Bellardini et al., available on arXiv

Abstract: We use FIRE-2 simulations to examine 3-D variations of gas-phase elemental abundances of [O/H], [Fe/H], and [N/H] in 11 Milky Way (MW) and M31-mass galaxies across their formation histories at z≤1.5 (tlookback≤9.4 Gyr), motivated by characterizing the initial conditions of stars for chemical tagging. Gas within 1 kpc of the disk midplane is vertically homogeneous to ≲0.008 dex at all z≤1.5. We find negative radial gradients (metallicity decreases with galactocentric radius) at all times, which steepen over time from ≈−0.01 dex kpc−1 at z=1 (tlookback=7.8 Gyr) to ≈−0.03 dex 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 dex at z=1, reducing to 0.05 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 (tlookback≳6.9 Gyr). Furthermore, elemental abundances are measurably homogeneous (to ≲0.05 dex) across a radial range of ΔR≈3.5 kpc at z≳1 and ΔR≈1.7 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.