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We construct time-evolving gravitational potential models for a Milky Way–mass galaxy from the FIRE-2 suite of cosmological-baryonic simulations using basis function expansions. These models capture the angular variation with spherical harmonics for the halo and azimuthal harmonics for the disk, and the radial or meridional plane variation with splines. We fit low-order expansions (four angular/harmonic terms) to the galaxy’s potential for each snapshot, spaced roughly 25 Myr apart, over the last 4 Gyr of its evolution, then extract the forces at discrete times and interpolate them between adjacent snapshots for forward orbit integration. Our method reconstructs the forces felt by simulation particles with high fidelity, with 95% of both stars and dark matter, outside of self-gravitating subhalos, exhibiting errors ≤4% in both the disk and the halo. Imposing symmetry on the model systematically increases these errors, particularly for disk particles, which show greater sensitivity to imposed symmetries. The majority of orbits recovered using the models exhibit positional errors ≤10% for 2–3 orbital periods, with higher errors for orbits that spend more time near the galactic center. Approximate integrals of motion are retrieved with high accuracy even with a larger potential sampling interval of 200 Myr. After 4 Gyr of integration, 43% and 70% of orbits have total energy and angular momentum errors within 10%, respectively. Consequently, there is higher reliability in orbital shape parameters such as pericenters and apocenters, with errors ∼10% even after multiple orbital periods. These techniques have diverse applications, including studying satellite disruption in cosmological contexts. 

Abstract Measurements of the accelerations of stars enabled by time-series extreme-precision spectroscopic observations, pulsar timing, and eclipsing binary stars in the solar neighborhood offer insights into the mass distribution of the Milky Way that do not rely on traditional equilibrium modeling. Given the measured accelerations, we can determine a total mass density and infer the amount of dark matter (DM) by accounting for the mass in stars, gas, and dust. Leveraging FIRE-2 simulations of Milky Way–mass galaxies we compare vertical acceleration profiles between cold DM (CDM) and self-interacting DM (SIDM) with a constant cross section of 1 cm²g⁻¹across three halos with diverse assembly histories. Notably, significant asymmetries in vertical acceleration profiles near the midplane at fixed radii are observed in both CDM and SIDM, particularly in halos recently affected by mergers with satellites of Sagittarius/SMC-like masses or greater. These asymmetries offer a unique window into exploring the merger history of a galaxy. We show that SIDM halos manifest a more oblate shape and consistently exhibit higher local stellar and DM densities and steeper vertical acceleration gradients, up to 10%–30% steeper near the solar neighborhood. However, similar magnitude changes can arise from azimuthal variations in the baryonic components at a fixed radius and external influences like mergers, making it difficult to distinguish between CDM and SIDM using acceleration measurements in a single galaxy. 

ABSTRACT We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. We quantify the relative importance of cosmological disc settling and post-formation dynamical heating. We identify three eras: a Pre-Disc Era (typically ≳ 8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disc Era (≈8–4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, σform; and a Late-Disc Era (≲ 4 Gyr ago), when stars formed with low σform. σform increased with time during the Pre-Disc Era, peaking ≈8 Gyr ago, then decreased throughout the Early-Disc Era as the disc settled and remained low throughout the Late-Disc Era. By contrast, the dispersion measured today, σnow, increases monotonically with age because of stronger post-formation heating for Pre-Disc stars. Importantly, most of σnow was in place at formation, not added post-formation, for stars younger than ≈10 Gyr. We compare the evolution of the three velocity components: at all times, σR, form > σϕ, form > σZ, form. Post-formation heating primarily increased σR at ages ≲ 4 Gyr but acted nearly isotropically for older stars. The kinematics of young stars in FIRE-2 broadly agree with the range observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time that the disc began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder discs. Including stellar cosmic-ray feedback does not significantly change disc rotational support at fixed stellar mass. 

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. 

ABSTRACT We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way-mass disc galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the discs at z ≈ 0 with pixel sizes ranging from 250 to 750 pc, analogous to modern integral field unit galaxy surveys, mapping the gas-phase metallicities in both the cold and dense gas and the ionized gas correlated with H ii regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity of the order of ≲0.1 dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at z ≈ 0: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving radially inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards. 

ABSTRACT We investigate stellar elemental abundance patterns at $$z$$ = 0 in eight low-mass ($$M_{*}=10^{6}{-}10^{9}\ \text{M}_{\odot }$$) galaxies in the Feedback in Realistic Environments cosmological simulations. Using magnesium (Mg) as a representative α-element, we explore stellar abundance patterns in magnesium-to-iron ([Mg/Fe]) versus iron-to-hydrogen ([Fe/H]), which follow an overall monotonic trend that evolved slowly over time. Additionally, we explore three notable secondary features in enrichment (in three different case-study galaxies) that arise from a galaxy merger or bursty star formation. First, we observe a secondary track with a lower [Mg/Fe] than the main trend. At $$z$$ = 0, stars from this track are predominantly found within 2–6 kpc of the centre; they were accreted in a 1:3 total-mass-ratio merger ∼0.4 Gyr ago. Second, we find a distinct elemental bimodality that forms following a strong burst in star formation in a galaxy at $$t_{\text{lookback}}\, \sim 10$$ Gyr. This burst quenched star formation for ∼0.66 Gyr, allowing Type Ia supernovae to enrich the system with iron (Fe) before star formation resumed. Third, we examine stripes in enrichment that run roughly orthogonal to the dominant [Mg/Fe] versus [Fe/H] trend; these stripes correspond to short bursts of star formation during which core-collapse supernovae enrich the surrounding medium with Mg (and Fe) on short time-scales. If observed, these features would substantiate the utility of elemental abundances in revealing the assembly and star-formation histories of dwarf galaxies. We explore the observability of these features for upcoming spectroscopic studies. Our results show that precise measurements of elemental abundance patterns can reveal critical events in the formation histories of low-mass galaxies. 

ABSTRACT The physics of magnetic fields (B) and cosmic rays (CRs) have recently been included in simulations of galaxy formation. However, significant uncertainties remain in how these components affect galaxy evolution. To understand their common observational tracers, we analyse the magnetic fields in a set of high-resolution, magnetohydrodynamic, cosmological simulations of Milky-Way-like galaxies from the FIRE-2 project. We compare mock observables of magnetic field tracers for simulations with and without CRs to observations of Zeeman splitting and rotation/dispersion measures. We find reasonable agreement between simulations and observations in both the neutral and the ionized interstellar medium (ISM). We find that the simulated galaxies with CRs show weaker ISM |B| fields on average compared to their magnetic-field-only counterparts. This is a manifestation of the effects of CRs in the diffuse, low density inner circumgalactic medium (CGM). We find that equipartition between magnetic and cosmic ray energy densities may be valid at large (> 1 kpc) scales for typical ISM densities of Milky-Way-like galaxies, but not in their haloes. Within the ISM, the magnetic fields in our simulated galaxies follow a power-law scaling with gas density. The scaling extends down to neutral hydrogen number densities < 300 cm−3, in contrast to observationally derived models, but consistent with the observational measurements. Finally, we generate synthetic rotation measure (RM) profiles for projections of the simulated galaxies and compare to observational constraints in the CGM. While consistent with upper limits, improved data are needed to detect the predicted CGM RMs at 10–200 kpc and better constrain theoretical predictions. 

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. 

ABSTRACT We develop a hybrid model of galactic chemical evolution that combines a multiring computation of chemical enrichment with a prescription for stellar migration and the vertical distribution of stellar populations informed by a cosmological hydrodynamic disc galaxy simulation. Our fiducial model adopts empirically motivated forms of the star formation law and star formation history, with a gradient in outflow mass loading tuned to reproduce the observed metallicity gradient. With this approach, the model reproduces many of the striking qualitative features of the Milky Way disc’s abundance structure: (i) the dependence of the [O/Fe]–[Fe/H] distribution on radius Rgal and mid-plane distance |z|; (ii) the changing shapes of the [O/H] and [Fe/H] distributions with Rgal and |z|; (iii) a broad distribution of [O/Fe] at sub-solar metallicity and changes in the [O/Fe] distribution with Rgal, |z|, and [Fe/H]; (iv) a tight correlation between [O/Fe] and stellar age for [O/Fe] > 0.1; (v) a population of young and intermediate-age α-enhanced stars caused by migration-induced variability in the Type Ia supernova rate; (vi) non-monotonic age–[O/H] and age–[Fe/H] relations, with large scatter and a median age of ∼4 Gyr near solar metallicity. Observationally motivated models with an enhanced star formation rate ∼2 Gyr ago improve agreement with the observed age–[Fe/H] and age–[O/H] relations, but worsen agreement with the observed age–[O/Fe] relation. None of our models predict an [O/Fe] distribution with the distinct bimodality seen in the observations, suggesting that more dramatic evolutionary pathways are required. All code and tables used for our models are publicly available through the Versatile Integrator for Chemical Evolution (VICE; https://pypi.org/project/vice). 

Abstract In the era of large-scale spectroscopic surveys in the Local Group, we can explore using chemical abundances of halo stars to study the star formation and chemical enrichment histories of the dwarf galaxy progenitors of the Milky Way (MW) and M31 stellar halos. In this paper, we investigate using the chemical abundance ratio distributions (CARDs) of seven stellar halos from the Latte suite of FIRE-2 simulations. We attempt to infer galaxies’ assembly histories by modeling the CARDs of the stellar halos of the Latte galaxies as a linear combination oftemplateCARDs from disrupted dwarfs, with different stellar massesM_⋆and quenching timest₁₀₀. We present a method for constructing these templates using present-day dwarf galaxies. For four of the seven Latte halos studied in this work, we recover the mass spectrum of accreted dwarfs to a precision of <10%. For the fraction of mass accreted as a function oft₁₀₀, we find the residuals of 20%–30% for five of the seven simulations. We discuss the failure modes of this method, which arise from the diversity of star formation and chemical enrichment histories that dwarf galaxies can take. These failure cases can be robustly identified by the high model residuals. Although the CARDs modeling method does not successfully infer the assembly histories in these cases, the CARDs of these disrupted dwarfs contain signatures of their unusual formation histories. Our results are promising for using CARDs to learn more about the histories of the progenitors of the MW and M31 stellar halos.