We characterize the 3D 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 $\approx 7 \, \rm {Gyr}$ 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 $\gtrsim 12 \, \rm {Gyr}$ ago. The strength of the present-day abundance gradient does not correlate with when the disc ‘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 deg scatter decreased over time, from $\lesssim 0.17 \, \rm {dex}$ at $t_{\rm lb} = 11.6 \, \rm {Gyr}$ to $\sim 0.04 \, \rm {dex}$ at present-day. Consequently, stars at $t_{\rm lb} \gtrsim 8 \, \rm {Gyr}$ formed in a disc with primarily azimuthal scatter in abundances. All stars formed in a vertically homogeneous disc, Δ[Fe/H]$\le 0.02 \, \rm {dex}$ within $1 \, \rm {kpc}$ of the galactic mid-plane, with the exception of the young stars in the inner $\approx 4 \, \rm {kpc}$ at z ∼ 0. These results generally agree with our previous analysis of gas-phase elemental abundances, which reinforces the importance of cosmological disc evolution and azimuthal scatter in the context of stellar chemical tagging. We provide analytic fits to our results for use in chemical-tagging analyses.
We analyse the stellar abundances of massive galaxies (log M*/M⊙ > 10.5) at redshift, z = 2, in the IllustrisTNG simulation with the goal of guiding the interpretation of current and future observations, particularly from JWST. We find that the effective size, Re, of galaxies strongly affects the abundance measurements: both [Mg/H] and [Fe/H] are anticorrelated with Re, while the relative abundance [Mg/Fe] slightly increases with Re. The α enhancement as tracked by [Mg/Fe] traces the formation time-scale of a galaxy weakly, and mostly depends on Re. Aperture effects are important: measuring the stellar abundances within 1 kpc instead of within Re can make a large difference. These results are all due to a nearly universal, steeply declining stellar abundance profile that does not scale with galaxy size – Small galaxies appear metal-rich because their stars live in the inner part of the profile where abundances are high. The slope of this profile is mostly set by the gas-phase abundance profile and not substantially modified by stellar age gradients. The gas-phase abundance profile, in turn, is determined by the strong radial dependence of the gas fraction and star-formation efficiency. We develop a simple model to describe the chemical enrichment, in which each radial bin of a galaxy is treated as an independent closed-box system. This model reproduces the gas-phase abundance profile of simulated galaxies, but not the detailed distribution of their stellar abundances, for which gas and/or metal transport are likely needed.
more » « less- NSF-PAR ID:
- 10416209
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 523
- Issue:
- 1
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- p. 849-864
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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