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

¹ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Via G. Sansone 1, 50019 Sesto F.no (Firenze), Italy
² Max-Planck-Institut für Astronomie, Konigstuhl 17, 69117 Heidelberg, Germany
³ Department of Physics and Astronomy, University of North Florida, 1 UNF Dr, Jacksonville, FL 32224, USA
⁴ Canadian Institute for Theoretical Astrophysics, University of Toronto, 60 St. George Street, Toronto, ON M5S 3H8, Canada
⁵ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁶ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ University of Wisconsin–Madison, Department of Astronomy, Madison, WI 53706, USA
⁹ Departamento de Astronomía, Facultad de Ciencias, Universidad de La Serena, Av. Raul Bitran 1302, La Serena, Chile
¹⁰ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800, BC, Mexico
¹¹ University of Colorado Boulder, Boulder, CO 80309, USA
¹² Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
¹³ Department of Space, Earth & Environment, Chalmers University of Technology, 41293 Gothenburg, Sweden
¹⁴ Instituto de Astronomía, Universidad Nacional Autónoma de México, Unidad Académica en Ensenada, Km 103 Carr. Tijuana-Ensenada, Ensenada 22860, Mexico
¹⁵ Apache Point Observatory and New Mexico State University, PO Box 59, Sunspot, NM 88349-0059, USA
¹⁶ Sternberg Astronomical Institute, Moscow State University, Moscow 119234, Russia
¹⁷ Department of Physics and Astronomy, Texas Christian University, Fort Worth, TX 76129, USA
¹⁸ Instituto de Astronomía, Universidad Católica del Norte, Av. Angamos 0610, Antofagasta, Chile
¹⁹ Department of Astronomy, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
²⁰ Instituto de Astronomía, Universidad Nacional Autónoma de México, A.P. 70-264, 04510 Mexico, D.F., Mexico
²¹ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA

^★ Corresponding author: eleonoramaria.zari@unifi.it

Received: 16 July 2025
Accepted: 1 October 2025

Abstract

We present a first large-scale kinematic map of ∼50 000 young OB stars (T_eff ≥ 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 (v_R). 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 v̄_R 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 v̄_R 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.