arXiv subjects

Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 19 recordsLinked to original sources

arXiv2026

The impact of Population III stars on the elemental enrichment of ultra-faint galaxies in the FIRE-3 zoom-in simulations

The observed stellar mass-metallicity relation for ultra-faint galaxies in the Local Group (LG) is in tension with many theoretical predictions, suggesting the need for modified or additional channels of elemental enrichment at low metallicities. We present a suite of FIRE-3 cosmological baryonic zoom-in simulations of isolated ultra-faint galaxies run to z=0 (maximum baryonic resolution ~ 30 Msun, 0.1 pc) and LMC-mass environments (maximum baryonic resolution ~ 110 Msun, 0.5 pc), with different sub-grid models for Population III stars. Under our Pop. III model(s), star particles forming below a critical metallicity Z_crit represent an initial mass function (IMF) consisting of only massive stars (8-100 Msun). We examine how various stellar feedback processes impact the star formation histories (SFHs) and elemental abundances of ultra-faint galaxies. We find that our Pop. III model variants introduce an effective metallicity floor in ultra-faints at z=0, at Z_floor ~ 1e-5 Zsun to 1e-3 Zsun or [Fe/H]_floor ~ -7 to -5 depending on the model. However, simply including the Pop. III model does not systematically guarantee that all (or even most) ultra-faints enrich up to [Fe/H] ~ -2.5 to fully resolve the tension with the observed mass-metallicity relation, even though we obtain some galaxies at that level. Crucially, the ultra-faints that do enrich in [Fe/H] to this level observed in the LG have extended SFHs after Pop. III enrichment and significant iron enrichment from white dwarf (Type Ia) supernovae, indicating that iron enrichment from Pop. III alone is not sufficient. We also examine the ways in which early-time Pop. III feedback impacts the metallicity distributions and [Mg/Fe] abundance ratios in ultra-faint galaxies, and make a prediction for the stellar mass - [Mg/H] metallicity relation from our simulations.

astro-ph.GA↗

arXiv2026

Bridging Scales: Multi-Zone GRMHD-Informed Anisotropic AGN Feedback in Galaxies

We present a galaxy-scale black hole (BH) accretion and feedback model incorporating results from multi-zone general relativistic magnetohydrodynamic (GRMHD) simulations spanning the event horizon to the Bondi radius. The model replaces a fixed suppression of the Bondi rate with a nuclear-environment-dependent factor, $κ^{\rm hot}_{\rm cho}$, set by the local ratio of Bondi to gravitational radius, and distributes outflow mass, kinetic energy, and thermal energy according to the angular structure measured in the large-dynamic-range GRMHD calculations. We apply it to idealized isolated simulations of an M87*-like massive elliptical and a Milky Way/Sgr A*-like galaxy, spanning over three orders of magnitude in BH mass without tuning accretion or feedback efficiencies to match observables. In the M87*-like system, the GRMHD-informed angle-dependent feedback model suppresses late-time stellar mass growth more effectively than the isotropic reference model, despite similar long-term BH accretion rates. Varying the uncertain mass loading of the collimated polar component has a modest impact on BH growth and star formation, indicating that feedback anisotropy matters more than its detailed mass loading. We find that $κ^{\rm hot}_{\rm cho}$ varies with nuclear environment and time, yielding BH growth histories markedly different from fixed-suppression models. For Sgr A*, this variable factor yields an accretion rate more consistent with Event Horizon Telescope (EHT) constraints than a fixed, suppressed Bondi model. These results show that nuclear-environment-dependent accretion and feedback angular structure affect BH-galaxy evolution, requiring subgrid models to include more than a universal Bondi suppression factor and a scalar feedback efficiency.

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arXiv2026

The Galactic Center Gamma-Ray Excess and the Look-Elsewhere Effect

The origin of the excess of GeV gamma-rays in the Milky Way's galactic center observed by Fermi-LAT has been an open question for nearly two decades. The two leading interpretations of this signal are that it is produced either by annihilating dark matter (DM), or arises from unresolved point sources, such as a novel population of millisecond pulsars. In both scenarios, uncertainties and limitations in the modeling of galactic diffuse gamma-ray emission, such as the steady-state approximation of cosmic ray (CR) propagation and assumption of an axisymmetric distribution of CRs, complicate the interpretation of the galactic center excess (GCE). Simulations with full magnetohydrodynamics (MHD) and CR transport can provide a new avenue for understanding the astrophysical phenomena that drive galactic diffuse gamma-ray emission. We utilize MHD simulations of Milky Way analogs with time-dependent, spectrally-resolved CR transport to reconstruct diffuse gamma-ray emission. Comparing instantaneous emission to a time-averaged baseline, we find residuals up to several times larger than the Fermi GCE, with a wide range of spectral shapes and spatial morphologies. At some simulation snapshots, the spectrum and morphology of the residuals resemble the Fermi GCE. Our results suggest that the Fermi GCE may be a manifestation of the look-elsewhere effect- a transient fluctuation in diffuse emission from stellar sources, observed at an inopportune moment- and motivate the development of new Fermi-LAT analysis templates that account for time-dependent CR transport.

astro-ph.HE↗

arXiv2026

Initial Conditions of Magnetized AGN Accretion Disks from Cloud Capture and Disruption

Accretion onto supermassive black holes (BHs) from the ISM can occur via capture of cold gas 'clouds' (GMCs, clumps, or overdense structures). Unlike material in stellar tidal disruptions or X-ray binaries, or classical Bondi accretion, ISM clouds are strongly magnetized (plasma $β\equiv P_{\rm thermal}/P_{\rm magnetic} \ll 1$), with supersonic turbulence, and cool rapidly. We derive analytic scalings for the initial conditions of accretion disks formed by capture/disruption of such clouds, in three regimes: (1) low-mass BHs ($M_{\rm BH} \ll M_{\rm cl}$), where turbulent Bondi-Hoyle or hierarchical 'leapfrog' growth via sub-cloud encounters dominates; (2) comparable-mass encounters ($M_{\rm BH} \sim M_{\rm cl}$); and (3) massive BHs tidally disrupting small clouds ($M_{\rm BH} \gg M_{\rm cl}$). In each, the resulting disk is predicted to be initially toroidally-magnetically-dominated ($|B_ϕ| \gg |B_{z}|$, $β\ll 1$), geometrically thick ($H/R \sim 0.1-1$), and gravitationally stable ($Q \gtrsim 1$), with super-sonic, trans-Alfvenic turbulence. These conditions differ qualitatively from those assumed in classical accretion/$α$ disk theory, 'chaotic cold accretion', stellar tidal disruption, or Bondi-Hoyle accretion. At least initially, the toroidal field should be sustained by flux-freezing on timescales shorter than Parker or turbulent resistive losses. We examine how cloud mass spectra drive accretion rate distributions, duty cycles, and fueling rates, with integrated BH growth dominated by clouds of mass comparable to the BH. Given known cosmological evolution, we suggest a scenario linking (a) inefficient growth and wandering BHs in dwarf galaxies, (b) hyper-Eddington growth in little red dots, (c) peak quasar activity in massive gas-rich systems at cosmic noon, and (d) lower-luminosity AGN in gas-poor galaxies at late times.

astro-ph.GA↗

arXiv2026

Magnetic field evolution in feedback-dominated galaxies

Over the last decade, magnetic fields (B-fields) have become a common ingredient in galaxy formation simulations. Although seemingly unimportant in matching observational constraints such as the $M_*-M_{\rm halo}$ relations, the B-field influences the circumgalactic medium (CGM) gas and determines cosmic ray (CR) propagation, making it an indispensable non-thermal component. Nevertheless, the intricate physics of B-fields, particularly how they evolve to the observed $μ$G-level field, remains largely elusive. In this work, we systematically address the evolution of B-fields over cosmic time using the FIRE-2 (Feedback In Realistic Environments) magnetohydrodynamics (MHD) simulations. We present results for both Milky Way (MW)-mass and dwarf galaxies, with and without CRs. We find that CRs have little impact on B-field properties, while the halo mass controls the saturated B-field strength at $z\sim 0$. However, all runs reach a halo-independent magnetic-to-turbulent ratio of $f_{\rm B}\sim 1-10\%$, though the methods used to measure turbulence could affect this value. We identify a three-stage evolutionary course for the B-field in tandem with galaxy formation: i. expansion, ii. amplification, and iii. saturation. The feature is universal, though the timings depend on the initial location and star formation history of the gas. Flux-freezing is the predominant driver for the first and last stages, exhibiting power relations $B\sim ρ^{2/3}$ and $ρ^{1/2}$, although the latter could also be explained by dynamical and energetic equilibria. The intermediate amplification stage, potentially driven by ``dynamos'', is fuelled by structure formation, feedback, and gas turbulence. We conjecture a numerically triggered, turbulence-driven amplification process to explain this phenomenon.

astro-ph.GA↗

arXiv2026

Exploring a Cosmic-Ray Inverse-Compton Origin to the SZ-to-X-Ray Pressure Deficit in the Cool Core Cluster ZwCl 3146

We explore the possibility that inverse-Compton (IC) scattering of cosmic microwave background photons by $\sim$GeV cosmic rays (CRs) injected by the central active galactic nucleus (AGN) in cool core (CC) clusters produces a non-negligible continuum-like X-ray signal that is easily misinterpreted as intracluster medium (ICM) thermal bremsstrahlung continuum. This is particularly relevant to the cooling flow problem--the lack of star formation relative to X-ray-inferred ICM cooling rates. Using ZwCl 3146, a relaxed CC system at $z=0.291$, we compare pressure profiles derived via X-rays and the thermal Sunyaev-Zel'dovich (SZ) effect. While SZ measurements probe only thermal ICM electrons, additional CR-IC emission would appear to boost the X-ray-inferred pressure. Relative to unity, we measure a $25\%$ decrement in $P_{SZ}/P_X$ within $100$ kpc of the ZwCl 3146 center at a statistical significance $\simeq3σ$, consistent with predicted deficits from CR-IC contamination in reasonable models of central AGN-driven CR injection. X-ray spectral fits of a two-component model with thermal ICM and CR-IC emission are consistent with CR-IC as the cause of this deficit. We test alternative explanations and systematics that could drive such a decrement, with leading-order systematics associated with halo triaxiality and AGN-driven cavities. On average, these systematics could bias $P_{SZ}/P_X$ low by $\simeq5\%$, a factor of two lower than our measurement uncertainty, with an object-to-object variation $\simeq5\%$. While our results establish that non-negligible CR-IC emission is plausible in ZwCl 3146, we stress that detailed studies of larger cluster samples are required to robustly assess whether CR-IC is relevant to the cooling flow problem.

astro-ph.HE↗

arXiv2026

The Impact of Simulation Resolution on Dwarf Galaxy Stellar Stream Populations in FIRE

Cosmological simulations allow us to study galaxy-progenitor stream populations, allowing comparisons with observations to constrain galaxy formation physics. Previous work found that present-day galaxy-progenitor streams in FIRE have larger pericenters than observed in the MW, which might indicate that they tidally disrupt at larger distances, where the Galactic tides are weaker, than in the MW. One possible explanation is that they tidally disrupt too quickly because of limited resolution. At lower resolution, low-mass dwarf galaxies can be puffier and more susceptible to disruption. We build and compare dwarf-galaxy stream catalogs in two FIRE-2 cosmological zoom-in simulations with identical initial conditions (m12i) but different particle mass resolution (7070 $M_\odot$ and 880 $M_\odot$). The higher-resolution run yields a larger dwarf galaxy stream population (22 streams versus 14) using the same cutoff of 100 star particles, primarily because it resolves lower-mass streams (down to $\sim10^{5}$ $M_\odot$). Both simulations form streams at galactocentric radii < 50 kpc, but the higher-resolution Triple Latte simulation produces more streams at these small radii. We find that dwarf galaxies in the higher-resolution simulation begin disrupting at smaller pericenters than in the lower-resolution simulation. By cross-matching stream candidates, we identify counterparts for $\sim$89$\%$ of the systems in the lower-resolution simulation. For matched systems that remain on similar orbits, the higher-resolution counterparts remain intact for longer. The orbital properties of streams in the higher-resolution simulation are broadly consistent with the MW dwarf-galaxy stream population, in contrast to the lower-resolution simulation.

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arXiv2026

The Virgo Cluster as a Case Study for Ancient Cosmic Ray Halos in Radio and X-rays

Virgo is the nearest cool core (CC) galaxy cluster, and is archetypal of the class of X-ray weak CCs. Despite the X-ray continuum implying a large cooling rate ($\gtrsim 10\,{\rm M_{\odot}\,yr^{-1}}$), measurements of cooler gas phases, dust, and star formation show almost no cooling outside the M87 nucleus. It is also known that M87$^{\ast}$ is a prodigious source of cosmic ray (CR) leptons observed in radio and $γ$-rays, and recent radio observations have shown that the M87 radio halo is low-frequency and its spectral index steepens rapidly away from the injection region+jets, consistent with an aging, low-energy CR population. We demonstrate that coupling X-ray and radio to newer Faraday rotation limits on the magnetic field, this implies the CRs within the CC radius ($\lesssim 40$kpc) must contribute significantly to the total pressure and soft X-ray emission, through a combination of Coulomb heating the gas and inverse-Compton (IC) scattering microwave background photons to $\sim$keV. We show this can explain the apparent correspondence between jet/radio power and X-ray cooling luminosity, and why the X-ray inferred cooling rate appears much larger than otherwise observed. This would also explain why the central X-ray inferred metallicity at $\lesssim 10\,$kpc appears to be suppressed relative to the UV/optical inferred metallicity (in a wavelength-dependent manner). Compared to more extreme radio+X-ray sources like Perseus, we find the observations require a softer 'escaping' CR spectrum (escaping or accelerated outside $\gtrsim 1\,$kpc from M87$^{\ast}$).

astro-ph.HE↗

arXiv2026

Star Formation Mitosis: The Effects of Particle Splitting on the Star Formation Outcome

Particle splitting can increase the dynamic range in Lagrangian astrophysical simulations by several orders of magnitude. However, such methods can introduce numerical noise that may affect the rate of convergence, particularly for collapse problems where fragmentation could be artificially suppressed or enhanced. Thus it is necessary to determine how these errors propagate to key quantities of interest, and to correct or mitigate them where possible. Using GIZMO's MFM-MHD method, we perform a set of $\approx 220$ simulations of a uniform-density turbulent gas sphere undergoing collapse, systematically varying the cloud mass and radius, feedback physics, initial resolution, the pace and cadence at which we split particles and the sink (accreting) or single-age stellar population (SSP; non-accreting) particle formation prescription. Crucially, we compare with control runs that start with uniformly-high resolution without splitting. While we find no evidence for artificial fragmentation caused by splitting particles, particle splitting runs have a smoother density field than their non-splitting counterparts. Due to this, particle splitting runs exhibit minor differences in the density and turbulence statistics. However, they form about the same total stellar mass as the control non-splitting runs, albeit with a slightly different mass spectrum. As a result, particle splitting runs have a different evolutionary history, compared to runs without particle splitting, especially when stellar feedback is present. Despite the changes seen in runs where we split particles, the numerical solution lies within the scatter inherently present in these chaotic systems, which we model by modifying the initial turbulent velocity field. Our results here argue that particle splitting generally ``works" for star formation applications, despite some systematic differences in the specifics.

astro-ph.GA↗

arXiv2026

Non-ideal MHD and protostellar feedback effects on disc formation and evolution in numerical simulations of star cluster formation

While recent surveys have resolved hundreds of nearby protostellar discs, numerical simulations assuming ideal magnetohydrodynamics (MHD) have historically struggled to achieve disc formation due to efficient angular momentum removal by magnetic torques. Non-ideal MHD effects, relevant at the low ionization fractions typical of molecular clouds, have been shown to reduce the effectiveness of magnetic braking and promote disc formation. In this work, we present the results from a suite of calculations following the gravitational collapse of 50 $M_{\odot}$ turbulent molecular cloud cores down to the formation and evolution of stellar systems and protostellar discs. We use the radiation-MHD code GIZMO including non-ideal MHD (Ohmic resistivity, ambipolar diffusion, and the Hall effect) and the STARFORGE numerical framework for modeling star formation and stellar feedback. We compare the effects of assuming ideal vs. non-ideal MHD and including sub-grid protostellar jet feedback on disc formation and evolution. Discs form in all of our models but are least massive in the model with ideal MHD and sub-grid jet feedback. Apart from the ideal MHD$+$jets model, we do not observe any significant differences in disc properties between the ideal and non-ideal MHD models; however, ideal MHD discs are embedded in smaller rotating envelopes. Disc sizes are in general agreement with those of observed discs. Jet feedback increases core fragmentation and reduces final stellar masses. Our results suggest that magnetic braking does not efficiently suppress disc formation, regardless of whether ideal or non-ideal MHD is assumed, under the dynamical conditions in which multiple stellar systems form.

astro-ph.SR↗

arXiv2026

Direct Tests of Black Hole Accretion Rate Prescriptions: I. Bondi Accretion at Different Scales

We present spatially resolved parsec-scale measurements of nuclear conditions (gas density and kinetic temperature) relevant for black hole accretion rate predictions in the Seyfert 2 galaxy, NGC 1068. We inject these parameters into the prescription for a Bondi-like accretion model, then compare the resulting accretion rate prediction to the empirical accretion rate derived from hard X-ray observations. Cosmological simulations have spatial resolution ranging from $\sim$10 pc to $\sim$kpc scales, and so for reasonable comparison we test these accretion rate predictions in pixel-sized radial steps out to 500 pc. Compared to warm H$_2$ gas, CO gas is the dominant mass carrier close to the SMBH. We find that the Bondi accretion rate ($\dot{\mathrm{M}}_{\mathrm{Bondi}}$) of cold molecular gas alone (measured using CO) overestimates the true accretion rate by up to 14 dex in a small aperture (r$\lesssim$5 pc) around the black hole, and by at least 8 dex inside large apertures (r$\lesssim$500 pc). These results are the first in a series of direct tests of accretion rate prescriptions, and they suggest that using a Bondi accretion formalism to model supermassive black hole accretion in Seyfert 2 galaxies may lead to overestimated accretion rates in simulations.

astro-ph.GA↗

arXiv2026

A Hot DOG Forged in FIRE: Nuclear and Starburst Spectral Decomposition of a Luminous Infrared Galaxy Simulation with a Resolved Dust Torus

Ultraluminous infrared galaxies are powered by a combination of rapid star formation and active galactic nucleus (AGN) emission, but their relative importance is not always observationally clear. We study the galactic continuum spectrum of a cosmologically simulated $\sim 4 \times 10^{10} M_\odot$ stellar mass starburst galaxy at redshift $z\sim 4.4$ that refines down to resolve beyond the dust sublimation boundary of its super-Eddington-accreting $\sim 10^7 M_\odot$ supermassive black hole. We find that this system resembles the rare class of hot dust-obscured galaxy (Hot DOG), with a roughly flat (in $νF_ν$) IR emission spectrum that sharply drops off at wavelengths $\lesssim 5~μ\mathrm{m}$. Our system also matches with the observational properties of many Hot DOGs, including undergoing multiple galaxy mergers and being the most massive galaxy within a dense cosmological environment. The distinctive Hot DOG spectral shape in our system is caused by AGN-heated mid-IR warm dust, predominately starburst-heated far-IR cold dust, and a steep near- to mid-IR cutoff caused by strong absorption in the dense ISM of the galactic nucleus, rather than the dust torus itself. This system is lower luminosity ($L_\mathrm{IR} \sim 2 \times 10^{12} L_\odot$) than those detected by the WISE survey at similar redshifts, but will be a prime target for future far-IR surveys such as PRIMA. Our results show that Hot DOGs can naturally result as a transitional phase during rapid AGN accretion, but before significant AGN-driven outflows clear optically thin paths.

astro-ph.GA↗

arXiv2026

Idealized Global Models of Accretion Disks with Strong Toroidal Magnetic Fields

We present global magnetohydrodynamic (MHD) simulations of idealized accretion disks with a strong toroidal magnetic field using an equation of state that fixes the gas thermal scale height. The disk forms from the inflow of a rotating magnetized gas cloud with a toroidal magnetic field. We find that the system maintains a moderately strong mean azimuthal field in the midplane, with plasma-$β\sim1$, trans-Alfvénic fluctuations, and large accretion stresses $α\sim0.1$. The azimuthal field in the disk is continuously escaping along the vertical direction but is also replenished via a local dynamo. The inflowing gas initially forms a strongly magnetized Keplerian disk with $β\ll1$ and $α\gg 1$. The disk gradually collapses from the inside out over $\sim 50-80$ orbits to form a moderately magnetized disk with $β\sim1$ and $α\sim0.1$. Radial advection of azimuthal magnetic field can maintain $β\lesssim1$ exterior to the circularization radius but not inside of it. Inclusion of a net initial vertical magnetic field can lead to an even more strongly magnetized disk midplane, consistent with previous work. When the gas thermal scale is not resolved ($\lesssim 4$ cells per thermal scale height), however, the disk remains highly magnetized with $β\ll 1 $. We discuss our results in the context of related shearing box simulations and other global disk simulations. The level of angular momentum transport found here is consistent with that inferred observationally in dwarf novae and X-ray transient outbursts, unlike simulations of weakly magnetized accretion disks.

astro-ph.HE↗

arXiv2026

Investigating black hole accretion and feedback self-regulation in Seyfert galaxies using the FIRE-3 cosmological hydrodynamic simulations

Recent observations of local Seyfert galaxies show an intriguing connection between Active Galactic Nuclei (AGN) luminosity and a deficit of molecular gas on ~50pc scales compared to 200pc, the plausible imprint of AGN feedback. Motivated by these findings, we investigate the interplay between supermassive black hole (BH) accretion, AGN feedback, and nuclear gas reservoirs using high-resolution cosmological hydrodynamic simulations implementing FIRE-3 multi-phase interstellar medium (ISM) physics and multi-component BH accretion and feedback models. Focusing on the late-time evolution of four Milky Way-mass galaxies, we find recurrent cycles of increased gas inflow toward the accretion disc, enhanced BH accretion, feedback self-regulation, and suppressed gas inflow rate until the next fueling event. AGN winds interact with the ISM and escape preferentially through low-density polar channels after opening central cavities on ~10-500pc scales, regulating BH growth and producing episodic behaviour on ~10-100Myr timescales. The simulations reproduce the observed diversity of nuclear morphologies, gas concentrations, and AGN luminosities in late-type Seyfert galaxies, but do not exhibit a clear anti-correlation between gas concentration and AGN luminosity. Higher-luminosity AGN ($L_X$~$10^{41.5-43}$ erg s$^{-1}$) powered by the accretion disc reservoir can coexist with feedback-driven cavities, consistent with observations, but they are more common in simulated galaxies with centrally-peaked gas distributions. Although differences in sample selection, tracer choice, spatial resolution, and stochasticity in AGN fueling may impact underlying concentration-luminosity trends, the apparent tension between simulations and observations points to the timing between gas inflow, accretion-disc depletion, and feedback-driven clearing on ~50-200pc scales as a key constraint on AGN self-regulation models.

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arXiv2026

Beyond Cloud-9: The case for discovering more HI-rich failed halos

HI-rich starless halos, should they exist, hold great promise for elucidating dark matter halo structure. Yet realizing this potential demands reliable theoretical predictions for their properties and abundances. Indeed, the recent identification of Cloud-9 as a strong HI-rich starless halo candidate in the nearby universe makes such predictions timely. This Letter examines HI-rich ($M_\mathrm{HI} \geq 10^6\,M_\odot$) starless (isolated/central) halos at $z=0$ across three cosmological simulations: FIREbox, Recal-EAGLE and NIVARIA-LG. All three successfully produce such objects, with $M_\mathrm{HI}$ extending up to $\sim\!1$--$2$ dex above Cloud-9, but with number densities that vary by a factor of $\sim30$. These populations span different regions of the $M_\mathrm{HI}$--$M_\mathrm{gas}$--$M_{200}$ space: NIVARIA-LG produces objects with higher $M_\mathrm{HI}$ and $M_\mathrm{gas}$ values ($\gtrsim 10^7$ and $\gtrsim 10^8\,M_\odot$), while FIREbox predicts they lie within extremely narrow ranges of $M_\mathrm{gas} \sim(1.1$--$1.6)\times10^8\,M_\odot$ and $M_{200} \sim(7.8$--$8.6)\times10^9\,M_\odot$. Recal-EAGLE and NIVARIA-LG exhibit a strong $M_\mathrm{gas}-M_{200}$ correlation, with similar slopes but different normalizations. The simulations predict numerical Cloud-9 analogs -- though similarities in the shapes of their HI column-density profiles may be driven by FAST's modest beam; halving it already reveals differences. Collectively, these inter-simulation discrepancies make a compelling case for discoveries beyond Cloud-9: a statistical sample of well-resolved HI-rich starless halos is needed to discriminate amongst competing predictions.

astro-ph.GA↗

arXiv2026

An Inverse-Compton-Boosted Cool Core Unifies Perseus's Radio and X-ray Halos

Perseus is the brightest X-ray strong cool-core (SCC) cluster, with a bright central radio and $γ$-ray source plus low-frequency radio mini and giant halos. It is the archetype of the cooling flow (CF) problem, with X-rays implying mass cooling rates orders-of-magnitude larger than observed in other channels. Recent work suggested that ancient ($\gtrsim$\,Gyr-old) cosmic ray (CR) halos (ACRHs), injected by the central source, would produce thermal-like soft X-ray inverse-Compton (CR-IC) emission 'boosting' the CC and alleviating the CF problem. We examine Perseus and show that a simple model of CRs injected by NGC 1275 (+satellites) simultaneously accounts for the excess CF luminosity and minihalo. The models reproduce Perseus's soft X-ray surface brightness and X-ray inferred density/temperature/pressure/metallicity/cooling time/mass deposition rates; $γ$-ray spectra; extended hard X-rays; and radio surface brightness and spectral index data, from kpc-Mpc. These also reproduce independent constraints on magnetic field strengths and mass/potential models. The evolution of the minihalo spectral index and surface brightness are predicted by an aging population of CRs boosting the apparent SCC luminosity via CR-IC, and match well the observed hard X-ray slopes. The 'giant' low-frequency halo can be predicted by the sum of ACRHs around satellites distributed throughout the cluster, dominating diffuse synchrotron at $\gtrsim 100\,$kpc. Re-acceleration is neither needed nor important in these models, and implied CR transport speeds are consistent with buoyant advection. Previous claims of upper limits to non-thermal X-rays and CR pressure relied on strong assumptions which are not valid at the CR energies of interest, e.g. a power-law spectrum of CRs. This could resolve many historical puzzles about Perseus, and makes new predictions for future observations.

astro-ph.HE↗

arXiv2026

Time-Dilation Methods for Extreme Multiscale Timestepping Problems

Many astrophysical simulations involve extreme dynamic range of timescales around 'special points' in the domain (e.g. black holes, stars, planets, disks, galaxies, shocks, mixing interfaces), where processes on small scales couple strongly to those on large scales. Adaptive resolution, multi-physics, and hybrid numerical methods have enabled tremendous progress on the spatial, physics, and numerical challenges involved. But often the limiter for following the long timescales of global evolution is the extremely short numerical timestep required in some subdomains (which leads to their dominating simulation costs). Recently several approaches have been developed for tackling this in problems where the short timescale solution is sampled and then projected as an effective subgrid model over longer timescales (e.g. 'zooming in and out'). We generalize these to a family of models where time evolution is modulated by a variable but continuous in space-and-time dilation/stretch factor $a({\bf x},\,t)$. This extends previous well-studied approaches (including reduced-speed-of-light and binary orbital dynamics methods), and ensures that the system comes to correct local steady-state solutions, and derive criteria that the dilation factor/timesteps/resolution must obey to ensure good behavior. We present a variety of generalizations to different physics or coupling scales. Compared to previous approaches, this method makes it possible to avoid imprinting arbitrary scales where there is no clear scale-separation, and couples well to Lagrangian or Eulerian methods. It is flexible and easily-implemented and we demonstrate its validity (and limitations) in test problems. We discuss the relationship between these methods and physical time dilation in GRMHD. We demonstrate how this can be used to obtain effective speedup factors exceeding $\gtrsim 10^{4}$ in multiphysics simulations.

astro-ph.IM↗

arXiv2026

Cosmic Rays on Galaxy Scales: Progress and Pitfalls for CR-MHD Dynamical Models

Recent years have seen many arguments for cosmic rays (CRs) as an important influence on galactic and circumgalactic (CGM) physics, star and galaxy formation. We present a pedagogical overview of state-of-the-art modeling of CR-magnetohydrodynamics (CR-MHD) on macro scales (~kpc), highlighting their fundamental dependence on the micro (< au) scales of CR gyro orbits and meso (~pc) scales of CR mean-free-paths, intended to connect the extragalactic, Galactic, and plasma CR transport modeling communities. We note the pitfalls and systematic errors that arise from older assumptions in CR modeling, including: use of a simple Fokker-Planck equation or ad-hoc two-moment formalisms for transport; assumption of leaky boxes or plane-parallel or shear-periodic boundaries for comparison to local interstellar medium (LISM) observations; ignoring detailed LISM constraints on CR spectra (e.g. focusing only on extragalactic observables or spectrally integrated models); assuming CR transport is mediated by classical models of advection, streaming from self-confinement (super-Alfvenic or Alfvenic), or extrinsic turbulence. We emphasize recent progress addressing these: development of rigorously-derived CR-MHD equations; use of global, 3D galaxy+halo models for LISM comparisons; new methods for full-spectrum dynamics; novel models for intermittent scattering and/or new drivers. We compile extragalactic+LISM observations to show how ~GeV CR transport is being rapidly constrained in the CGM, and present phenomenological models which can be used in future simulations. We conclude by highlighting critical open questions for micro, meso, and macro-scale CR-MHD simulations.

astro-ph.GA↗