[Paper Review] 3D elemental abundances of stars at formation across the histories of Milky Way-mass galaxies in the FIRE simulations

Matthew A. Bellardini, Andrew Wetzel+2 more|arXiv (Cornell University)|Mar 7, 2022

Galaxies: Formation, Evolution, Phenomena132 references37 citations

TL;DR

This study uses high-resolution FIRE-2 cosmological simulations to model 3D elemental abundances ([Fe/H], [Mg/H], [Mg/Fe]) of stars at formation in 11 Milky Way- and M31-mass galaxies. It reveals that radial abundance gradients steepen over time, azimuthal scatter decreases from ~0.17 dex at 11.6 Gyr ago to ~0.04 dex today, and stars formed after ~8 Gyr ago exhibit primarily azimuthal abundance variations, providing critical constraints for chemical tagging in galactic archaeology.

ABSTRACT

We characterize the 3-D spatial variations of [Fe/H], [Mg/H], and [Mg/Fe] in stars at the time of their formation, across 11 simulated Milky Way (MW)- and M31-mass galaxies in the FIRE-2 simulations, to inform initial conditions for chemical tagging. The overall scatter in [Fe/H] within a galaxy decreased with time until $\approx 7$ Gyr ago, after which it increased to today: this arises from a competition between a reduction of azimuthal scatter and a steepening of the radial gradient in abundance over time. The radial gradient is generally negative, and it steepened over time from an initially flat gradient $\gtrsim 12$ Gyr ago. The strength of the present-day abundance gradient does not correlate with when the disk `settled'; instead, it best correlates with the radial velocity dispersion within the galaxy. The strength of azimuthal variation is nearly independent of radius, and the 360 degree scatter decreased over time, from $\lesssim 0.17$ dex at $t_{ m lb} = 11.6$ Gyr to $\sim 0.04$ dex at present day. Consequently, stars at $t_{ m lb} \gtrsim 8$ Gyr formed in a disk with primarily azimuthal scatter in abundances. All stars formed in a vertically homogeneous disk, $\Delta$[Fe/H] $\leq 0.02$ dex within $1$ kpc of the galactic midplane, with the exception of the young stars in the inner $\approx 4$ kpc at $z \sim 0$. These results generally agree with our previous analysis of gas-phase elemental abundances, which reinforces the importance of cosmological disk evolution and azimuthal scatter in the context of stellar chemical tagging. We provide analytic fits to our results for use in chemical-tagging analyses.

Motivation & Objective

• To characterize the 3D spatial distribution of elemental abundances at the time of star formation in Milky Way-mass galaxies.
• To quantify how radial, vertical, and azimuthal abundance variations evolve over cosmic time.
• To provide analytic fits for initial abundance conditions to improve chemical tagging in galactic archaeology.
• To test the assumption that stars form with abundances representative of their local gas, and assess the role of azimuthal scatter in birth conditions.
• To determine the transition time when azimuthal abundance homogeneity becomes a valid assumption for chemical tagging.

Proposed method

• Simulates 11 Milky Way- and M31-mass galaxies using the FIRE-2 cosmological simulation suite with high-resolution hydrodynamics and stellar feedback.
• Tracks elemental abundances ([Fe/H], [Mg/H], [Mg/Fe]) of stars at the time of their formation using gas-phase enrichment models.
• Measures radial, vertical, and azimuthal abundance scatter across different lookback times and galactocentric radii.
• Fits piecewise linear functions to radial abundance profiles to quantify gradient evolution.
• Compares stellar abundance scatter with gas-phase scatter to assess the validity of the star-gas abundance equivalence assumption.
• Derives analytic fits for radial gradients and azimuthal scatter as functions of lookback time for use in chemical tagging models.

Experimental results

Research questions

• RQ1 How do radial, vertical, and azimuthal gradients in stellar elemental abundances evolve over time in Milky Way-mass galaxies?
• RQ2 What is the timescale over which azimuthal abundance scatter in stars decreases, and how does it compare to gas-phase scatter?
• RQ3 How do the strengths of radial and azimuthal abundance variations correlate with galaxy kinematics and structural evolution?
• RQ4 To what extent do stars formed at different times exhibit homogeneous or inhomogeneous abundance distributions in 3D space?
• RQ5 At what lookback time does the assumption of azimuthal abundance homogeneity become valid for chemical tagging?

Key findings

• The radial abundance gradient in [Fe/H] was initially flat (>12 Gyr ago) and steepened over time, becoming most negative around 7 Gyr ago before slightly flattening to the present.
• The azimuthal scatter in [Fe/H] decreased from ≲0.17 dex at 11.6 Gyr ago to ∼0.04 dex at present day, indicating that stars formed after ~8 Gyr ago were born in a disk with primarily azimuthal abundance variations.
• Vertical abundance gradients were minimal, with ∆[Fe/H] ≤ 0.02 dex within 1 kpc of the midplane, except in the inner ~4 kpc where young stars show a small vertical gradient.
• The strength of the present-day radial abundance gradient correlates best with the radial velocity dispersion of the galaxy, not with the time when the disk settled.
• The azimuthal scatter in stars is systematically smaller than in the gas at large lookback times and large radii, indicating that star-forming gas is more spatially heterogeneous than the stars it produces.
• The transition lookback time for azimuthal abundance homogeneity in stars is ~8 Gyr ago, ~1 Gyr earlier than the corresponding transition time for gas, suggesting stars form with more spatially uniform abundances than their parent gas.