[Paper Review] LYRA II: Cosmological dwarf galaxy formation with inhomogeneous Population III enrichment

Thales A. Gutcke, Rüdiger Pakmor+2 more|arXiv (Cornell University)|Oct 12, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy89 references36 citations

TL;DR

This study presents a high-resolution cosmological simulation of a 2×10⁹ M⊙ dwarf galaxy using the LYRA model in AREPO, resolving individual supernovae and inhomogeneous Population III (PopIII) enrichment. It shows that host halo mass for PopIII stars critically shapes the galaxy's star formation history and metallicity distribution, with higher-mass PopIII hosts enabling earlier, denser star formation and sustained gas accretion enabling late-time re-ignition of star formation, while dark matter profiles remain cuspy despite hierarchical assembly.

ABSTRACT

We present the simulation of a $2 imes10^9 M_\odot$ halo mass cosmological dwarf galaxy run to $z=0$ at 4 solar mass gas resolution with resolved supernova feedback. We compare three simple subgrid implementations for the inhomogeneous chemical enrichment from Population III stars and compare them to constraints from Local Group dwarf galaxies. The employed model, LYRA, is a novel high resolution galaxy formation model built for the moving mesh code AREPO, which is marked by a resolved multi-phase interstellar medium, single stars and individual supernova events. The resulting reionization relic is characterized by a short ($<1.5$ Gyr) star formation history that is repeatedly brought to a standstill by violent bursts of feedback. Star formation is reignited for a short duration due to a merger at $z\approx4$ and then again at $z\approx0.2-0$ after sustained gas accretion. Our model $z=0$ galaxy matches the stellar mass, size, stellar kinematics and metallicity relations of Local Group dwarf galaxies well. The dark matter profile does not exhibit a core in any version of the model. We show that the host halo masses of Population III stars affect the assembly history of dwarf galaxies. This manifests itself through the initial gaseous collapse in the progenitor halos, affecting the central density of the stellar component and through the accretion of luminous substructure.

Motivation & Objective

• To investigate how inhomogeneous chemical enrichment from Population III stars affects the formation and evolution of cosmological dwarf galaxies.
• To test whether different host halo masses for the first stars (PopIII) influence the star formation history, metallicity distribution, and structural properties of z = 0 dwarf galaxies.
• To assess the impact of PopIII enrichment on dark matter profile morphology, particularly whether it leads to cored profiles.
• To compare simulation results with observational constraints from Local Group dwarf galaxies, including stellar mass, size, kinematics, and metallicity relations.

Proposed method

• Simulates a 2×10⁹ M⊙ cosmological dwarf galaxy at 4 M⊙ gas resolution using the high-resolution galaxy formation model LYRA in the AREPO moving-mesh code.
• Resolves the multi-phase interstellar medium, individual supernova events, and radiation feedback from young stars.
• Implements three subgrid models for inhomogeneous PopIII enrichment: homogeneous enrichment, enrichment at MPopIII = 10⁶ M⊙, and enrichment at MPopIII = 10⁷ M⊙, with an additional model adding a 3 km/s velocity kick to compensate for unresolved stellar motions.
• Uses on-the-fly halo finding and metallicity floor assignment based on halo mass thresholds to model the onset of PopIII enrichment.
• Tracks the accretion of luminous substructures and ex-situ stellar fractions to assess hierarchical assembly effects.
• Compares simulated galaxy properties (stellar mass, size, kinematics, metallicity, dark matter profile) to observational data from the Local Group and Solo Dwarf Survey.

Experimental results

Research questions

• RQ1 How does the host halo mass of the first Population III stars influence the star formation history of a cosmological dwarf galaxy?
• RQ2 To what extent does inhomogeneous PopIII enrichment shape the metallicity distribution function in dwarf galaxies?
• RQ3 Can sustained gas accretion at low rates reignite star formation in low-mass dwarfs, and what conditions are required for this to occur?
• RQ4 Does the hierarchical assembly of dwarf galaxies via accretion of smaller halos lead to observable differences in stellar substructure and ex-situ fractions?
• RQ5 Do the simulated dark matter profiles exhibit cores, and how do they compare to observations of Local Group dwarfs?

Key findings

• The star formation history is short (<1.5 Gyr) and quenched early by reionization and supernova feedback, with two brief re-ignition episodes at z ≈4 and z ≈0.2–0.3 due to mergers and sustained gas accretion.
• The total stellar mass is robust across models (~10⁸ M⊙), but the star formation history varies significantly: earlier onset occurs in models with lower MPopIII (10⁶ M⊙), while higher MPopIII (10⁷ M⊙) delays initial star formation.
• The metallicity distribution is bi-modal in the MPopIII = 10⁶ M⊙ model due to short, isolated bursts in small halos, while the homogeneous model yields a uni-modal distribution.
• Sustained gas accretion over gigayears can reignite star formation at extremely low rates (10⁻⁶–10⁻⁵ M⊙ yr⁻¹), but only if the central potential is deep enough, which is enabled by a central stellar over-density (likely a nuclear star cluster).
• The ex-situ stellar fraction exceeds 95% due to hierarchical assembly from small, star-forming progenitor halos, and the number of accreted luminous substructures is sensitive to PopIII host halo mass.
• No statistically significant core is formed in the dark matter profile across any model, despite high stellar-to-halo mass ratio and high star formation density threshold, indicating that PopIII enrichment and feedback do not drive core formation in this system.