Our Galaxy's youngest disc

TL;DR

This study models the young stellar disc of the Milky Way using Gaia data and distribution functions, revealing insights into its structure, kinematics, and the influence of dust models on spatial predictions.

Contribution

Introduces a new family of distribution functions to analyze Gaia data, improving understanding of the young disc's structure and kinematics, and highlights the importance of accurate dust modeling.

Findings

The young disc's surface density peaks at about 5.5 kpc from the Galactic center.

Outer disc shows disturbances with velocity deviations around 10 km/s.

Measured line-of-sight velocities are significantly affected by binary orbital motions.

Abstract

We investigate the structure of our Galaxy's young stellar disc by fitting the distribution functions (DFs) of a new family to five-dimensional Gaia data for a sample of $47\,000$ OB stars. Tests of the fitting procedure show that the young disc's DF would be strongly constrained by Gaia data if the distribution of Galactic dust were accurately known. The DF that best fits the real data accurately predicts the kinematics of stars at their observed locations, but it predicts the spatial distribution of stars poorly, almost certainly on account of errors in the best-available dust map. We argue that dust models could be greatly improved by modifying the dust model until the spatial distribution of stars predicted by a DF agreed with the data. The surface density of OB stars is predicted to peak at $R\simeq5.5\mbox{kpc}$, slightly outside the reported peak in the surface density of molecular gas; we suggest that the latter radius may have been under-estimated through the use of poor kinematic distances. The velocity distributions predicted by the best-fit DF for stars with measured line-of-sight velocities $v_\parallel$ reveal that the outer disc is disturbed at the level of 10 $\mbox{km}~\mbox{s}^{-1}$ in agreement with earlier studies, and that the measured values of $v_\parallel$ have significant contributions from the orbital velocities of binaries. Hence the outer disc is colder than it is sometimes reported to be.