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

Abstract

The inner $\sim5$ kiloparsec (kpc) region of the Milky Way is complex. 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 ($r < 5$ 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 $\sim1.5$ kpc, is comprised of stars on nearly-radial orbits, 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; the chemical/dynamic properties of the knot suggest it could constitute a classical bulge formed via secular evolution. Moreover, our results show that the bar is more slowly rotating than previously thought, with a pattern speed of $Ω_{\mathrm{bar}}=24\pm3$ km s$^{-1}$ kpc$^{-1}$. This new estimate suggests that the influence of the bar extends beyond the solar radius, with $R_{\mathrm{CR}}\sim9.4-9.8$ kpc, depending on the adopted Milky Way rotation curve; it also suggests a ratio of corotation to bar length of $\mathcal{R}\sim1.8-2$. 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.