The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar

TL;DR

The paper tackles the complex chemodynamical structure of the Milky Way's inner 5 kpc by combining Gaia DR3 RVS data with APOGEE DR17, using a novel hybrid-CNN approach to derive stellar parameters for bulge stars. It performs full orbit integrations in a barred potential and applies orbital frequency analysis to robustly separate coexisting components, revealing a pressure-supported spheroidal bulge with a MDF peak at $[ ext{Fe/H}]\approx -0.70$ and a high-$[\alpha/\text{Fe}]$ population, distinct from the bar and inner thick disc. It also identifies an inner thick disc, a multi-component Galactic bar with orbit-family–dependent chemistry, and a retrograde $X_4$ population, some of which resemble Gaia-Enceladus/Sausage inner-halo remnants. The results provide strong evidence for a multi-component, chemodynamical inner Galaxy, offering robust constraints on bulge formation scenarios and informing future surveys such as 4MIDABLE-LR.

Abstract

Studying the composition and origin of the inner region of our Galaxy -- the "Galactic bulge" -- is crucial for understanding the formation and evolution of the Milky Way and other galaxies. We present new observational constraints based on a sample of around 18,000 stars in the inner Galaxy, combining Gaia DR3 RVS and APOGEE DR17 spectroscopy. Gaia-RVS complements APOGEE by improving sampling of the metallicity, [Fe/H] in the -2.0 to -0.5 dex range. This work marks the first application of Gaia-RVS spectroscopy to the bulge region, enabled by a novel machine learning approach (hybrid-CNN) that derives stellar parameters from intermediate-resolution spectra with precision comparable to APOGEE's infrared data. We performed full orbit integrations using a barred Galactic potential and applied orbital frequency analysis to disentangle the stellar populations in the inner Milky Way. For the first time, traced by the field stars, we are able to robustly identify the long-sought pressure supported bulge. We show this stellar population to be chemically and kinematically distinct from the other main components co-existing in the same region. The spheroidal bulge has a metallicity distribution function (MDF) peak at around -0.70 dex extending to solar value, is dominated by a high-[alpha/Fe] population with almost no dependency on metallicity, consistent with very rapid early formation, predating the thick disc and the bar. We find evidence that the bar has influenced the dynamics of the spheroidal bulge, introducing a mild triaxiality and radial extension. We identify a group of stars on X4 orbits, likely native to the early spheroid, as this population mimics the chemistry of the spheroidal bulge, with a minor contamination from the more metal-poor ([Fe/H] < -1.0) halo. We find the inner-thick disc to be kinematically hotter (mean Vphi ~125 km/s) than the local-thick disc. ...

Figures (29)

• Figure 1: Sample selection schema. I) Distance from the Galactic midplane (Z) vs the galactocentric radius (R) distribution of the parent inner-galaxy sample. The green (RPM) and black (RVS) colours represent the two spectroscopic sources. The background shows the 2D density distribution for the full RVS data, which is mostly concentrated at the solar neighbourhood. The red stars represents the current position of the Sun. II) Z vs R for the bulge sample stars [ Total (9661) = RPM(6211) + RVS (3450) ] confined to within 5 kpc. Unless otherwise stated, “bulge sample” refers to this group. The fraction of stars that are in the bulge sample from the parent inner-galaxy sample are also shown. III) Similar to panel I), but for stars that do not remain confined within 5 kpc.
• Figure 2: General properties of the bulge sample stars: I) Distance from the Galactic midplane (Z) vs the galactocentric radius (R) of the bulge sample, along with histograms showing the individual distributions for the RPM (green) and RVS (black) stars; II) Kiel diagram ($\textit{T}_{\text{eff}}$ vs $\text{log}(\textit{g})$ ); III) $[\alpha/\text{Fe}]$ vs [Fe/H] diagram for the bulge sample plus the individual histograms.
• Figure 3: Sky distribution (in Galactic coordinates) of the bulge sample stars, RPM (green), and RVS (black), above the extinction map covering the Bulge region obtained with results from Anders2022. Right: Zoom-in view of the Baade's window (black circle). The pencil-beam-like observation of the APOGEE survey is evident in the sky distribution of the targets, while the Gaia spacecraft observes all-sky and covers a much larger area.
• Figure 4: Comparison of the $\textit{T}_{\text{eff}}$, $\text{log}(\textit{g})$, [Fe/H], $[\alpha/\text{Fe}]$, distances, extinctions, ecc, and $\mathrm{Z_{max}}$ for the 36 common stars in the RPM and RVS bulge samples. For the RPM bulge sample, $\textit{T}_{\text{eff}}$, $\text{log}(\textit{g})$, [Fe/H] , and $[\alpha/\text{Fe}]$ are from APOGEE DR17. The distances and extinctions correspond to the StarHorse estimates from Queiroz2021, and the orbital parameters, ecc and $\mathrm{Z_{max}}$, were re-estimated using new galactic potential. For the RVS bulge sample, [Fe/H] and $[\alpha/\text{Fe}]$ are from rvs_cnn_2023; the distances and extinctions correspond to the StarHorse estimates from current work; and the orbital parameters, ecc and $\mathrm{Z_{max}}$, were estimated using the new galactic potential.
• Figure 5: [Fe/H] vs spatial distribution of the bulge sample. Top: $\mathrm{Z_{gal}}$ as a function of [Fe/H] shown as a kernel density estimate (KDE) for the RVS (black) and RPM (green) samples. Bottom: [Fe/H] as a function of $\mathrm{R_{gal}}$.