The Milky Way is a less massive galaxy--new estimates of the Milky Way's local and global stellar masses

Abstract

Stellar mass is the most fundamental property of a galaxy. While it has been robustly measured for millions of external galaxies, it remains poorly constrained for the Milky Way because of the strong selection effect from our inside perspective. In this work, we reconstruct the intrinsic vertical mass density profile in the solar neighborhood and the radial mass density profile across the entire Galaxy using data from the Gaia and APOGEE surveys, following careful correction for the selection function. The local density profile exhibits strong north-south asymmetry in the geometric thick disk regime and increases steeply toward the disk mid-plane, favoring an exponential model over the sech$^2$ model. Integrating the local vertical density profile yields a surface stellar mass density of 31.563$\pm$2.813(syst.)$\pm$0.024(stoch.)~${\rm M_{\odot}pc^{-2}}$, of which 25.074 and 6.489~${\rm M_{\odot}pc^{-2}}$ correspond to living stars and stellar remnants, respectively. The radial surface mass density profile {of the Milky Way} shares the same flat inner component as observed in the brightness profile. With this mass density profile {and local mass density from Gaia}, we derive a new estimate of the total stellar mass of the Milky Way of 2.607$\pm$0.353(syst.)$\pm$0.085(stoch.)${\rm \times10^{10}M_{\odot}}$, a factor of two lower than the previous results. This discrepancy arises primarily from the inner disk profile, which was previously unavailable and extrapolated from the outer disk profile. The lower stellar mass estimate of the Milky Way significantly reduces its rarity in terms of supermassive black hole mass among external galaxies and implies a larger dark matter-to-baryon mass ratio in the inner Galaxy.