Expanding stellar associations as Galactic accelerometers

Abstract

The gravitational potential of the Milky Way is fundamental for understanding the evolution of our Galaxy and the nature of dark matter. We introduce a new method to constrain the Galactic potential using expanding young stellar associations. We exploit the physical constraint that these stars share a common, compact origin to reconstruct their most likely orbits and infer the gravitational potential in which they have evolved. We define the size of an association using the trace and determinant of its position covariance matrix. By integrating synthetic associations backward in different trial potentials, we show how the true potential can be identified as the one that minimises these metrics. We demonstrate that, while current observational errors are still too large, upcoming observations will allow us to distinguish between different potentials. Our results suggest that with Gaia DR4 astrometry and radial velocity errors below 0.2 km/s, the halo mass can be constrained with a precision of 0.6 trillion solar masses and the concentration with a precision of < 0.8 using a single association, albeit with significant degeneracies. In addition, we show that the inferred dynamical traceback age is sensitive to the gravitational potential, suggesting that independent age information can help break existing degeneracies, but also that traceback-age estimates are not independent of the assumed potential. Expanding stellar associations carry information about the gravitational potential in which they have evolved. With the arrival of next-generation astrometry and high-precision radial velocities, they will provide a complementary tool for constraining the Galactic potential.