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