A Spectroscopic and Photometric Exploration of the C/M Ratio in the Disk of M31

TL;DR

This study investigates the C/M ratio in M31's disk using spectroscopy and photometry, revealing its dependence on stellar metallicity and other parameters, and confirming its link to stellar properties across diverse populations.

Contribution

It provides the first detailed analysis of the C/M ratio in M31's disk, demonstrating its correlation with metallicity and other stellar parameters using combined spectroscopic and photometric data.

Findings

C/M ratio correlates with stellar metallicity.

The trend between log(C/M) and metallicity matches that in Local Group satellites.

C/M ratio is primarily governed by stellar properties, independent of star formation history.

Abstract

We explore the ratio (C/M) of carbon-rich to oxygen-rich thermally pulsing asymptotic giant branch(TP-AGB) stars in the disk of M31 using a combination of moderate-resolution optical spectroscopy from the Spectroscopic Landscape of Andromeda's Stellar Halo (SPLASH) survey and six-filter Hubble Space Telescope photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) survey.Carbon stars were identified spectroscopically. Oxygen-rich M-stars were identifed using three different photometric definitions designed to mimic, and thus evaluate, selection techniques common in the literature. We calculate the C/M ratio as a function of galactocentric radius, present-day gas-phase oxygen abundance, stellar metallicity, age (via proxy defined as the ratio of TP-AGB stars to red giant branch, RGB, stars), and mean star formation rate over the last 400 Myr. We find statistically significant correlations between log(C/M) and all parameters. These trends are consistent across different M-star selection methods, though the fiducial values change. Of particular note is our observed relationship between log(C/M) and stellar metallicity, which is fully consistent with the trend seen across Local Group satellite galaxies. The fact that this trend persists in stellar populations with very different star formation histories indicates that the C/M ratio is governed by stellar properties alone.