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Abstract Recent measurements of the Milky Way rotation curve found a sharp decline at around 15–20 kpc from the center of the Galaxy, suggesting that the Galactic dark matter halo is much less massive than predicted by other dynamical tracers. To address this tension, we study the validity of the assumptions made in calculating the Milky Way’s rotation curve. To do so, we apply the Jeans equation, the current standard approach of measuring rotation curves, to three cosmological zoom-in simulations of Milky Way–like galaxies from theFIRE-2 Latte suite. Using synthetic Gaia surveys, we replicate the sample selection process and calculation employed in measuring the Milky Way rotation curve. We examine four failure modes of this calculation and find that the measured curves deviate from the true curve by 5%–20% rather than below 5%, as estimated by previous works. Interestingly, there is a large galaxy-to-galaxy variance, and different systematics dominate different galaxies. We rederive the Milky Way’s dark matter density profile with the rotation curve while incorporating systematics from the simulations. The posterior distribution of the density profiles is consistent with a fiducial Navarro–Frenk–White (NFW) profile when assuming a generalized NFW profile for dark matter. We find that the virial mass is $7.32_{-1.53}^{+1.98}\times 10^{11}$M_⊙, consistent with other probes of the Milky Way’s mass. However, we recommend that the field move away from relying solely on the rotation curve when studying the dark matter profile and adopt methods that incorporate additional probes and/or do not heavily depend on assumptions described in this study. 

ABSTRACT The dark matter (DM) distribution in dwarf galaxies provides crucial insights into both structure formation and the particle nature of DM. GraphNPE (Graph Neural Posterior Estimator), first introduced in Nguyen et al. (2023), is a novel simulation-based inference framework that combines graph neural networks and normalizing flows to infer the DM density profile from line-of-sight stellar velocities. Here, we apply GraphNPE to satellite dwarf galaxies in the FIRE-2 Latte simulation suite of Milky Way-mass haloes, testing it against both Cold and Self-Interacting DM scenarios. Our method demonstrates superior precision compared to conventional Jeans-based approaches, recovering DM density profiles to within the 95 per cent confidence level even in systems with as few as 30 tracers. Moreover, we present the first evaluation of mass modelling methods in constraining two key parameters from realistic simulations: the peak circular velocity, $$V_\mathrm{max}$$, and the peak virial mass, $$M_\mathrm{200m}^\mathrm{peak}$$. Using only line-of-sight velocities, GraphNPE can reliably recover both $$V_\mathrm{max}$$ and $$M_\mathrm{200m}^\mathrm{peak}$$ within our quoted uncertainties, including those experiencing tidal effects ($$\gtrsim 63~{{\rm per\ cent}}$$ of systems are recovered within our 68 per cent confidence intervals and $$\gtrsim 92~{{\rm per\ cent}}$$ within our 95 per cent confidence intervals). The method achieves $$10-20~{{\rm per\ cent}}$$ accuracy in $$V_\mathrm{max}$$ recovery, while $$M_\mathrm{200m}^\mathrm{peak}$$ is recovered to $$0.1-0.4 \, \mathrm{dex}$$ accuracy. This work establishes GraphNPE as a robust tool for inferring DM density profiles in dwarf galaxies, offering promising avenues for constraining DM models. The framework’s potential extends beyond this study, as it can be adapted to non-spherical and disequilibrium models, showcasing the broader utility of simulation-based inference and graph-based learning in astrophysics. 

Abstract Spatial patterns of stellar elemental abundances encode rich information about a galaxy’s formation history. We analyze the radial, vertical, and azimuthal variations of metals in stars, both today and at formation, in the FIRE-2 cosmological simulations of Milky Way (MW)-mass galaxies, and we compare them with the MW. The radial gradient today is steeper (more negative) for younger stars, which agrees with the MW, although radial gradients are shallower in FIRE-2. Importantly, this age dependence was present already at birth: radial gradients today are only modestly (≲0.01 dex kpc⁻¹) shallower than at birth. Disk vertical settling gives rise to negative vertical gradients across all stars, but vertical gradients of mono-age stellar populations are weak. Similar to the MW, vertical gradients in FIRE-2 are shallower at larger radii, but they are overall shallower in FIRE-2. This vertical dependence was present already at birth: vertical gradients today are only modestly (≲0.1 dex kpc⁻¹) shallower than at birth. Azimuthal scatter is nearly constant with radius, and it is nearly constant with age ≲8 Gyr ago but increases for older stars. Azimuthal scatter is slightly larger (≲0.04 dex) today than at formation. Galaxies with larger azimuthal scatter have a stronger radial gradient, implying that azimuthal scatter today arises primarily from the radial redistribution of gas and stars. Overall, spatial variations of stellar metallicities show only modest differences between formation and today; spatial variations today primarily reflect the conditions of stars at birth, with spatial redistribution of stars after birth contributing secondarily. 

ABSTRACT We study the morphology of hundreds of simulated central galaxies in the stellar mass range $$M_\star =$$ 107.5–1011  $$\rm M_\odot$$ from the FIREbox cosmological volume. We demonstrate that FIREbox is able to predict a wide variety of morphologies, spanning from disc-dominated objects to spheroidal galaxies supported by stellar velocity dispersion. However, the simulations predict a strong relation between morphology (degree of rotational support) and stellar mass: galaxies comparable to the Milky Way are often disc-dominated while the presence of stellar discs mostly vanishes for dwarfs with $$M_\star < 10^9 ~$$\rm M_\odot$$. This defines a ‘morphology transition’ regime for galaxies with $$10^9 < M_\star /\rm {M_\odot }< 10^{10}$$ in which discs become increasingly common, but below which discs are rare. We show that burstiness in the star formation history and the deepening of the gravitational potential strongly correlate in our simulations with this transition regime, with discs forming in objects with lower levels of burstiness in the last $$\sim 6$$ Gyr and haloes with mass $$\sim 10^{11} ~ \rm {{\rm M}_{\odot }}$$ and above. While observations support a transition towards thicker discs in the regime of dwarfs, our results are in partial disagreement with observations of at least some largely rotationally supported gas discs in dwarfs with $$M_\star < 10^9$$\rm M_\odot$$. This study highlights dwarf morphology as a fundamental benchmark for testing future galaxy formation models. 

Open clusters (OCs) act as key probes that can be leveraged to constrain the formation and evolution of the Milky Way (MW)’s disk, as each has a unique chemical fingerprint and well-constrained age. Significant Galactic dynamic interactions can leave imprints on the orbital properties of OCs, allowing us to use the present-day properties of long-lived OCs to reconstruct the MW’s dynamic history. To explore these changes, we identify OC analogs in FIRE-2 simulations of MW-mass galaxies. For this work, we focus on one particular FIRE-2 OC, which we identify as an analog to the old, subsolar, distant, and high-Galactic-latitude MW OC, Berkeley 20. Our simulated OC resides ∼6 kpc from the galactic center and ultimately reaches a height $\mid Z_{\max }\mid >2$kpc from the galactic disk, similar to Berkeley 20. We trace the simulated cluster’s orbital and environmental history, identifying key perturbative episodes, including (1) an interaction with a gas overdensity in a spiral arm that prompts an outward migration event and (2) a substantial interaction with a Sagittarius Dwarf Spheroidal Galaxy–mass satellite that causes significant orbital modification. Our simulated OC shows significant resilience to disruption during both its outward migration and the satellite-driven heating event that causes subsequent inward migration. Ultimately, we find these two key processes—migration and satellite heating—are essential to include when assessing OC orbital dynamics in the era of Gaia. 

ABSTRACT We analyse the stellar distributions on the [Fe/H]–[Mg/Fe] plane for 11 Milky Way-mass galaxies from the FIRE-2 cosmological baryonic zoom-in simulations. Alpha-element bimodality, in the form of two separate sequences on the [Fe/H]–[Mg/Fe] plane, is not a universal feature of disc galaxies. Five galaxies demonstrate double sequences with the $$\alpha$$-enriched one being older and kinematically hotter, in qualitative agreement with the high-$$\alpha$$ and low-$$\alpha$$ populations in the Milky Way disc; three galaxies have unimodal distribution, two show weakly bimodal features where low-$$\alpha$$ sequence is visible only over a short range of metallicities, and one show strong bimodality with a different slope of high-$$\alpha$$ population. We examine the galaxies’ gas accretion history over the last 8 Gyr, when bimodal sequences emerge, and demonstrate that the presence of the low-$$\alpha$$ sequence in the bimodal galaxies is related to the recent infall of metal-poor gas from the circumgalactic medium that joins the galaxy in the outskirts and induces significant growth of the gas discs compared to their non-bimodal counterparts. We also analyse the sources of the accreted gas and illustrate that both gas-rich mergers and smooth accretion of ambient gas can be the source of the accreted gas, and create slightly different bimodal patterns. 

Abstract Using the FIRE-2 cosmological zoom-in simulations, we investigate the temporal evolution of gas-phase metallicity radial gradients of Milky Way–mass progenitors in the redshift range of 0.4 <z< 3. We pay special attention to the occurrence of positive (i.e., inverted) metallicity gradients—where metallicity increases with galactocentric radius. This trend, contrary to the more commonly observed negative radial gradients, has been frequently seen in recent spatially resolved grism observations. The rate of occurrence of positive gradients in FIRE-2 is about ∼7% for 0.4 <z< 3 and ∼13% at higher redshifts (1.5 <z< 3), broadly consistent with observations. Moreover, we investigate the correlations among galaxy metallicity gradient, stellar mass, star formation rate (SFR), and degree of rotational support. Metallicity gradients show a strong correlation with both sSFR and the rotational-to-dispersion velocity ratio (v_c/σ), implying that starbursts and kinematic morphology of galaxies play significant roles in shaping these gradients. The FIRE-2 simulations indicate that galaxies with high sSFR ( $\log \left(\mathrm{sSFR}\right[{\mathrm{yr}}^{-1}\left]\right)\gtrsim -9.2$) and weak rotational support (v_c/σ≲ 1) are more likely—by ∼15%—to develop positive metallicity gradients. This trend is attributed to galaxy-scale gas flows driven by stellar feedback, which effectively redistribute metals within the interstellar medium. Our results support the important role of stellar feedback in governing the chemo-structural evolution and disk formation of Milky Way–mass galaxies at the cosmic noon epoch. 

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. 

Open-star clusters are the essential building blocks of the Galactic disk; “strong chemical tagging”—the premise that all star clusters can be reconstructed given chemistry information alone—is a driving force behind many current and upcoming large Galactic spectroscopic surveys. In this work, we characterize the abundance patterns for nine elements (C, N, O, Ne, Mg, Si, S, Ca, and Fe) in open clusters (OCs) in three galaxies (m12i, m12f, and m12m) from the Latte suite of FIRE-2 simulations, to investigate the feasibility of strong chemical tagging in these simulations. We select young massive (≥10^4.6M_⊙) OCs formed in the last ∼100 Myr and calculate the intra- and intercluster abundance scatter for these clusters. We compare these results with analogous calculations drawn from observations of OCs in the Milky Way. We find the intracluster scatter of the observations and simulations to be comparable. While the abundance scatter within each cluster is minimal (≲0.020 dex), the mean abundance patterns of different clusters are not unique. We also calculate the chemical difference in intra- and intercluster star pairs and find it, in general, to be so small that it is difficult to distinguish between stars drawn from the same OC or from different OCs. Despite tracing three distinct nucleosynthetic families (core-collapse supernovae, white dwarf supernovae, and stellar winds), we conclude that these elemental abundances do not provide enough discriminating information to use strong chemical tagging for reliable OC membership.