Towards Understanding the Milky Way's Typicality: Assessing the Chemodynamics of M31's Bulge & Bar, Thick & Thin Discs

TL;DR

This study introduces a new spectral modeling framework to analyze the chemodynamical properties of M31's inner regions, revealing multiple stellar populations and their similarities to the Milky Way's disc structures.

Contribution

A novel spectral decomposition method applied to M31, identifying multiple stellar components and their chemodynamical properties, comparable to Milky Way's disc populations.

Findings

Identified classical bulge and bar components in M31 with distinct kinematics and abundances.

Detected thick and thin disc analogs in M31's inner disc, similar to the Milky Way.

Observed differences in the south disc potentially due to recent galactic interactions.

Abstract

We describe a novel framework to model galaxy spectra with two cospatial stellar populations, such as may represent a bulge & bar or thick & thin disc, and apply it to APOGEE spectra in the inner $\sim$2 kpc of M31, as well as to stacked spectra representative of the northern and southern parts of M31's disc ($R\sim4-7$ kpc). We use a custom M31 photometric decomposition and A-LIST spectral templates to derive the radial velocity, velocity dispersion, metallicity, and $\alpha$ abundance for both components in each spectrum. In the bulge, one component exhibits little net rotation, high velocity dispersion ($\sim$170 km s$^{-1}$), near-solar metallicity, and high $\alpha$ abundance ([$\alpha$/M] = 0.28), while the second component shows structured rotation, lower velocity dispersion ($\sim$121 km s$^{-1}$), and slightly higher abundances ([M/H] = 0.09, [$\alpha$/M] = 0.3). We tentatively associate the first component with the classical bulge and the second with the bar. In the north disc we identify two distinct components: the first with hotter kinematics, lower metallicity, and higher $\alpha$ abundance than the second ([M/H] = 0.1 and 0.39, [$\alpha$/M] = 0.29 and 0.07). These discs appear comparable to the Milky Way's ''thick'' and ''thin'' discs, providing the first evidence that M31's inner disc has a similar chemodynamical structure. We do not identify two distinct components in the south, potentially due to effects from recent interactions. Such multi-population analysis is crucial to constrain galaxy evolution models that strive to recreate the complex stellar populations found in the Milky Way.