Axial Asymmetry Studies in Gaia Data Release 2 Yield the Pattern Speed of the Galactic Bar

TL;DR

This study uses Gaia DR2 star count asymmetries to determine the Milky Way's bar pattern speed and corotation radius, revealing a likely weak and fast bar with potential non-steady-state effects.

Contribution

It introduces a novel method to measure the Galactic bar's pattern speed using asymmetry sign flips in star counts, refining previous estimates.

Findings

OLR radius at 0.96 R0 with 0.03 R0 uncertainty

Pattern speed of 49.3 km/s/kpc with 2.2 km/s/kpc uncertainty

Galactic bar likely weak and fast

Abstract

Our recent studies of axial-symmetry breaking in the nearby ($d <3 \,{\rm kpc}$) star counts are sensitive to the distortions of stellar orbits perpendicular and parallel to the orientation of the bar just within and beyond the outer Lindblad resonance (OLR) radius. Using the location of the sign flip in the left-right asymmetry in stars counts about the anticenter line to determine the OLR radius $R_{\rm OLR}$, and treating the bar as if it were a weakly non-axisymmetric effect, we use $R_{\rm OLR}$ and recent measurements of the Galactic rotation curve and the Sun-Galactic-center distance $R_{0}$ to determine the pattern speed $\Omega_{\rm p}$ of the Galactic bar, as well as the Galactic corotation radius $R_{\rm CR}$. After removing the effect of the Large and Small Magellanic clouds from our asymmetry measurement, we find that $R_{\rm OLR}=(0.96 \pm 0.03)R_0 = 7.85 \pm 0.25 \ \rm kpc$, $\Omega_{\rm p} = 49.3 \pm 2.2 \ \rm km \ s^{-1} \ kpc^{-1}$, $R_{\rm CR}=(0.58 \pm 0.04)R_0 = 4.76 \pm 0.27 \ \rm kpc$, revealing, as we shall show, that the Milky Way's bar is likely both weak and fast, though we also note possible evidence for non-steady-state effects in the bar region.