Was the Milky Way a chain galaxy? Using the IGIMF theory to constrain
  the thin-disk star formation history and mass
  The observed present-day stellar mass function (PDMF) of the solar
neighborhood is a mixture of stellar populations born in star-forming events
that occurred over the life-time of the thin disk 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 SFR and metallicity. Here, the shape of the PDMF for stars
more massive than $1\,M_\odot$ in combination with the mass density in low-mass
stars is used to constrain the current star-formation rate (SFR), the star
formation history (SFH) and the current stellar plus remnant mass ($M_*$) in
the Galactic thin disk. This yields the current SFR, $\dot{M}_*=
4.1^{+3.1}_{-2.8}~M_\odot$yr$^{-1}$, a declining SFH and
$M_*=2.1^{+3.0}_{-1.5}\times 10^{11}M_\odot$, respectively, with a V-band
stellar mass-to-light ratio of $M_*/L_V=2.79^{+0.48}_{-0.38}$.These values are
consistent with independent measurements. We also quantify the surface density
of black holes and neutron stars in the Galactic thin disk. 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 IGMF 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.
