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ABSTRACT 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.
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3D gas-phase elemental abundances across the formation histories of Milky Way-mass galaxies in the FIRE simulations: initial conditions for chemical tagging
ABSTRACT We use FIRE-2 simulations to examine 3D variations of gas-phase elemental abundances of [O/H], [Fe/H], and [N/H] in 11 MW and M31-mass galaxies across their formation histories at z ≤ 1.5 ($$t_{\rm lookback} \le 9.4 \, \rm {Gyr}$$), motivated by characterizing the initial conditions of stars for chemical tagging. Gas within $$1 \, \rm {kpc}$$ of the disc mid-plane is vertically homogeneous to $$\lesssim 0.008 \, \rm {dex}$$ at all z ≤ 1.5. We find negative radial gradients (metallicity decreases with galactocentric radius) at all times, which steepen over time from $$\approx \! -0.01 \, \rm {dex}\, \rm {kpc}^{-1}$$ at z = 1 ($$t_{\rm lookback} = 7.8 \, \rm {Gyr}$$) to $$\approx \! -0.03 \, \rm {dex}\, \rm {kpc}^{-1}$$ at z = 0, and which broadly agree with observations of the MW, M31, and nearby MW/M31-mass galaxies. Azimuthal variations at fixed radius are typically $$0.14 \, \rm {dex}$$ at z = 1, reducing to $$0.05 \, \rm {dex}$$ at z = 0. Thus, over time radial gradients become steeper while azimuthal variations become weaker (more homogeneous). As a result, azimuthal variations were larger than radial variations at z ≳ 0.8 ($$t_{\rm lookback} \gtrsim 6.9 \, \rm {Gyr}$$). Furthermore, elemental abundances are measurably homogeneous (to ≲0.05 dex) across a radial range of $$\Delta R \approx 3.5 \, \rm {kpc}$$ at z ≳ 1 and $$\Delta R \approx 1.7 \, \rm {kpc}$$ at z = 0. We also measure full distributions of elemental abundances, finding typically negatively skewed normal distributions at z ≳ 1 that evolve to typically Gaussian distributions by z = 0. Our results on gas abundances inform the initial conditions for stars, including the spatial and temporal scales for applying chemical tagging to understand stellar birth in the MW.
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- Award ID(s):
- 1715216
- PAR ID:
- 10278860
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 505
- Issue:
- 3
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 4586 to 4607
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/mnras/stab1606
