Disentangling the Galaxy’s Gordian knot: evidence from <i>APOGEE–Gaia</i> for a knotted and slower bar in the Milky Way
Danny Horta, Michael Petersen, Jorge Peñarrubia
Abstract
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 ($r &lt; 5~\mathrm{kpc}$) 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.