Disentangling the Galaxy's Gordian knot: evidence from APOGEE-Gaia for a knotted and slower bar in the Milky Way

Abstract

The inner $\sim 5$ kiloparsec (kpc) region of the Milky Way is complex. However, unravelling the evolution of the Galaxy requires precise understanding of the formation of this region. We report a study focused on disentangling the inner Galaxy ($r < 5~\mathrm{kpc}$) using the measured positions, velocities, and element abundance ratios of red giant stars from the APOGEE-Gaia surveys. After removing the stellar halo, inner Galaxy populations can be grouped into three main components based on their angular momentum: bar, disc, and a previously unreported 'knot' component. The knot has a spheroidal shape, is concentrated in the inner $\sim 1.5~\mathrm{kpc}$, and contains stars with super-solar [Fe/H] element abundances. The chemical compositions of the knot are qualitatively similar to the Galactic bar and inner disc, suggestive that these three populations share a common genesis. Moreover, our results show that the bar is more slowly rotating than previously thought, with a pattern speed of $\Omega _{\mathrm{bar}}=24\pm 3~\mathrm{km\, s}^{-1}~\mathrm{kpc}^{-1}$. This new estimate suggests that the influence of the bar extends beyond the solar radius, with $R_{\mathrm{CR}}\sim 9.4-9.8~\mathrm{kpc}$, depending on the adopted Milky Way rotation curve; it also suggests a ratio of corotation to bar length of $\mathcal {R}\sim 1.8\!-\!2$. Our findings help place constraints on the formation and evolution of inner Galaxy populations, and directly constrain dynamical studies of the Milky Way bar and stars in the solar neighbourhood.