The ANDROIDS programme has used the Mega...

Abstract. Our proximity and external vantage point make M31 an ideal testbed for under-standing 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 ugriJKs bands with CFHT using a careful sky calibration. We use Bayesian modelling of the optical-infrared spec-tral 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 ∼ 0.2 dex at 40 kpc. The best-constrained stellar population models use the full ugriJKs SED but are also consistent with optical-only fits. Therefore, while NIR data can be success-fully 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/Li via g − i colour found in the literature. We advocate a stellar mas