2019MNRAS.483...46H

The observed present-day stellar mass function (PDMF) of the solar neighbourhood is a mixture of stellar populations born in star-forming events that occurred over the lifetime of the thin disc of the Galaxy. Assuming stars form in embedded clusters which have stellar initial mass functions (IMFs) which depend on the metallicity and density of the star-forming gas clumps, the integrated galaxy-wide IMF (IGIMF) can be calculated. The shape of the IGIMF thus depends on the star formation rate (SFR) and metallicity. Here, the shape of the PDMF for stars more massive than 1 M_☉in combination with the mass density in low-mass stars is used to constrain the current SFR, the star formation history (SFH), and the current stellar plus remnant mass (M_*) in the Galactic thin disc. This yields the current SFR, {dot}M_*=4.1^+3.1_–2.8 M_☉yr^–1, a declining SFH, and M_*=2.1+3.0^_–1.5×10¹¹ M_☉, respectively, with a V-band stellar mass-to-light ratio of M_*/LV=2.79^+0.48_–0.38. These values are consistent with independent measurements. We also quantify the surface density of black holes and neutron stars in the Galactic thin disc. The invariant canonical IMF can reproduce the PDMF of the Galaxy as well as the IGIMF, but in the universal IMF framework it is not possible to constrain any of the above Galactic properties. Assuming the IGIMF theory is the correct framework and in combination with the vertical velocity dispersion data of stars, it follows that the Milky Way would have appeared as a chain galaxy at high redshift.