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The dawn is quiet II: Gaia XP constraints on the Milky Way’s proto-Galaxy from very metal-poor MDF tails

Year: 2026
Type: article
Abstract: The earliest phase of the Milky Way’s evolution involved a transition from a dispersion-supported proto-galaxy to a rotationally supported disk. A key chemical signature of this transition is the moderate rise in [ α /Fe] near [ F e / H ] ≈ − 1.3 , which we previously interpreted as evidence for α -enhanced gas accretion fueling early disk formation. However, this trend alone does not uniquely constrain the trade-off between initial gas mass, inflow rate, and star formation efficiency (SFE), leaving the physical condition of the proto-Milky Way uncertain. To break this degeneracy, we analyze the metal-poor tail ( − 3 < [ F e / H ] < − 2 ) of the Milky Way’s metallicity distribution function (MDF) using Gaia DR3 BP/RP (XP) metallicities from ten catalogs. After applying recommended quality cuts, all catalogs exhibit a single-slope exponential tail with slopes k ≃ 0.6 –2.0. Comparison with one-zone galactic chemical-evolution (GCE) models that replicated the [ α /Fe]-rise from Paper I shows that shallow tails ( k ≃ 0.6 ) require a massive initial cold gas reservoir ( ≳ 10 9 M ⊙ ), while steeper tails ( k ≳ 1 ) arise from small reservoirs that built up over time with weak inflow. MDFs with k ≃ 1.0 are best reproduced under our GCE framework, which favor a proto-Galaxy with a moderate gas reservoir ( 10 8 – 10 9 M ⊙ ) sustained through weak continuous inflow ( ∼ 2 M ⊙ y r − 1 ) and SFE comparable to today’s value (a few × 10 − 10 y r − 1 ) during the first Gyr. This scenario is reinforced by MDFs of 30 Milky Way analogs in the Auriga simulations, which exhibit similar slopes ( k ≈ 1.25 ). The metal-poor MDF tail thus provides a quantitative constraint on the Milky Way’s early gas accretion and star formation history.
DOI: 10.33232/001c.159462
Language: English
Open Access status: diamond
OpenAlex ID: W7140175685
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