The Prevalence of the $\alpha$-bimodality: First JWST $\alpha$-abundance Results in M31

Abstract

We present initial results from our JWST NIRSpec program to study the $\alpha$-abundances in the M31 disk. The Milky Way has two chemically-defined disks, the low-$\alpha$ and high-$\alpha$ disks, which are closely related to the thin and thick disks, respectively. The origin of the two populations and the $\alpha$-bimodality between them is not entirely clear, although there are now several models that can reproduce the observed features. To help constrain the models and discern the origin, we have undertaken a study of the chemical abundances of the M31 disk using JWST NIRSpec, in order to determine whether stars in M31's disk also show an $\alpha$-abundance bimodality. Approximately 100 stars were observed in our single NIRSpec field at a projected distance of 18 kpc from the M31 center. The 1-D extracted spectra have an average signal-to-noise ratio of 85 leading to statistical metallicity precision of 0.016 dex, $\alpha$-abundance precision of 0.012 dex, and a radial velocity precision 8 km/s. The initial results indicate that, in contrast to the Milky Way, there is no $\alpha$-bimodality in the M31 disk, and no low-$\alpha$ sequence. The entire stellar population falls along a single chemical sequence very similar to the MW's high-alpha component which had a high star formation rate. While this is somewhat unexpected, the result is not that surprising based on other studies that found the M31 disk has a larger velocity dispersion than the MW and is dominated by a thick component. M31 has had a more active accretion and merger history than the MW which might explain the chemical differences.