The large-scale kinematics of young stars in the Milky Way disc: first results from SDSS-V

We present a first large-scale kinematic map of 50, 000 young OB stars (T ₄₅₅ 10, 000 K), based on BOSS spectroscopy from the Milky Way Mapper OB program in the ongoing Sloan Digital Sky Survey V (SDSS-V). Using photogeometric distances, line-of-sight velocities and Gaia DR3 proper motions, we map 3D Galactocentric velocities across the Galactic plane to 5 kpc from the Sun, with a focus on radial motions (vR). Our results reveal mean radial motion with amplitudes of 30 km/s that are coherent on kiloparsec scales, alternating between inward and outward motions. These vR amplitudes are considerably higher than those observed for older, red giant populations. These kinematic patterns show only a weak correlation with spiral arm over-densities. Age estimates, derived from MIST isochrones, indicate that 85% of the sample is younger than 300 Myr and that the youngest stars (30 Myr) align well with density enhancements. The age-dependent vR in Auriga makes it plausible that younger stars exhibits different velocity variations than older giants. The origin of the radial velocity features remains uncertain, and may result from a combination of factors, including spiral arm dynamics, the Galactic bar, resonant interactions, or phase mixing following a perturbation. The present analysis is based on approximately one-third of the full target sample. The completed survey will enable a more comprehensive investigation of these features and a detailed dynamical interpretation.