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

Abstract

Our recent studies of axial-symmetry breaking in the nearby ($d<3\mathrm{kpc}$) 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 $R_{\mathrm{OLR}}$, and treating the bar as if it were a weakly non-axisymmetric effect, we use $R_{\mathrm{OLR}}$ and recent measurements of the Galactic rotation curve and the Sun-Galactic-center distance $R_0$ to determine the pattern speed $\hat{I}{\text{c◯}}_{\mathrm{p}}$ of the Galactic bar, as well as the Galactic corotation radius $R_{\mathrm{CR}}$. After removing the effect of the Large and Small Magellanic clouds from our asymmetry measurement, we find that $R_{\mathrm{OLR}}=(0.96\pm 0.03)R_0=7.85\pm 0.25\mathrm{kpc}$, $\hat{I}{\text{c◯}}_{\mathrm{p}}=49.3\pm 2.2\mathrm{km}{\mathrm{s}}^{-1}\mathrm{kp}{\mathrm{c}}^{-1}$, $R_{\mathrm{CR}}=(0.58\pm 0.04)R_0=4.76\pm 0.27\mathrm{kpc}$, 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.

12 pages, 2 figures (3 panels), accepted for publication in ApJ Letters