in the outer disk ($>$18 kpc). The appearance of the $\alpha$/Fe bimodality depends on the merging history at various galactocentric radii, and wide-field multi-object spectroscopy is required for unveiling the history of M31.