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

TL;DR

This paper addresses how young stars in the Milky Way disc respond to non-axisymmetric structures by constructing a large-scale kinematic map from SDSS-V OB spectroscopy combined with Gaia proper motions and photogeometric distances. It presents a first-by-area analysis of about 50,000 OB stars, deriving 3D velocities and Bayesian ages to reveal coherent radial velocity patterns with amplitudes up to approximately ±30 km s$^{-1}$ across the disc, larger than in older giant populations. The radial-velocity features show only a weak association with spiral-arm overdensities, suggesting a complex interplay of spiral arms, the Galactic bar, resonant interactions, and phase mixing. Age estimates indicate most stars are very young (~<320 Myr), with the youngest (<30 Myr) tracing density enhancements, implying a stronger dynamical response for younger cohorts. As the SDSS-V survey progresses toward its full target set, these maps will enable more robust dynamical modeling and interpretation of the Milky Way’s non-axisymmetric structure.

Abstract

We present a first large-scale kinematic map of $\sim$50,000 young OB stars ($T_{\rm eff} \geq 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 $\sim$5 kpc from the Sun, with a focus on radial motions ($v_R$). Our results reveal mean radial motion with amplitudes of $\pm 30$ km/s that are coherent on kiloparsec scales, alternating between inward and outward motions. These $\bar{v}_R$ 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 $\sim300$ Myr and that the youngest stars ($\le 30$ Myr) align well with density enhancements. The age-dependent $\bar{v}_R$ 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.

Figures (18)

• Figure 1: Distribution in the sky (Galactic coordinates) of the sources of the SDSS-V mwm_ob program observed until Feb 2025 (top, gray, 165,460 sources) and of the filtered sample described in Section \ref{['sec:filtered']} (bottom, orange, around 45,487 stars). The sources of the mwm_ob program are mostly located in the Galactic plane and the MCs.
• Figure 2: Flowchart illustrating the steps to construct the HSS described in Sec. \ref{['sec:filtered']}. The notation "v.theta-3*" indicates that the numbers are provisional and refer to the specific Robostrategy version named "theta-3" Blanton2025. The survey efficiency has improved and the final number of observed targets will likely increase in the future.
• Figure 3: Comparison between the BOSS Net parameters and Gaia DR3 ESP-HS parameters for the Hot Stars Sample. The dashed orange lines correspond to the 1:1 relation.
• Figure 4: Kiel diagram of the sources in the mwm_ob program (gray histogram) passing the quality cuts described in Section \ref{['sec:filtered']} and of those in the HSS (coloured histogram). The dashed vertical lines mark the following temperatures: 31,000 K (O9.5V type star), 17,000 (B3V type star), 10,400 K (B9V type star), 7,400 K (A9V type star), and 6,050 K (F9V type star).
• Figure 5: Comparison between Gaia DR3 and SDSS-V XCSAO/Boss Net line of sight velocities (in km/s). The orange dashed line corresponds to the 1:1 relation.