The spiral structure of the Milky Way

TL;DR

This review discusses recent advances in understanding the Milky Way's spiral structure through astrometric data from VLBI and Gaia, revealing detailed spiral arms and fundamental Galactic parameters.

Contribution

It synthesizes recent high-precision astrometric measurements to refine the Milky Way's spiral morphology and fundamental parameters for the first time in detail.

Findings

Detailed mapping of Perseus, Local, Sagittarius, and Scutum arms.

Estimated Galactic center distance as 8.35 kpc.

Measured rotation speed at the Sun as 240 km/s.

Abstract

The morphology and kinematics of the spiral structure of the Milky Way is a long-standing problem in astrophysics. In this review we firstly summarize various methods with different tracers used to solve this puzzle. The astrometry of Galactic sources is gradually alleviating this difficult situation caused mainly by large distance uncertainties, as we can currently obtain accurate parallaxes (a few $\mu$as) and proper motions ($\approx$ 1 km~s$^{-1}$) by using Very Long Baseline Interferometry (VLBI). On the other hand, Gaia mission is providing the largest, uniform sample of parallaxes for O-type stars in the entire Milky Way. Based upon the VLBI maser and Gaia O-star parallax measurements, nearby spiral structures: the Perseus, Local, Sagittarius and Scutum arms are determined in unprecedented detail. Meanwhile, we estimate fundamental Galactic parameters, the distance to the Galactic center, $R_0$, to be 8.35 $\pm$ 0.18 kpc, a circular rotation speed at the Sun, $\Theta_0$, to be 240 $\pm$ 10 km~s$^{-1}$. We found kinematic differences between O stars and interstellar masers: the O stars, on average, rotate faster, $>$~8~ km s$^{-1}$ than maser-traced high-mass star forming regions.