SF-R You Sure? The Conflicting Role of Star Formation Rates in Constraining the Evolution of Milky Way Analogs in Cosmological Simulations

Abstract Milky Way analogs (MWAs) have long been studied by astronomers to place our Galaxy within an extragalactic context. With the power of cosmological simulations, we are now able to not only characterize MWAs today, but also watch as they evolve through cosmic time. We use the Evolution and Assembly of GaLaxies and their Environments (EAGLE) and Illustris The Next Generation (IllustrisTNG) simulations to study a group of MWAs defined by their stellar mass (SM) and star formation rate (SFR). We trace these galaxies back along their evolution to investigate the star-forming and mass assembly tracks taken by a galaxy to become an MWA today in light of these chosen parameters. We also take mock-observations of “MWAs” at z > 0 and trace them forward in time to determine if galaxies that looked similar to the Milky Way earlier in their evolution still look like the Milky Way today, thus quantifying a selection efficiency that could inform future observational studies of MWAs. We find that most galaxies with Milky Way–SM follow a similar evolution regardless of present-day SFR, although MWAs in IllustrisTNG generally have not quenched, leading to star formation histories that produce “too-blue” galaxies today. Additionally, we find contamination by MWA-“imposters” in our mock-observations, with low selection efficiency at high redshift due to the tight constraint requiring convergence to the Milky Way’s present-day SFR. Our work suggests present-day SM may suffice as a stand-alone selection parameter and helps to clarify how MWAs should be selected, and thus will be an important reference for future studies of both simulated and observed MWAs.