The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar
Samir Nepal (Sydney Institute for Astronomy)
Wednesday 15/07/2026 @ 11:00, Sala Antonio Sollima (IV piano Battiferro)
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. 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. For the first time, traced by the field stars, we are able to robustly identify the long-sought pressure supported bulge component. 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-? population with almost no dependency on metallicity, consistent with very rapid early formation, predating the thick disc and the bar. 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 halo. The inner-thick disc is kinematically hotter (mean V? ?125 km/s) than the local-thick disc and is predominantly metal-poor with an MDF peak at [Fe/H] ? -0.45 dex and includes a large fraction of stars with sub-solar [Fe/H] and intermediate-?. The Galactic bar is dynamically well mixed and contains both metal-rich and metal-poor stars, as well as high-? and low-? in nearly equal measure, but their relative contributions vary across different orbital families. Despite their spatial overlap, the spheroidal bulge, thick disc, and bar occupy distinct regions in both phase space and chemical abundance.