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1. Response of the LMC’s Bar to a Recent SMC Collision and Implications for the SMC’s Dark Matter Profile

   https://doi.org/10.3847/1538-4357/ade0ae  

   Rathore, Himansh; Besla, Gurtina; Daniel, Kathryne J; Beraldo_e_Silva, Leandro (July 2025, The Astrophysical Journal)

   Abstract The LMC’s stellar bar is offset from the outer disk center, tilted from the disk plane, and does not drive gas inflows. These properties are atypical of bars in gas-rich galaxies, yet the LMC bar’s strength and radius are similar to typical barred galaxies. UsingN-body hydrodynamic simulations, we show that the LMC’s unusual bar is explainable if there was a recent collision (impact parameter ≈2 kpc) between the LMC and SMC. Pre-collision, the simulated bar is centered and coplanar. Post-collision, the simulated bar is offset (≈1.5 kpc) and tilted (≈8 $\stackrel{\text{°}}{.}$6). The simulated bar offset reduces with time, and comparing with the observed offset (≈0.8 kpc) suggests the timing of the true collision to be 150–200 Myr ago. Then, 150 Myr post-collision, the LMC’s bar is centered with its dark matter (DM) halo, whereas the outer disk center is separated from the DM center by ≈1 kpc. The SMC collision produces a tilted-ring structure for the simulated LMC, consistent with observations. Post-collision, the simulated LMC bar’s pattern speed decreases by a factor of 2. We also provide a generalizable framework to quantitatively compare the LMC’s central gas distribution in different LMC–SMC interaction scenarios. We demonstrate that the SMC’s torques on the LMC’s bar during the collision are sufficient to explain the observed bar tilt, provided the SMC’s total mass within 2 kpc was (0.8–2.4) × 10⁹M_⊙. Therefore, the LMC bar’s tilt constrains the SMC’s pre-collision DM profile, and requires the SMC to be a DM-dominated galaxy. 

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2. A dark matter profile to model diverse feedback-induced core sizes of ΛCDM haloes

   https://doi.org/10.1093/mnras/staa2101  

   Lazar, Alexandres; Bullock, James S; Boylan-Kolchin, Michael; Chan, T K; Hopkins, Philip F; Graus, Andrew S; Wetzel, Andrew; El-Badry, Kareem; Wheeler, Coral; Straight, Maria C; et al (September 2020, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We analyse the cold dark matter density profiles of 54 galaxy haloes simulated with Feedback In Realistic Environments (FIRE)-2 galaxy formation physics, each resolved within $$0.5{{\ \rm per\ cent}}$$ of the halo virial radius. These haloes contain galaxies with masses that range from ultrafaint dwarfs ($$M_\star \simeq 10^{4.5}\, \mathrm{M}_{\odot }$$) to the largest spirals ($$M_\star \simeq 10^{11}\, \mathrm{M}_{\odot }$$) and have density profiles that are both cored and cuspy. We characterize our results using a new, analytic density profile that extends the standard two-parameter Einasto form to allow for a pronounced constant density core in the resolved innermost radius. With one additional core-radius parameter, rc, this three-parameter core-Einasto profile is able to characterize our feedback-impacted dark matter haloes more accurately than other three-parameter profiles proposed in the literature. To enable comparisons with observations, we provide fitting functions for rc and other profile parameters as a function of both M⋆ and M⋆/Mhalo. In agreement with past studies, we find that dark matter core formation is most efficient at the characteristic stellar-to-halo mass ratio M⋆/Mhalo ≃ 5 × 10−3, or $$M_{\star } \sim 10^9 \, \mathrm{M}_{\odot }$$, with cores that are roughly the size of the galaxy half-light radius, rc ≃ 1−5 kpc. Furthermore, we find no evidence for core formation at radii $$\gtrsim 100\ \rm pc$$ in galaxies with M⋆/Mhalo < 5 × 10−4 or $$M_\star \lesssim 10^6 \, \mathrm{M}_{\odot }$$. For Milky Way-size galaxies, baryonic contraction often makes haloes significantly more concentrated and dense at the stellar half-light radius than DMO runs. However, even at the Milky Way scale, FIRE-2 galaxy formation still produces small dark matter cores of ≃ 0.5−2 kpc in size. Recent evidence for a ∼2 kpc core in the Milky Way’s dark matter halo is consistent with this expectation. 

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3. Aggressively Dissipative Dark Dwarfs: The Effects of Atomic Dark Matter on the Inner Densities of Isolated Dwarf Galaxies

   https://doi.org/10.3847/1538-4357/adb02b  

   Roy, Sandip; Shen, Xuejian; Barron, Jared; Lisanti, Mariangela; Curtin, David; Murray, Norman; Hopkins, Philip F (March 2025, The Astrophysical Journal)

   Abstract We present the first suite of cosmological hydrodynamical zoom-in simulations of isolated dwarf galaxies for a dark sector that consists of cold dark matter and a strongly dissipative subcomponent. The simulations are implemented in GIZMO and include standard baryons following the FIRE-2 galaxy formation physics model. The dissipative dark matter is modeled as atomic dark matter (aDM), which forms a dark hydrogen gas that cools in direct analogy to the Standard Model. Our suite includes seven different simulations of ∼10¹⁰M_⊙systems that vary over the aDM microphysics and the dwarf’s evolutionary history. We identify a region of aDM parameter space where the cooling rate is aggressive and the resulting halo density profile is universal. In this regime, the aDM gas cools rapidly at high redshifts, and only a small fraction survives in the form of a central dark gas disk; the majority collapses centrally into collisionless dark “clumps,” which are clusters of subresolution dark compact objects. These dark clumps rapidly equilibrate in the inner galaxy, resulting in an approximately isothermal distribution that can be modeled with a simple fitting function. Even when only a small fraction (∼5%) of the total dark matter is strongly dissipative, the central densities of classical dwarf galaxies can be enhanced by over an order of magnitude, providing a sharp prediction for observations. 

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4. Orientations of Dark Matter Halos in FIRE-2 Milky Way–mass Galaxies

   https://doi.org/10.3847/1538-4357/acea79  

   Baptista, Jay; Sanderson, Robyn; Huber, Dan; Wetzel, Andrew; Sameie, Omid; Boylan-Kolchin, Michael; Bailin, Jeremy; Hopkins, Philip F.; Faucher-Giguere, Claude-André; Chakrabarti, Sukanya; et al (November 2023, The Astrophysical Journal)

   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. 

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5. Testable Predictions of Outside-in Age Gradients in Dwarf Galaxies of All Types

   https://doi.org/10.3847/1538-4357/ad8b1e  

   Riggs, Claire L; Brooks, Alyson M; Munshi, Ferah; Christensen, Charlotte R; Cohen, Roger E; Quinn, Thomas R; Wadsley, James (November 2024, The Astrophysical Journal)

   Abstract We use a sample of 73 simulated satellite and central dwarf galaxies spanning a stellar mass range of 10^5.3–10^9.1M_⊙to investigate the origin of their stellar age gradients. We find that dwarf galaxies often form their stars “inside-out,” i.e., the stars form at successively larger radii over time. However, the oldest stars get reshuffled beyond the star-forming radius by fluctuations in the gravitational potential well caused by stellar feedback (the same mechanisms that cause dwarfs to form dark matter cores). The result is that many dwarfs appear to have an “outside-in” age gradient atz= 0, with younger stellar populations more centrally concentrated. However, for the reshuffled galaxies with the most extended star formation, young stars can form out to the large radii to which the old stars have been reshuffled, erasing the age gradient. We find that major mergers do not play a significant role in setting the age gradients of dwarfs. We find similar age gradient trends in satellites and field dwarfs, suggesting that environment plays only a minor role, if any. Finally, we find that the age gradient trends are imprinted on the galaxies at later times, suggesting that the stellar reshuffling dominates after the galaxies have formed 50% of their stellar mass. The later reshuffling is at odds with results from thefire-2simulations. Hence, age gradients offer a test of current star formation and feedback models that can be probed via observations of resolved stellar populations. 

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