The Stellar Mass of M31 as inferred by the Andromeda Optical & Infrared Disk Survey

TL;DR

This study uses optical and infrared data to model M31's stellar populations and mass distribution, revealing inside-out disk formation and a metallicity gradient, with implications for galaxy evolution understanding.

Contribution

It provides the first detailed stellar mass profile of M31 using Bayesian SED fitting across multiple bands, highlighting the effectiveness of optical data alone.

Findings

Stellar mass of M31 within 30 kpc is approximately 10.3 billion solar masses.

The i-band mass-to-light ratio decreases from 0.5 dex in the bulge to 0.2 dex at 40 kpc.

Optical-only SED fits are nearly as effective as full optical-infrared fits for mass estimation.

Abstract

Our proximity and external vantage point make M31 an ideal testbed for understanding the structure of spiral galaxies. The Andromeda Optical and Infrared Disk Survey (ANDROIDS) has mapped M31's bulge and disk out to R=40 kpc in $ugriJK_s$ bands with CFHT using a careful sky calibration. We use Bayesian modelling of the optical-infrared spectral energy distribution (SED) to estimate profiles of M31's stellar populations and mass along the major axis. This analysis provides evidence for inside-out disk formation and a declining metallicity gradient. M31's $i$-band mass-to-light ratio ($M/L_i^*$) decreases from 0.5 dex in the bulge to $\sim 0.2$ dex at 40 kpc. The best-constrained stellar population models use the full $ugriJK_s$ SED but are also consistent with optical-only fits. Therefore, while NIR data can be successfully modelled with modern stellar population synthesis, NIR data do not provide additional constraints in this application. Fits to the $gi$-SED alone yield $M/L_i^*$ that are systematically lower than the full SED fit by 0.1 dex. This is still smaller than the 0.3 dex scatter amongst different relations for $M/L_i$ via $g-i$ colour found in the literature. We advocate a stellar mass of $M_*(30\mathrm{kpc})=10.3^{+2.3}_{-1.7}\times 10^{10}\mathrm{M}_\odot$ for the M31 bulge and disk.