2024/10/21 by Danny Horta, Ricardo P. Schiavon, Horta, Danny +1 · 3 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Gravitation Theories #FOS: Physical sciences #Scientific Research and Discoveries
paper · pdf · doi:10.48550/arxiv.2410.16374
openalex publication_date 2024/10/21 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
Unravelling galaxy formation theory requires understanding galaxies both at high and low redshifts. A possible way to connect both realms is by studying the oldest stars in the Milky Way (i.e., the proto-Galaxy). We use the APOGEE-Gaia surveys to perform a purely chemical dissection of Milky Way (MW) stellar populations, and identify samples of stars likely belonging to proto-Galactic fragments. The metallicity dependence of the distribution of old MW stars in the [Mg/Mn]-[Al/Fe] enables the distinction of at least two populations in terms of their star formation histories: a rapidly evolved population likely associated with the main progenitor system of the proto-MW; and populations characterised by less efficient, slower, star formation. In the Solar neighbourhood less efficient star forming populations are dominated by the Gaia-Enceladus/Sausage accretion debris. In the inner Galaxy, they are largely associated with the Heracles structure. We model the density of chemically defined proto-Galaxy populations, finding that they are well represented by a Plummer model with a scale radius of a∼3.5 kpc, and an oblate ellipsoid with flattening parameters [p∼0.8; q∼0.6]; this finding indicates that the MW plausibly hosts a low-mass, metal-poor, bulge component. We integrate this density for chemically unevolved stars between -2 < [Fe/H] < -0.5 to obtain a minimum stellar mass for the proto-Galaxy of M* (r<10~kpc) = 9.1±0.2×108 M\odot. Our results suggest the proto-Milky Way is at least comprised of two significant fragments: the main in situ progenitor and the Heracles structure.