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Abstract The physical mechanisms responsible for bar formation and destruction in galaxies remain a subject of debate. While we have gained valuable insight into how bars form and evolve from isolated idealized simulations, in the cosmological domain, galactic bars evolve in complex environments, with mergers and gas accretion events occurring in the presence of the turbulent interstellar medium with multiple star formation episodes, in addition to coupling with their host galaxies’ dark matter halos. We investigate the bar formation in 13 Milky Way–mass galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulations. 8 of the 13 simulated galaxies form bars at some point during their history: three from tidal interactions and five from internal evolution of the disk. The bars in FIRE-2 are generally shorter than the corotation radius (mean bar radius ∼1.53 kpc), have a wide range of pattern speeds (36–97 km s⁻¹kpc⁻¹), and live for a wide range of dynamical times (2–160 bar rotations). We find that the bar formation in FIRE-2 galaxies is influenced by satellite interactions and the stellar-to-dark-matter mass ratio in the inner galaxy, but neither is a sufficient condition for bar formation. Bar formation is more likely to occur, with the bars formed being stronger and longer-lived, if the disks are kinematically cold; galaxies with high central gas fractions and/or vigorous star formation, on the other hand, tend to form weaker bars. In the case of the FIRE-2 galaxies, these properties combine to produce ellipsoidal bars with strengthsA₂/A₀∼ 0.1–0.2. 

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. 

Dark matter subhalos are predicted to perturb stellar streams; stream morphologies and dynamics can, therefore, constrain the mass distribution of subhalos. Using FIRE-2 simulations of Milky Way–mass galaxies, we demonstrate that the presence of an LMC analog significantly changes stream–subhalo encounter rates. The LMC analog brings in many subhalos, increasing encounter rates for streams near the massive satellite by 10%–40%. Additionally, the LMC analog displaces the host from its center of mass (inducing reflex motion), which causes a north–south asymmetry in the density and radial velocity distributions of subhalos. This asymmetry, combined with the presence of LMC-analog subhalos, causes encounter rates at the same distance to vary by 50%–70% across the sky, particularly in regions opposite the LMC analog. Furthermore, the LMC analog induces a density wake in the host's dark matter halo, further boosting the encounter rates near the LMC analog. We also explore how stream orbital properties affect encounter rates, finding up to a 50% increase for streams moving retrograde to the LMC analog’s orbit in the opposite quadrant. Finally, we report the encounter rates for Milky Way streams within the context of our simulations, both with and without the presence of an LMC analog. The dependence of encounter rates on stream location, orbit, and their position relative to the LMC has important implications for where to search for streams with spurs and gaps in the Milky Way. 

ABSTRACT Observational studies are finding stars believed to be relics of the earliest stages of hierarchical mass assembly of the Milky Way (i.e. proto-galaxy). In this work, we contextualize these findings by studying the masses, ages, spatial distributions, morphology, kinematics, and chemical compositions of proto-galaxy populations from the 13 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. Our findings indicate that proto-Milky Way populations: (i) can have a stellar mass range between 1 × 108 < M⋆ < 2 × 1010 [M⊙], a virial mass range between 3 × 1010 < M⋆ < 6 × 1011 [M⊙], and be as young as 8 ≲ Age ≲ 12.8 [Gyr] (1 ≲ z ≲ 6); (ii) are pre-dominantly centrally concentrated, with $$\sim 50~{{\ \rm per\ cent}}$$ of the stars contained within 5–10 kpc; (iii) on average show weak but systematic net rotation in the plane of the host’s disc at z = 0 (i.e. 0.25 ≲ 〈κ/κdisc〉 ≲ 0.8); (iv) present [α/Fe]-[Fe/H] compositions that overlap with the metal-poor tail of the host’s old disc; and (v) tend to assemble slightly earlier in Local Group-like environments than in systems in isolation. Interestingly, we find that $$\sim 60~{{\ \rm per\ cent}}$$ of the proto-Milky Way galaxies are comprised by 1 dominant system (1/5 ≲M⋆/M⋆, proto-MilkyWay≲ 4/5) and 4–5 lower mass systems (M⋆/M⋆, proto-MilkyWay≲ 1/10); the other $$\sim 40~{{\ \rm per\ cent}}$$ are comprised by 2 dominant systems and 3–4 lower mass systems. These massive/dominant proto-Milky Way fragments can be distinguished from the lower mass ones in chemical-kinematic samples, but appear (qualitatively) indistinguishable from one another. Our results could help observational studies disentangle if the Milky Way formed from one or two dominant systems. 

Abstract The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions (HALO7D) survey measures the kinematics and chemical properties of stars in the Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope–measured proper motions of MW halo main-sequence turnoff stars in the four Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey fields. HALO7D consists of deep pencil beams, making it complementary to other contemporary wide-field surveys. We present the [Fe/H] and [α/Fe] abundances for 113 HALO7D stars in the Galactocentric radial range of ∼10–40 kpc along four separate pointings. Using the full 7D chemodynamical data (3D positions, 3D velocities, and abundances) of HALO7D, we measure the velocity anisotropy,β, of the halo velocity ellipsoid for each field and for different metallicity-binned subsamples. We find that two of the four fields have stars on very radial orbits, while the remaining two have stars on more isotropic orbits. Separating the stars into high-, mid-, and low-[Fe/H] bins at −2.2 and −1.1 dex for each field separately, we find differences in the anisotropies between the fields and between the bins; some fields appear dominated by radial orbits in all bins, while other fields show variation between the [Fe/H] bins. These chemodynamical differences are evidence that the HALO7D fields have different fractional contributions from the progenitors that built up the MW stellar halo. Our results highlight the additional information available on smaller spatial scales compared to results from a spherical average of the stellar halo. 

Abstract In the Λ-Cold Dark Matter model of the universe, galaxies form in part through accreting satellite systems. Previous works have built an understanding of the signatures of these processes contained within galactic stellar halos. This work revisits that picture using seven Milky Way–like galaxies in the Latte suite of FIRE-2 cosmological simulations. The resolution of these simulations allows a comparison of contributions from satellites above M * ≳ 10 × 7 M ⊙ , enabling the analysis of observable properties for disrupted satellites in a fully self-consistent and cosmological context. Our results show that the time of accretion and the stellar mass of an accreted satellite are fundamental parameters that in partnership dictate the resulting spatial distribution, orbital energy, and [ α /Fe]-[Fe/H] compositions of the stellar debris of such mergers at present day. These parameters also govern the resulting dynamical state of an accreted galaxy at z = 0, leading to the expectation that the inner regions of the stellar halo ( R GC ≲ 30 kpc) should contain fully phase-mixed debris from both lower- and higher-mass satellites. In addition, we find that a significant fraction of the lower-mass satellites accreted at early times deposit debris in the outer halo ( R GC > 50 kpc) that are not fully phased-mixed, indicating that they could be identified in kinematic surveys. Our results suggest that, as future surveys become increasingly able to map the outer halo of our Galaxy, they may reveal the remnants of long-dead dwarf galaxies whose counterparts are too faint to be seen in situ in higher-redshift surveys. 

Abstract The Large Magellanic Cloud (LMC) will induce a dynamical friction (DF) wake on infall to the Milky Way (MW). The MW’s stellar halo will respond to the gravity of the LMC and the dark matter (DM) wake, forming a stellar counterpart to the DM wake. This provides a novel opportunity to constrain the properties of the DM particle. We present a suite of high-resolution, windtunnel-style simulations of the LMC's DF wake that compare the structure, kinematics, and stellar tracer response of the DM wake in cold DM (CDM), with and without self-gravity, versus fuzzy DM (FDM) withm_a= 10⁻²³eV. We conclude that the self-gravity of the DM wake cannot be ignored. Its inclusion raises the wake’s density by ∼10%, and holds the wake together over larger distances (∼50 kpc) than if self-gravity is ignored. The DM wake’s mass is comparable to the LMC’s infall mass, meaning the DM wake is a significant perturber to the dynamics of MW halo tracers. An FDM wake is more granular in structure and is ∼20% dynamically colder than a CDM wake, but with comparable density. The granularity of an FDM wake increases the stars’ kinematic response at the percent level compared to CDM, providing a possible avenue of distinguishing a CDM versus FDM wake. This underscores the need for kinematic measurements of stars in the stellar halo at distances of 70–100 kpc. 

Abstract The shape and orientation of dark matter (DM) halos are sensitive to the microphysics of the DM particles, yet in many mass models, the symmetry axes of the Milky Way’s DM halo are often assumed to be aligned with the symmetry axes of the stellar disk. This is well motivated for the inner DM halo, but not for the outer halo. We use zoomed-in cosmological baryonic simulations from the Latte suite of FIRE-2 Milky Way–mass galaxies to explore the evolution of the DM halo’s orientation with radius and time, with or without a major merger with a Large Magellanic Cloud analog, and when varying the DM model. In three of the four cold DM halos we examine, the orientation of the halo minor axis diverges from the stellar disk vector by more than 20° beyond about 30 galactocentric kpc, reaching a maximum of 30°–90°, depending on the individual halo’s formation history. In identical simulations using a model of self-interacting DM withσ= 1 cm²g⁻¹, the halo remains aligned with the stellar disk out to ∼200–400 kpc. Interactions with massive satellites (M≳ 4 × 10¹⁰M_⊙at pericenter;M≳ 3.3 × 10¹⁰M_⊙at infall) affect the orientation of the halo significantly, aligning the halo’s major axis with the satellite galaxy from the disk to the virial radius. The relative orientation of the halo and disk beyond 30 kpc is a potential diagnostic of self-interacting DM, if the effects of massive satellites can be accounted for. 

Abstract The total mass of the Local Group (LG) is a fundamental quantity that enables interpreting the orbits of its constituent galaxies and placing the LG in a cosmological context. One of the few methods that allows inferring the total mass directly is the “Timing Argument,” which models the relative orbit of the Milky Way (MW) and M31 in equilibrium. The MW itself is not in equilibrium, a byproduct of its merger history and including the recent pericentric passage of the Large Magellanic Cloud (LMC), and recent work has found that the MW disk is moving with a lower bound “travel velocity” of ∼32 km s⁻¹with respect to the outer stellar halo. Previous Timing Argument measurements have attempted to account for this nonequilibrium state, but have been restricted to theoretical predictions for the impact of the LMC specifically. In this paper, we quantify the impact of a travel velocity on recovered LG mass estimates using several different compilations of recent kinematic measurements of M31. We find that incorporating the measured value of the travel velocity lowers the inferred LG mass by 10%–12% compared to a static MW halo. Measurements of the travel velocity with more distant tracers could yield even larger values, which would further decrease the inferred LG mass. Therefore, the newly measured travel velocity directly implies a lower LG mass than from a model with a static MW halo and must be considered in future dynamical studies of the Local Volume. 

Abstract In the Gaia era it is increasingly apparent that traditional static, parameterized models are insufficient to describe the mass distribution of our complex, dynamically evolving Milky Way (MW). In this work, we compare different time-evolving and time-independent representations of the gravitational potentials of simulated MW-mass galaxies from the FIRE-2 suite of cosmological-baryonic simulations. Using these potentials, we calculate actions for star particles in tidal streams around three galaxies with varying merger histories at each snapshot from 7 Gyr ago to the present day. We determine the action-space coherence preserved by each model using the Kullback–Leibler divergence to gauge the degree of clustering in actions and the relative stability of the clusters over time. We find that all models produce a clustered action space for simulations with no significant mergers. However, a massive (mass ratio prior to infall more similar than 1:8) interacting galaxy not present in the model will result in mischaracterized orbits for stars most affected by the interaction. The locations of the action space clusters (i.e., the orbits of the stream stars) are only preserved by the time-evolving model, while the time-independent models can lose significant amounts of information as soon as 0.5–1 Gyr ago, even if the system does not undergo a significant merger. Our results imply that reverse-integration of stream orbits in the MW using a fixed potential is likely to give incorrect results if integrated longer than 0.5 Gyr into the past.