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1. Cosmic-Ray Feedback from Supernovae in a Parker-unstable Medium

   https://doi.org/10.3847/1538-4357/adf4d7  

   Habegger, Roark; Zweibel, Ellen G (August 2025, The Astrophysical Journal)

   Abstract Supernova energy drives interstellar medium (ISM) turbulence and can help launch galactic winds. What difference does it make if 10% of the energy is initially deposited into cosmic rays? To help answer this question and study cosmic-ray feedback, we perform galactic patch simulations of a stratified ISM in a low star formation rate, high magnetic field regime. We compare two magnetohydrodynamic and cosmic-ray simulations, which are identical except for how each supernova’s energy is injected. In one, 10% of the energy is injected as cosmic-ray energy. In the other case, energy injection is strictly thermal and kinetic. We find that cosmic-ray injections drive a faster, hotter, and more massive outflow long after the injections occur. Both simulations show the formation of cold clouds (with a total mass fraction > 50%) through the Parker instability and thermal instability. The Parker instability simultaneously produces high mass loading factorsη> 10³as it does not directly require star formation and supernovae. We also show how the Parker instability naturally leads to a decorrelation of cosmic-ray pressure and gas density. This decorrelation leads to a significant decrease in the calorimetric fraction for injected cosmic rays, but it depends on having a highly resolved magnetic field. 

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2. Stress-testing cosmic ray physics: the impact of cosmic rays on the surviving disc of ram-pressure-stripped galaxies

   https://doi.org/10.1093/mnras/stac794  

   Farber, Ryan J.; Ruszkowski, Mateusz; Tonnesen, Stephanie; Holguin, Francisco (March 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Cluster spiral galaxies suffer catastrophic losses of the cool, neutral gas component of their interstellar medium due to ram pressure stripping, contributing to the observed quenching of star formation in the disc compared to galaxies in lower density environments. However, the short-term effects of ram pressure on the star formation rate and active galactic nucleus (AGN) activity of galaxies undergoing stripping remain unclear. Numerical studies have recently demonstrated cosmic rays can dramatically influence galaxy evolution for isolated galaxies, yet their influence on ram pressure stripping remains poorly constrained. We perform the first cosmic ray magnetohydrodynamic simulations of an L* galaxy undergoing ram pressure stripping, including radiative cooling, self-gravity of the gas, star formation, and stellar feedback. We find the microscopic transport of cosmic rays plays a key role in modulating the star formation enhancement experienced by spirals at the outskirts of clusters compared to isolated spirals. Moreover, we find that galaxies undergoing ram pressure stripping exhibit enhanced gas accretion on to their centres, which may explain the prevalence of AGNs in these objects. In agreement with observations, we find cosmic rays significantly boost the global radio emission of cluster spirals. Although the gas removal rate is relatively insensitive to cosmic ray physics, we find that cosmic rays significantly modify the phase distribution of the remaining gas disc. These results suggest observations of galaxies undergoing ram pressure stripping may place novel constraints on cosmic ray calorimetry and transport. 

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3. Suppressed Cosmic-Ray Energy Densities in Molecular Clouds from Streaming Instability-regulated Transport

   https://doi.org/10.3847/1538-4357/ad675a  

   Fitz_Axen, Margot; Offner, Stella; Hopkins, Philip_F; Krumholz, Mark_R; Grudić, Michael_Y (September 2024, The Astrophysical Journal)

   Abstract Cosmic rays (CRs) are the primary driver of ionization in star-forming molecular clouds (MCs). Despite their potential impacts on gas dynamics and chemistry, no simulations of star cluster formation following the creation of individual stars have included explicit cosmic-ray transport (CRT) to date. We conduct the first numerical simulations following the collapse of a 2000M_⊙MC and the subsequent star formation including CRT using the STAR FORmation in Gaseous Environments framework implemented in the GIZMO code. We show that when CRT is streaming-dominated, the CR energy in the cloud is strongly attenuated due to energy losses from the streaming instability. Consequently, in a Milky Way–like environment the median CR ionization rate in the cloud is low (ζ≲ 2 × 10⁻¹⁹s⁻¹) during the main star-forming epoch of the calculation and the impact of CRs on the star formation in the cloud is limited. However, in high-CR environments, the CR distribution in the cloud is elevated (ζ≲ 6 × 10⁻¹⁸), and the relatively higher CR pressure outside the cloud causes slightly earlier cloud collapse and increases the star formation efficiency by 50% to ∼13%. The initial mass function is similar in all cases except with possible variations in a high-CR environment. Further studies are needed to explain the range of ionization rates observed in MCs and explore star formation in extreme CR environments. 

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4. Cosmic-Ray Diffusion in the Turbulent Interstellar Medium: Effects of Mirror Diffusion and Pitch-angle Scattering

   https://doi.org/10.3847/1538-4357/ade4c8  

   Barreto-Mota, Lucas; de_Gouveia_Dal_Pino, Elisabete M; Xu, Siyao; Lazarian, Alexandre (July 2025, The Astrophysical Journal)

   Abstract Cosmic rays (CRs) interact with turbulent magnetic fields in the interstellar medium (ISM), generating nonthermal emission. After many decades of studies, the theoretical understanding of their diffusion in the ISM continues to pose a challenge. This study numerically explores a recent prediction termed “mirror diffusion” and its synergy with the traditional diffusion mechanism based on gyroresonant scattering. Our study combines 3D MHD simulations of star-forming regions with test particle simulations to analyze CR diffusion. We demonstrate the significance of mirror diffusion in CR diffusion parallel to the magnetic field when the mirroring condition is satisfied. Our results support the theoretical expectation that the resulting particle propagation arising from mirror diffusion in combination with much faster diffusion induced by gyroresonant scattering resembles a Levy-flight-like propagation. Our study highlights the necessity to reevaluate the diffusion coefficients traditionally adopted in the ISM based on gyroresonant scattering alone. For instance, our simulations imply a diffusion coefficient ∼10²⁷cm²s^–1for particles with a few hundred TeV within regions spanning a few parsecs around the source. This estimate is in agreement with gamma-ray observations, which show the relevance of our results for the understanding of gamma-ray emission in star-forming regions. 

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5. Metallicity Dependence of Pressure-regulated Feedback-modulated Star Formation in the TIGRESS-NCR Simulation Suite

   https://doi.org/10.3847/1538-4357/ad59ab  

   Kim, Chang-Goo; Ostriker, Eve C; Kim, Jeong-Gyu; Gong, Munan; Bryan, Greg L; Fielding, Drummond B; Hassan, Sultan; Ho, Matthew; Jeffreson, Sarah_M R; Somerville, Rachel S; et al (August 2024, The Astrophysical Journal)

   Abstract We present a new suite of numerical simulations of the star-forming interstellar medium (ISM) in galactic disks using the TIGRESS-NCR framework. Distinctive aspects of our simulation suite are (1) sophisticated and comprehensive numerical treatments of essential physical processes including magnetohydrodynamics, self-gravity, and galactic differential rotation, as well as photochemistry, cooling, and heating coupled with direct ray-tracing UV radiation transfer and resolved supernova feedback and (2) wide parameter coverage including the variation in metallicity over $Z\prime \equiv Z/Z_{\odot }\sim 0.1-3$, gas surface density Σ_gas∼ 5–150M_⊙pc⁻², and stellar surface density Σ_star∼ 1–50M_⊙pc⁻². The range of emergent star formation rate surface density is Σ_SFR∼ 10⁻⁴–0.5M_⊙kpc⁻²yr⁻¹, and ISM total midplane pressure isP_tot/k_B= 10³–10⁶cm⁻³K, withP_totequal to the ISM weight $\mathcal{W}$. For given Σ_gasand Σ_star, we find ${\mathrm{\Sigma }}_{\mathrm{SFR}}\propto Z{\prime }^{0.3}$. We provide an interpretation based on the pressure-regulated feedback-modulated (PRFM) star formation theory. The total midplane pressure consists of thermal, turbulent, and magnetic stresses. We characterize feedback modulation in terms of the yield ϒ, defined as the ratio of each stress to Σ_SFR. The thermal feedback yield varies sensitively with both weight and metallicity as ${\mathrm{\Upsilon }}_{\mathrm{th}}\propto {\mathcal{W}}^{-0.46}Z{\prime }^{-0.53}$, while the combined turbulent and magnetic feedback yield shows weaker dependence ${\mathrm{\Upsilon }}_{\mathrm{turb}+\mathrm{mag}}\propto {\mathcal{W}}^{-0.22}Z{\prime }^{-0.18}$. The reduction in Σ_SFRat low metallicity is due mainly to enhanced thermal feedback yield, resulting from reduced attenuation of UV radiation. With the metallicity-dependent calibrations we provide, PRFM theory can be used for a new subgrid star formation prescription in cosmological simulations where the ISM is unresolved. 

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