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 disc's vertical structure. From the well measured volume mass density and surface mass density, we derive local volume and surface mass-to-light ratios for the Galactic disc, in the bands B, V and I. We obtain mass-to-light ratios 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 turn-off stars and giants. Our results on the colours and mass-to-light ratios 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 under-luminous by about 1-sigma with respect to the main locus of the Tully-Fisher relation, as observed for external galaxies.