Watching Our Galaxy Grow Up: The Mass and Color Evolution of the Milky Way

Using our rich observations within the Milky Way to better understand galaxy evolution requires understanding what the Milky Way looks like “as a galaxy”—that is, its “true” shape and abundance profiles (unskewed by observational biases), signatures of past mergers and significant accretion events, and even its total stellar mass and integrated spectral energy distribution, which have historically been difficult to constrain. We present a new approach to determining the Milky Way’s integrated mass and colors, using recent measurements of the intrinsic density profiles of stellar populations spanning nearly 13 Gyr in time and 1.5 dex in metallicity (representing nearly all of the Galaxy’s stars). We trace the evolution of the Milky Way in various diagnostic spaces, explore the impact of specific events on the present-day Milky Way’s integrated properties, and use TNG50 simulations to identify “young” Galactic analogs and their eventual fates, compared to the real Milky Way’s path. From the simulation comparisons, we find strong evidence for an earlier-than-average stellar mass assembly of the Milky Way, and that present-day Milky Way analogs follow a similar growth history, albeit at slightly later times; we also find that analogs of the early Milky Way are in no way guaranteed to follow the Milky Way's subsequent path. This empirical study offers new constraints on our “Galaxy as a galaxy”—today and across cosmic time—and on its place in the general galactic population.