The Milky Way's middle-aged inner ring

Abstract

Aims. We investigate the metallicity, age, and orbital anatomy of the inner Milky Way specifically focussing on the outer bar region. Methods. We integrate a sample of apogee DR16 inner Galaxy stars in a state of the art bar-bulge potential with a slow pattern speed and investigate the link between the resulting orbits and their [Fe/H] and ages. By superimposing the orbits we build density, [Fe/H], and age maps of the inner Milky Way, which we divide further using the orbital parameters eccentricity, |X max |, and |Z max |. Results. We find that at low heights from the Galactic plane the Galactic bar gradually transitions into a radially thick, vertically thin, elongated inner ring with average solar [Fe/H]. This inner ring is mainly composed of stars with AstroNN ages between 4 and 9 Gyr with a peak in age between 6 and 8 Gyr, making the average age of the ring ∼6 Gyr. The vertical thickness of the ring decreases markedly towards younger ages. We also find very large L4 Lagrange orbits that have average solar to super-solar metallicities and intermediate ages. Lastly, we confirm a clear X-shape in the [Fe/H] and density distributions at large Galactic heights. Conclusions. The orbital structure obtained for the apogee stars reveals that the Milky Way hosts an inner ring-like structure between the planar bar and corotation. This structure is on average metal rich, intermediately aged, and enhances the horizontal gradient along the bar's major axis.

A barred Milky Way surrogate from an N-body simulation

“…If the onset and evolution of our model is indicative of the Milky Way, the bar first emerged in the last 3 − 4 Gyr. This time scale is significantly shorter than a recent estimate of the bar age of ∼ 6 Gyr from the quenching of the star formation in the bar compared to the inner ring stars (Wylie et al 2021).…”

The stellar mass distribution of the Milky Way's bar: an analytic model

“…The distinction between "bar" and "long bar" in our fitting does not have an immediate physical meaning (e.g. in the sense that the two components correspond to two distinct orbital families, (Skokos et al 2002;Harsoula & Kalapotharakos 2009;Wylie et al 2021), and it should be merely seen as a convenient way of parametrising the density distribution. The bar + long bar components together can be considered as a meaningful component.…”

“…Having an easily-computable representation of the density distribution of the Galactic bar and its associated gravitational field is important for a number of applications, such as orbit integrations (Stolte et al 2014;Habing 2016;Price-Whelan et al 2016;Queiroz et al 2020;Wylie et al 2021), hydrodynamical calculations of the response of the interstellar gas to a bar potential (Armillotta et al 2019;Tress et al 2020;Sormani et al 2020), and the study of the effect of the bar resonances (Dehnen 2000;Monari et al 2019;Binney 2020;. However, there is a lack of a synthetic model that summarises in a concise way our current knowledge of the mass distribution of the Milky Way bar resulting from the huge body of work mentioned above.…”