On the mass-to-light ratio of the local Galactic disc and the optical luminosity of the Galaxy

We measure the volume luminosity density and surface luminosity density generated by the Galactic disc, using accurate data on the local luminosity function and the vertical structure of the disc. From the well-measured volume mass density and surface mass density, we derive local volume and surface mass-to-light ratios (M/L) for the Galactic disc, in the bands B, V and I. We obtain M/L for the local column of stellar matter of (M/L)(B) = 1.4 +/- 0.2, (M/L)(V) = 1.5 +/- 0.2 and (M/L)(I) = 1.2 +/- 0.2. The dominant contributors to the surface luminosity in these bands are main-sequence turnoff stars and giants. Our results on the colours and M/L for the 'solar cylinder' well agree with population synthesis predictions using initial mass functions typical of the solar neighbourhood. Finally, we infer the global luminosity of the Milky Way, which appears to be underluminous by about 1 sigma with respect to the main locus of the Tully-Fisher relation, as observed for external galaxies.