[Paper Review] Dead man tells tales: metallicity distribution of the Milky Way stellar halo reveals the past of the GSE progenitor galaxy

Sergey Khoperskov, Ivan Minchev+5 more|arXiv (Cornell University)|Oct 8, 2023

Astronomy and Astrophysical ResearchPhysics and Astronomy3 citations

TL;DR

This study uses APOGEE DR17 data and N-body simulations to show that the Gaia-Sausage-Enceladus (GSE) progenitor galaxy had a radial metallicity gradient of ≈−0.1 dex/kpc, inferred from the observed −0.014 dex/kpc gradient in GSE debris within the Milky Way halo. The result reveals chemical structure preserved in accreted stars, offering insights into the GSE's pre-accretion properties and validating chemical tagging as a tool for reconstructing galactic merger histories.

ABSTRACT

The Gaia-Sausage-Enceladus~(GSE) stands out as the largest known ancient accretion event in the Milky Way~(MW) history. Despite this significance, the parameters of its progenitor galaxy are still poorly constrained. We identify GSE stars from the APOGEE DR17 using Gaussian mixture models and recover a negative radial metallicity gradient for the GSE debris within the MW stellar halo, with a magnitude of $\approx -0.014^{-0.002}_{-0.022}$ dex/kpc. We argue that this gradient reflects the radial metallicity gradient of the GSE galaxy progenitor before it was disrupted by the MW. By investigating the cosmological HESTIA simulations and $N$-body models of galaxy mergers, we constrain the radial metallicity gradient of the GSE-progenitor to be $\approx -0.1^{-0.06}_{-0.15}$ dex/kpc. We, therefore, propose that a chemical tagging of accreted stars using their integrals of motion, although they are not conserved during mergers, provide essential information about the structure and the past of systems accreted onto the MW.

Motivation & Objective

• To constrain the radial metallicity gradient of the Gaia-Sausage-Enceladus (GSE) progenitor galaxy, which remains poorly understood despite its significance in Milky Way assembly.
• To determine whether the observed metallicity gradient in GSE debris within the Milky Way stellar halo reflects the original chemical structure of the GSE progenitor before disruption.
• To test the hypothesis that integrals of motion—though not conserved during mergers—can still preserve information about the progenitor's internal structure through chemical tagging.
• To use cosmological HESTIA simulations and N-body merger models to calibrate the relationship between a progenitor's intrinsic metallicity gradient and the resulting debris gradient in the halo.
• To improve understanding of the formation and evolution of massive, ancient dwarf galaxies like GSE by linking observed chemical patterns to their pre-accretion properties.

Proposed method

• Identified GSE stars in APOGEE DR17 using Gaussian mixture models to separate kinematically distinct populations in phase space.
• Measured the radial metallicity gradient of GSE debris in the Milky Way stellar halo using observed [Fe/H] and galactocentric radius, yielding ∇R[Fe/H] ≈ −0.014−0.022+0.002 dex/kpc.
• Conducted 1,000 N-body simulations of galaxy mergers to establish a statistical relationship between the progenitor’s radial metallicity gradient and the resulting debris gradient in the halo.
• Validated the N-body results using self-consistent HESTIA cosmological simulations of the Local Group, ensuring robustness across different modeling frameworks.
• Used the calibrated relationship to infer the intrinsic radial metallicity gradient of the GSE progenitor, constrained to ≈−0.1−0.15+0.06 dex/kpc.
• Applied chemical tagging via orbital integrals to reconstruct the progenitor’s chemical structure despite non-conservation of motion during the merger.

Experimental results

Research questions

• RQ1 What was the radial metallicity gradient of the Gaia-Sausage-Enceladus (GSE) progenitor galaxy before its accretion onto the Milky Way?
• RQ2 Can the observed metallicity gradient in GSE debris within the Milky Way stellar halo be used to infer the original chemical structure of the GSE progenitor?
• RQ3 To what extent do integrals of motion in accreted stars preserve information about the progenitor’s internal chemical gradients despite non-conservation during mergers?
• RQ4 How do simulated merger outcomes compare with observed debris gradients, and can they be used to calibrate the relationship between progenitor and debris metallicity gradients?
• RQ5 How does the inferred metallicity gradient of the GSE progenitor compare with those of present-day dwarf galaxies, and what does this imply about its formation and evolution?

Key findings

• The GSE debris in the Milky Way stellar halo exhibits a radial metallicity gradient of ∇R[Fe/H] ≈ −0.014−0.022+0.002 dex/kpc, indicating a slight decrease in metallicity with increasing galactocentric distance.
• The radial metallicity gradient of the GSE progenitor galaxy is constrained to ≈−0.1−0.15+0.06 dex/kpc using N-body simulations and HESTIA cosmological models.
• The inferred gradient is consistent with those observed in nearby dwarf galaxies with old stellar populations, suggesting similar chemical evolution patterns despite differences in mass.
• The study demonstrates that chemical tagging using orbital integrals—despite their non-conservation during mergers—can successfully recover the progenitor’s internal chemical structure.
• The results imply that massive, ancient dwarf galaxies like GSE likely had radial metallicity gradients similar to present-day dwarfs, but with significantly higher masses, indicating distinct evolutionary pathways.
• The work establishes a robust method to infer the pre-accretion properties of accreted galaxies using chemical and kinematic data, enhancing Galactic archaeology.