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 (Teff ≥ 10 000 K), based on BOSS spectroscopy from the Milky Way Mapper OB programme in the ongoing Sloan Digital Sky Survey V (SDSS-V). Using photogeometric distances, line-of-sight velocities, and Gaia DR3 proper motions, we mapped 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 v̄R 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 v̄R in Auriga makes it plausible that younger stars exhibit different velocity variations than older giants. The origin of the radial-velocity features remains uncertain, and it 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.