Radial redistribution of stellar orbits in FIRE simulations of Milky-Way-mass galaxies

Abstract

A central question in galactic dynamics and galactic archeology is: how much do the orbits of stars redistribute (migrate) after birth? We use the FIRE-2 cosmological zoom-in simulations of 11 Milky Way-mass galaxies to quantify the change in the orbital specific angular momentum, j_phi, orbital radius, R_orbit, and azimuthal velocity, v_phi, of stars from birth to today. We examine the dependences on stellar age, present-day R_orbit, and birth R_orbit, characterizing both the median (net) change, Delta R_orbit, and its scatter, sigma(Delta R_orbit). We comprehensively compare five ways of measuring orbital radius; we find generally consistent trends, but only when measuring radius today and radial redistribution self-consistently. Stars selected by their birth R_orbit typically decreased in R_orbit, j_phi, and v_phi since birth. The trend for stars at a given R_orbit today depends on age: those younger than ~5 Gyr generally decreased in R_orbit, j_phi, and v_phi since birth, while those older generally increased in R_orbit, j_phi, and v_phi since birth. sigma(Delta R_orbit), a standard metric of radial redistribution, increases with stellar age only up to ~ 3 Gyr; it saturates at sigma(Delta R_orbit) ~2 kpc for older stars. This saturation contradicts a common expectation of a monotonic increase with age. Our results broadly agree with recent observational inferences of Delta R_orbit and sigma(Delta R_orbit) in the Milky Way. Across our FIRE-2 sample, the timing of disk formation does not correlate with sigma(Delta R_orbit), but it correlates with (net) Delta R_orbit.