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ABSTRACT Galaxies like the Milky Way are surrounded by complex populations of satellites at all stages of tidal disruption. In this paper, we present a dynamical study of the disrupting satellite galaxies in the Auriga simulations that are orbiting 28 distinct Milky Way-mass hosts across three resolutions. We find that the satellite galaxy populations are highly disrupted. The majority of satellites that remain fully intact at present day were accreted recently without experiencing more than one pericentre ($$n_{\rm peri} \lesssim 1$$) and have large apocentres ($$r_{\rm apo} \gtrsim 200 \mathrm{\, kpc}$$) and pericentres ($$r_{\rm peri} \gtrsim 50 \mathrm{\, kpc}$$). The remaining satellites have experienced significant tidal disruption and, given full knowledge of the system, would be classified as stellar streams. We find stellar streams in Auriga across the range of pericentres and apocentres of the known Milky Way dwarf galaxy streams and, interestingly, overlapping significantly with the Milky Way intact satellite population. We find no significant change in satellite orbital distributions across resolution. However, we do see substantial halo-to-halo variance of $$(r_\text{peri}, r_\text{apo})$$ distributions across host galaxies, as well as a dependence of satellite orbits on host halo mass–systems disrupt at larger pericentres and apocentres in more massive hosts. Our results suggest that either cosmological simulations (including, but not limited to, Auriga) are disrupting satellites far too readily, or that the Milky Way’s satellites are more disrupted than current imaging surveys have revealed. Future observing facilities and careful mock observations of these systems will be key to revealing the nature of this apparent discrepancy. 

Abstract ΛCDM cosmology predicts the hierarchical formation of galaxies, which build up mass by merger events and accreting smaller systems. The stellar halo of the Milky Way (MW) has proven to be useful a tool for tracing this accretion history. However, most of this work has focused on the outer halo where dynamical times are large and the dynamical properties of accreted systems are preserved. In this work, we investigate the inner galaxy regime, where dynamical times are relatively small and systems are generally completely phase mixed. Using the FIRE-2 and Auriga cosmological zoom-in simulation suites of MW-mass galaxies, we find the stellar density profiles along the minor axis (perpendicular to the galactic disk) within the Navarro–Frenk–White scale radii (R ≈ 15 kpc) are best described as an exponential disk with scale height < 0.3 kpc and a power-law component with slopeα ≈ −4. The stellar density amplitude and slope for the power-law component are not significantly correlated with metrics of the galaxy’s accretion history. Instead, we find the stellar profiles strongly correlate with the dark matter profile. Across simulation suites, the galaxies studied in this work have a stellar-to-dark-matter mass ratio that decreases as 1/r²along the minor axis. 

Abstract The large-scale morphology of Milky Way (MW)–mass dark matter halos is shaped by two key processes: filamentary accretion from the cosmic web and interactions with massive satellites. Disentangling their contributions is essential for understanding galaxy evolution and constructing accurate mass models of the MW. We analyze the time-dependent structure of MW-mass halos from zoomed cosmological-hydrodynamical simulations by decomposing their mass distribution into spherical harmonic expansions. We find that the dipole and quadrupole moments dominate the gravitational power spectrum, encoding key information about the halo’s shape and its interaction with the cosmic environment. While the dipole reflects transient perturbations from infalling satellites and damps on dynamical timescales, the quadrupole—linked to the halo’s triaxiality—is a persistent feature. We show that the quadrupole’s orientation aligns with the largest filaments, imprinting a long-lived memory on the halo’s morphology even in its inner regions (∼30 kpc). At the virial radius, the quadrupole distortion can reach 1–2 times the spherical density, highlighting the importance of environment in shaping MW-mass halos. Using multichannel singular spectrum analysis, we successfully disentangle the effects of satellite mergers and filamentary accretion on quadrupole. We find that, compared to isolated MW–LMC simulations that typically use a spherical halo, the LMC-mass satellite induces a quadrupolar response that is an order of magnitude larger in our cosmological halo. This highlights the need for models that incorporate the MW’s asymmetry and time evolution, with direct consequences for observable structures such as disk warps, the LMC-induced wake, and stellar tracers—particularly in the era of precision astrometry. 

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. 

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. 

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 A variety of observational campaigns seek to test dark matter models by measuring dark matter subhaloes at low masses. Despite their predicted lack of stars, these subhaloes may be detectable through gravitational lensing or via their gravitational perturbations on stellar streams. To set measurable expectations for subhalo populations within Lambda cold dark matter, we examine 11 Milky Way (MW)-mass haloes from the FIRE-2 baryonic simulations, quantifying the counts and orbital fluxes for subhaloes with properties relevant to stellar stream interactions: masses down to $$10^{6}\, \text{M}_\odot$$, distances ≲50 kpc of the galactic centre, across z = 0 − 1 (tlookback = 0–8 Gyr). We provide fits to our results and their dependence on subhalo mass, distance, and lookback time, for use in (semi)analytical models. A typical MW-mass halo contains ≈16 subhaloes $$\gt 10^{7}\, \text{M}_\odot$$ (≈1 subhalo $$\gt 10^{8}\, \text{M}_\odot$$) within 50 kpc at z ≈ 0. We compare our results with dark matter-only versions of the same simulations: because they lack a central galaxy potential, they overpredict subhalo counts by 2–10×, more so at smaller distances. Subhalo counts around a given MW-mass galaxy declined over time, being ≈10× higher at z = 1 than at z ≈ 0. Subhaloes have nearly isotropic orbital velocity distributions at z ≈ 0. Across our simulations, we also identified 4 analogues of Large Magellanic Cloud satellite passages; these analogues enhance subhalo counts by 1.4–2.1 times, significantly increasing the expected subhalo population around the MW today. Our results imply an interaction rate of ∼5 per Gyr for a stream like GD-1, sufficient to make subhalo–stream interactions a promising method of measuring dark subhaloes. 

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 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.