arXiv subjects

Christoph Pfrommer

Publications and source records attributed to Christoph Pfrommer.

At least 19 recordsLinked to original sources

arXiv2026

Identifying heating processes in simulations with an entropy-based scheme: A single jet episode in a galaxy cluster

Understanding heating processes in galaxy clusters is essential for predicting the regulation of radiative cooling and star formation, and for clarifying the mechanisms underlying active galactic nucleus (AGN) feedback in cool-core clusters. We investigate the processes through which AGN jets deposit heat into the intracluster medium (ICM) by tracking passive entropy scalars in magneto-hydrodynamic (MHD) simulations. This enables us to systematically disentangle the contributions from different heating channels. We successfully validate this method with several idealized tests, including turbulent heating, heating by anisotropic Braginskii viscosity, dissipative and adiabatic heating by shocks using in-situ shock-detection methods, and cosmic ray (CR) heating through the excitations and damping of Alfvén waves. Using this methodology, we simulate single-epoch outbursts of high-power jets with varying densities in a cluster environment. Light jets produce wider bubbles, displacing a larger fraction of the gas in the cluster's core, whereas comparatively denser jets propagate more efficiently to larger distances without significantly disturbing the central region. During early evolution, shock heating of the ICM prevails irrespective of jet density. At later times, light jets primarily heat the ICM through turbulently induced heating (which includes dissipation of vortices emerging from a turbulent cascade and mixing gas of different temperatures) and compressible dissipation of sound waves and very weak shocks, while denser jets continue to dissipate most of their energy via shocks. Particularly, turbulently induced heating by solenoidal-shear motions prevails inside the cocoon, whereas shocks and acoustic compressions dominate outside. In light jets, the forward shock weakens rapidly, whereas dense jets can sustain strong bow shocks to large distances.

astro-ph.HE↗

arXiv2026

The impact of nonlinear Landau damping on cosmic ray streaming and the role of external turbulence

Cosmic rays (CRs) streaming faster than the Alfvén speed generate Alfvén waves that scatter them. The saturated wave amplitude is regulated by a damping mechanism. This damping controls the CR scattering rate and drift velocity, and thus the rate at which CRs transfer momentum and energy to the ambient plasma, which sets the strength of CR feedback. In the ionized interstellar medium, nonlinear Landau damping (NLLD) is commonly invoked as the dominant damping mechanism for Alfvén waves. We test this assumption with fluid-PIC simulations in which thermal ions and electrons are modeled as fluids with optional Landau closures, allowing us to compare otherwise identical runs with and without NLLD. When waves are self-generated by monoenergetic CRs, runs with and without Landau closures are indistinguishable: NLLD is inefficient and the CRs isotropize in the wave frame. Once high-amplitude Alfvén waves mimicking external turbulence are seeded, NLLD curbs wave growth and equalizes forward- and backward-propagating waves. We show that the saturated drift velocity, which falls below the Alfvén speed, can be accurately predicted from the wave spectra alone. If the CR distribution is undersampled, a numerical instability corrupts the largest resonant scales: unphysical, unresolved pitch-angle gradients resonantly generate backward-propagating waves, driving an inverse cascade that should be absent from purely streaming-driven waves. While power-law CR distributions may permit stronger NLLD, our results indicate that NLLD is unlikely to be an intrinsic saturation mechanism of the streaming instability; instead, its effectiveness hinges on the interplay of streaming-generated modes with those of external origin.

astro-ph.HE↗

arXiv2026

Characterizing turbulence in galaxy clusters: Defining turbulent energies and assessing multi-scale versus fixed-scale filters

Disentangling turbulence and bulk motions in the intracluster medium (ICM) of galaxy clusters is inherently ambiguous, as the plasma is continuously stirred by different processes on disparate scales. This poses a serious problem in the interpretation of both observations and numerical simulations. In this paper, we use filtering operators in real space to separate bulk motion from turbulence at different scales. We show how filters can be used to define consistent kinetic and magnetic energies for the bulk and turbulent component. We apply our GPU-accelerated filtering pipeline to a simulation of a major galaxy cluster merger, which is part of the PICO-Clusters suite of zoom-in cosmological simulations of massive clusters using the moving mesh code Arepo and the IllustrisTNG galaxy formation model. We find that during the merger the turbulent pressure fraction on physical scales of $\lesssim$50 kpc reaches a maximum of 5\%, before decreasing to 2\% after $\sim$1.3 Gyr from the core passage. These low values are consistent with recent observations of clusters with XRISM, and suggest that unless a cluster was recently perturbed by a major merger, turbulence levels are low. We then reexamine the popular multi-scale iterative filter method. In our tests, we find that its use can introduce artifacts, and that it does not reliably disentangle fluctuations living on widely separated length scales. Rather, we believe it is more fruitful to use fixed-scale filters and turbulent energies to compare between simulations and observations. This work significantly improves our understanding of turbulence generation by major mergers in galaxy clusters, which can be probed by XRISM and next-generation X-ray telescopes, allowing us to connect high-resolution cosmological simulations to observations.

astro-ph.GA↗

arXiv2026

Towards robust simulations of the galactic dynamo: a cross-code comparison between Eulerian and Lagrangian schemes

Galactic magnetic fields are thought to be seeded by microscopic processes such as the Biermann battery and are primarily amplified to observed microgauss strengths by dynamo processes. Much of this amplification likely occurs within proto-galaxies, where turbulent and rotational kinetic energy is converted into magnetic energy via dynamo action. However, two aspects of this process remain poorly constrained in simulations: the saturation level of the amplified field and the timescale on which it is reached. In particular, different numerical methods, resolution strategies, and subgrid prescriptions have not converged to similar results, making it difficult to establish robust, code-independent conclusions about dynamo amplification in galaxies. In this work, we present the first systematic comparison of both Eulerian and Lagrangian codes that employ diverse MHD schemes, including ART, RAMSES, AREPO, and SWIFT. Using identical initial conditions for Milky Way-type galaxies and without including star formation and stellar feedback, we find that the amplification history of the magnetic field within the galactic disk depends strongly on the choice of hydrodynamical scheme, with orders-of-magnitude differences between Eulerian and Lagrangian approaches. We further demonstrate that reducing the mesh velocity in the moving-mesh code AREPO leads to results consistent with those obtained using the static grid codes ART and RAMSES. These results highlight the importance of carefully assessing numerical effects when interpreting magnetic field amplification in galaxy simulations.

astro-ph.GA↗

arXiv2026

Saturation Mechanism of Cosmic Ray Streaming Instabilities with a Shell Distribution

Cosmic rays (CRs) drive galactic winds and regulate galaxy growth while contributing to heating the central cooling plasma in dense galaxy clusters, making the nonlinear saturation of the instabilities that govern their transport a problem of central importance in astrophysics. Using fully kinetic particle-in-cell (PIC) simulations, we find that the streaming instability driven by CRs with a shell momentum distribution saturates through gyro-phase bunching around driven Alfvén waves. Fluid-PIC (FPIC) simulations with both ideal and Landau closures reproduce the PIC saturation amplitude, and the same gyro-phase bunching mechanism, indicating that nonlinear Landau damping (NLLD) does not determine the saturation level at our simulation parameters ($v_{\rm A} = 0.01c$, $n_{\rm CR}/n_i = 0.01$). Notably, the CR ions fully isotropize in the Alfvén-wave frame in all simulations, including the FPIC run with an ideal closure in which NLLD is entirely absent, showing that this isotropization does not require NLLD. We additionally confirm, using the FPIC ideal closure, that this saturation amplitude is unchanged across a tenfold increase in domain size, indicating that the mechanism is local rather than dependent on domain-scale processes. Together, these results indicate that, at least in this regime, NLLD is not the dominant saturation mechanism for the CR streaming instability, in tension with common assumptions built into CR transport models used in galaxy formation and interstellar medium simulations. Whether this conclusion extends to the lower CR densities and Alfvén speeds characteristic of the interstellar medium remains an open question that we address in an upcoming work.

astro-ph.GA↗

arXiv2026

Collisionless relaxation as the origin of the anisotropic, non-thermal, and multi-temperature momentum distributions observed in space plasmas

Anisotropic, non-thermal, and multi-temperature distributed particle momenta are commonly observed in collisionless space plasmas, such as the solar wind. Using Liouville's theorem, we argue that anisotropic compression or expansion of the plasma, followed by a relaxation of the resulting anisotropic stress must lead to non-equilibrium states that exhibit either anisotropic, non-thermal distribution functions, different electron and ion temperatures, or a combination of these effects. We present arguments showing that a plasma in thermal equilibrium undergoing anisotropic compression or expansion cannot return to thermal equilibrium in the absence of particle collisions. Since most astrophysical plasmas are practically collisionless and experience significant anisotropic compression or expansion, we expect anisotropic, non-thermal, and multi-temperature particle distributions to be ubiquitous, in agreement with solar wind measurements.

physics.plasm-ph↗

arXiv2026

High-spectral-resolution Observations of the [S II] Emission-line Doublet in the Filamentary Nebula Surrounding NGC 1275

We analyze new high-spectral resolution SITELLE observations (R = $λ/Δλ$ = 7000) of the filamentary nebula surrounding NGC 1275, central galaxy of the Perseus cluster. We present here analysis of the \sii$\lambda6716$ and \sii$\lambda6731$ emission line doublet, using its ratio to determine the electron density of the optically emitting filaments. We compare these measurements with electron densities derived from deep Chandra X-ray observations of the intra-cluster medium (ICM) to determine if any correlations in density can be found. We report the detection of a clear dichotomy between the outer filaments, displaying on average lower \sii\text{ }emission line ratio of $\sim 1.1$ and the inner filaments displaying higher ratios of $\sim 1.3$. These results indicate that most of the gaseous filaments lie close to the low-density threshold for the density measurement of $\sim 10^2\text{ cm}^{-3}$. Using radial profiles, we find that the inner filaments have a roughly constant density, whereas the ICM density decreases with radius. In the outer filaments, we observe hints of local connections between the densities of the ICM and optical filaments, but no clear correlation seems to be observed overall. We also combined these density measurements with cold molecular CO gas observations to derive a relationship between temperature, density and pressure for the multiphase environment surrounding NGC 1275. Finally, we investigated potential models to explain the observed density measurements and explored similar studies of filamentary nebula around other central galaxies of cool-core galaxy clusters.

astro-ph.GA↗

arXiv2026

Robust magnetic field estimates in star-forming galaxies with the equipartition formula in the absence of equipartition

The equipartition model is widely used to estimate magnetic field strength from synchrotron intensity in radio galaxies, yet the validity of its underlying assumptions remains uncertain. Using an Arepo simulation which incorporates a two-moment cosmic ray (CR) transport scheme and a multiphase interstellar medium, we compare magnetic fields inferred from synthetic synchrotron emission maps with the true fields in the simulation. Starting from the derivation of the equipartition formula, we find that the deviation between the equipartition magnetic field and the true magnetic field depends only weakly on the ratio of the magnetic to the CR energy density. In practice, for both face-on and edge-on projections, the equipartition model slightly overestimates the total synchrotron-weighted magnetic field with mean offsets of 32% (0.17 dex) and 36% (0.2 dex), even though the energy equipartition between CR protons and magnetic fields does not hold locally. Beyond these average offsets, a clear trend emerges in edge-on projections that the model underestimates the field in the disk and overestimates it in the halo. Our results demonstrate that the validity of the equipartition model depends only weakly on the strict fulfillment of energy equipartition, and that the equipartition model remains a practical method for estimating magnetic field strengths in face-on projection maps based on our CR-magnetohydrodynamics simulation.

astro-ph.GA↗

arXiv2026

An HI+radio continuum study of local cluster galaxies: Intercepting the early stages of environmental processing with MeerKAT

The evolution of galaxies in clusters is driven by their interaction with the environment, which deeply affects and alters the properties of the multi-phase interstellar medium. Here we make use of MeerKAT observations to study the properties of the neutral and nonthermal interstellar medium, traced respectively by the HI 21 cm line and by the radio continuum emission at 1.4 GHz, out to about twice the virial radius of three local ($z\simeq0.04$) galaxy clusters, namely IIZW108, A4059 and A3558, with $M_{200}\simeq2-9\times10^{14}~M_\odot$. We assemble a sample of 61 galaxies, including the so-called jellyfish and unwinding galaxies, detected in both HI and radio continuum, and derive their optical properties from IR-to-UV photometry using ancillary data. We find that cluster galaxies in our sample are on average $\sim2.7$ times more star-forming than galaxies with the same stellar and HI mass in the field, indicating that the HI + radio-continuum selection has intercepted galaxies at the very early stages of their environmental processing when the external pressure has not yet removed the gas, but the resulting fast compression has enhanced both the star formation and the radio continuum luminosity. The study also reveals that galaxies at the initial stage can features a radio luminosity excess due to the old relativistic electrons permeating the interstellar medium. We show that unwinding galaxies are characterized by high stellar and HI masses, arguing that their peculiar morphology may results from their large baryonic mass. Finally, via the stacking analysis of 677 optically-selected cluster members we quantitatively show that cluster galaxies are more HI-poor than in the field, and the deficiency steadily grows approaching the cluster center where galaxies have, on average, a factor $\sim3\times$ less HI mass than those in the field.

astro-ph.GA↗

arXiv2026

A comparison of numerical schemes for driven subsonic MHD turbulence

Turbulence is ubiquitous in astrophysical systems, and since most cosmic gas is ionised, it supports magnetic fields. In turbulent environments, these fields are rapidly amplified through a small-scale dynamo. Multi-scale astrophysical simulations, however, rarely resolve this process adequately. Limited spatial dynamic range makes small-scale amplification sensitive to the numerical choices made in the hydrodynamics and magnetic field solvers. Here, we investigate idealised periodic boxes of driven subsonic turbulence with a weak seed magnetic field. These simulations with purely numerical dissipation provide a simple environment in which an efficient small-scale dynamo is expected. We aim to systematically compare the three widely used magnetohydrodynamics (MHD) codes \textsc{Arepo}, \textsc{Athena}, and \textsc{Ramses} across different divergence-control schemes: constrained transport, Powell cleaning, and Dedner cleaning. To minimise comparison bias, we adopt identical turbulent driving and analysis pipelines across all runs. At sufficient resolution, every code and scheme we test exhibits dynamo-like exponential amplification of the seed field until saturation. The structural properties of the magnetic field in both the kinematic and saturated regimes are consistent across schemes (with the exception of \textsc{Arepo}'s constrained transport) and agree with theoretical expectations. Residual differences, particularly in kinematic amplification rates and saturation strengths at fixed resolution, appear attributable to varying levels of numerical diffusion. Notably, we find for this setup no systematic advantage of constrained transport over divergence-cleaning methods. We stress that this comparison, conducted in a highly idealised setting, represents a first step. Future extensions to more complex and physically realistic configurations remain essential.

astro-ph.IM↗

arXiv2026

CRexit observed: probing cosmic ray transport in the circumgalactic medium with absorption line spectra

Cosmic rays (CRs) likely provide dynamically important non-thermal pressure support in the circumgalactic medium (CGM), but how their transport physics shapes observable absorption signatures remains uncertain. We investigate whether absorption-line diagnostics can distinguish between different CR transport regimes in CR-pressure-dominated halos. Using high-resolution simulations, we generate synthetic spectra along large ensembles of sightlines and measure column densities, equivalent widths, covering fractions (CFs), velocity widths, abundance ratios, and stacked absorption profiles for ions tracing cool, warm, and hot gas. We find that the effective CR transport speed strongly regulates the multiphase structure of the CGM. Efficient CR transport enhances the formation of cool ($T\sim10^4$ K) and warm ($T\sim10^5$ K) gas, leading to deeper and broader absorption lines of low- and intermediate-ionization species. The two-moment CR transport model produces the strongest MgII and SiII absorption and reaches MgII CFs consistent with the range inferred for star-forming galaxies. In contrast, slow CR transport underproduces cool, low-ionization gas and yields substantially weaker absorption. We also find that the origin of CIV-bearing gas changes with CR transport: slow transport mainly produces extended warm halo gas, whereas efficient transport shifts much of the CIV absorption into mixing layers around cool clouds. The high-ionization tracer OVI responds more weakly, indicating that CR transport primarily regulates the cool condensed phase and its interfaces rather than the volume-filling hot halo. These findings suggest that absorption-line measurements of cool and transition-phase gas can provide valuable constraints on the effective transport of CRs through the CGM.

astro-ph.GA↗

arXiv2026

Active galactic nucleus driven jet feedback in cosmologically forming cool-core galaxy clusters I: The effect of hierarchical assembly on intra-cluster medium properties

The atmospheres of cool-core galaxy clusters are excellent probes of astrophysical plasmas. However, how the interplay between assembly and active galactic nucleus (AGN) feedback leads to the observed gas profiles remains uncertain. We study the impact of hierarchical assembly on the intra-cluster medium (ICM) in cool-core galaxy clusters using hydrodynamic simulations as part of the PICO-Cluster project. We compare cosmological zoom simulations employing an explicit AGN jet model against PICO-Cluster simulations with IllustrisTNG kinetic AGN feedback, as well as against isolated galaxy cluster simulations using jet feedback. The stellar and gas fractions of our cosmological galaxy cluster simulations with jet feedback are in excellent agreement with observed galaxy clusters, and the ICM thermodynamic profiles resemble those of local cool-core galaxy clusters while those run with IllustrisTNG kinetic AGN feedback do not match these observations. In all simulations, cosmological and isolated, the AGN heating roughly balances the cooling losses, with star formation being significantly suppressed. The most notable differences between the cosmological and isolated simulations are the resulting velocity and multi-phase structure: gas at radii $> 50$ kpc is shaped by satellite galaxies rather than jet feedback originating form the central galaxy. This leads to significant differences in non-thermal pressure support, with only the cosmological simulations being consistent with recent observations. A second notable difference is the abundance of warm ($<10^5$ K) gas beyond the core region, which is absent in our isolated simulation. Our results highlight the need for taking cosmological assembly into account in comparisons of the ICM dynamics and its multi-phase nature, while self-regulation is altered by hierarchical assembly via merger-driven growth of the central supermassive black hole.

astro-ph.GA↗

arXiv2026

The PICO-Cluster Project: presenting the galaxy cluster sample and studying magnetic field growth, Faraday rotation and Braginskii heating

Galaxy clusters constitute a microcosm of the Universe and offer a unique laboratory for studying plasma astrophysics, encompassing processes such as cosmic-ray acceleration and non-thermal radio emission, turbulence, weakly collisional plasma physics, and transformative mechanisms in galaxy evolution. To investigate these phenomena, we introduce the PICO-Cluster project, studying 'Plasmas In COsmological Clusters' using a suite of high-resolution cosmological zoom-in simulations of massive galaxy clusters with masses $\gtrsim10^{15}$M$_\odot$ selected from a parent simulation box with a comoving side length of 1 $h^{-1}$Gpc. In this work, we present 24 baseline simulations performed with the moving-mesh AREPO code and the IllustrisTNG galaxy formation model, achieving a baryonic mass resolution of up to $1.4\times10^{6}\mathrm{M}_\odot$. The initial conditions are carefully designed to exclude low-resolution particle contamination within the high-resolution region; as a result, all clusters remain free of such contamination out to at least 2.7 $R_{200}$ at all times. Our galaxy and cluster properties agree with recent simulations and many observational constraints, including scaling relations and thermodynamic profiles. The magnetic energy within the cluster is numerically converged once the small-scale dynamo has saturated, yielding a remarkably tight volume-averaged plasma-beta of $β\approx100$ inside $R_{200}$ across our sample after redshift $z\sim1.2$. Faraday rotation measure profiles, which trace the line-of-sight magnetic field and electron density, decline with cylindrical radius; notably, the mean decreases more rapidly than the root-mean-square due to the increasing relative contribution of galaxies at larger radii. Finally, viscous heating rates in Braginskii theory are highly intermittent and, on average, approach radiative cooling rates in the cluster outskirts.

astro-ph.GA↗

arXiv2026

Resolving Star Cluster Formation in Galaxy Simulations with Cosmic Ray Feedback

Star clusters host the massive stars responsible for feedback in star-forming galaxies. Stellar feedback shapes the interstellar medium (ISM), affecting the formation of future star clusters. To self-consistently capture the interplay between feedback and star formation, a model must resolve the parsec-scale star formation sites and the multiphase ISM. Additionally, the dynamical impact of cosmic rays (CRs) on star formation rates (SFRs) must also be considered. We present the first simulations of the formation of an ensemble of star clusters with dynamically-coupled CRs, near-individual star particles, and a feedback-regulated ISM. We analyze tallbox simulations performed using the CRISP model in the moving-mesh code AREPO. We apply varied implementations of CR transport under the theory of self-confinement. We find that CRs simultaneously reduce the SFR, the power law slope of the cluster mass function, and the cluster formation efficiency. Each simulation is compatible with observations, and CR feedback tends to move results along observed star cluster relations. We see only modest changes in cluster radius and velocity dispersions, but significant differences in the virial parameters. Ultimately, the primary impact of CRs is to reduce SFRs. Lower SFRs imply fewer supernovae, and consequently a lower turbulent energy budget for gas. Star clusters formed in a CR-regulated ISM have lower velocity dispersions, and are therefore more bound under self-gravity. The effective clustering of supernovae is unchanged by CRs. Despite the idealized setup, the CRISP feedback model recovers many key aspects of star cluster formation.

astro-ph.GA↗

arXiv2026

CRexit: how different cosmic ray transport modes affect thermal instability in the circumgalactic medium

The circumgalactic medium (CGM) plays a critical role in galaxy evolution, influencing gas flows, feedback processes, and galactic dynamics. Observations show a substantial cold gas reservoir in the CGM, but the mechanisms driving its formation and evolution remain unclear. Cosmic rays (CRs), as a source of non-thermal pressure, are increasingly recognized as key regulators of cold gas dynamics. This study explores how CRs affect cold clouds that condense from the hot CGM via thermal instability (TI). Using 3D CR-magnetohydrodynamic (CRMHD) simulations with AREPO, we assess the impact of various CR transport models on cold gas evolution. Under purely advective CR transport, CR pressure significantly suppresses the collapse of thermally unstable regions, altering the CGM's structure. In contrast, realistic CR transport models reveal that CRs escape collapsing regions via streaming and diffusion along magnetic fields, diminishing their influence on the thermal and dynamic structure of the cold CGM. The ratio of the CR escape time to the cloud collapse time emerges as a critical factor in determining the impact of CRs on TI. CRs remain confined within cold clouds when effective CR diffusion is slow which maximizes their pressure support and inhibits collapse. Fast effective CR diffusion, as realized in our 2-moment CRMHD model, facilitates rapid CR escape, reducing their stabilizing effect. This realistic CR transport model shows a wide dynamic range of the effective CR diffusion coefficient, ranging from $10^{29}$ to $10^{30}\,\mathrm{cm^{2}\,s^{-1}}$ for thermally- to CR-dominated atmospheres, respectively. In addition to these CR transport-related effects, we demonstrate that high numerical resolution is crucial to avoid spuriously large clouds formed in low-resolution simulations, which would result in overly long CR escape times and artificially amplified CR pressure support.

astro-ph.GA↗

arXiv2026

Simulating realistic radio morphologies of Fanaroff-Riley I jets in a self-regulating cool-core cluster

Active galactic nucleus (AGN) jets radiate radio synchrotron emission displaying a wide range of morphologies. At the same time, they provide heat to prevent cooling flows in cool-core galaxy clusters. We produce mock radio observations of AGN jets in a self-regulating cool-core galaxy cluster. To this end, we employ magneto-hydrodynamical simulations of an idealised Perseus-like galaxy cluster, in which accretion-powered low-density jets accelerate cosmic ray protons and electrons by means of a sub-grid model. Cosmic ray electron spectra are spatially and temporally evolved along Lagrangian tracer trajectories using the Fokker-Planck solver Crest to produce radio synchrotron emission. Self-regulated AGN jets stabilize the cool-core cluster against cooling flows and produce realistic Fanaroff-Riley I (FRI) and disturbed lobe morphologies, in contrast to symmetrical lobe structures obtained with a single jet outburst of fixed power. Our mock radio observations are viewed in a blazar configuration - along the jet axis - and exhibit complex radio-emitting lobe structures despite this. This highlights the strong deflection of light jets by cold gas structures and suggests that small-scale black hole and jet properties cannot be inferred from kpc-scale FRI radio lobe morphologies. Combining self-consistently evolved magnetic fields and electron spectra enables us to explain a known observational phenomenon, whereby radio observations of AGN lobes on galaxy cluster scales occasionally display similar spatial extents at different frequencies: in 1-50 $μ$G magnetic fields obtained in our cool-core environment, both freshly accelerated and hundreds-of-Myr-old electrons are able to contribute to the 150 MHz - 1.4 GHz frequency range.

astro-ph.GA↗

arXiv2026

The diffuse gamma-ray sky of a Milky Way analogue: Local diversity and global constraints

Diffuse gamma-ray emission is a key tracer of cosmic rays (CRs) in galaxies, encoding information about their transport, energetics, and interactions with the interstellar medium. Interpreting the Milky Way gamma-ray sky is challenging because the observed emission depends on the three-dimensional distributions of CRs and gas, as well as the observer location within the Galaxy. Using the Rhea suite of CR-MHD simulations of a Milky Way analogue, we study how pion-decay gamma-ray emission varies with galactic environment, local conditions, and CR transport physics. Emission is computed in post-processing under steady-state assumptions, enabling analysis of luminosities, spectra, full-sky maps, and angular power spectra (APS) for multiple observer positions, including those inside Local-Bubble-like cavities. The simulated galaxy reproduces Milky Way-like gamma-ray luminosities and spectral slopes without parameter tuning. While total luminosity is relatively stable, the morphology of the gamma-ray sky varies strongly with observer location due to the complex local gas distribution, consistent with observations. For all observers, the APS traces gas column density rather than the smoother CR energy density, in agreement with previous studies. Comparisons with Fermi-LAT data show good agreement in both the all-sky spectrum and APS, with a diffusion coefficient scaling proportional to E^0.5 providing the best match. These results show that key features of Galactic gamma-ray emission arise naturally in self-consistent CR-MHD simulations. Gas density fluctuations primarily shape emission morphology, while CR transport governs spectral and structural properties. The Rhea simulations provide a physically grounded framework for interpreting diffuse gamma-ray observations and highlight the importance of local environment in tracing Galactic CR physics.

astro-ph.HE↗

arXiv2026

Effects of Resolution and Local Stability on Galactic Disks: I. Multiple Spiral Mode Formation via Swing Amplification

We investigate the formation of multiple spiral modes in Milky Way-like disk-halo systems without explicitly exciting perturbations. We explore how numerical resolution, the level of local disk stability, and the presence of a live halo influence both the initial appearance and the subsequent evolution of these modes. To characterize spiral structure, we compute Fourier amplitudes for modes $m=1$-$6$. In marginally unstable, lower-resolution disks ($N_\star=5\times10^6$, $N_{\rm DM}=1.14\times10^7$), faint features appear within the first $0.5$ Gyr due to numerical noise, in contrast to high-resolution models where perturbations emerge later. Across all sufficiently resolved, live-halo models with $m_{\rm DM}/m_\star \le 10$, the spirals exhibit a cascading sequence in both mode number and radius: higher-$m$ modes form and decay first, followed by the delayed emergence of lower-$m$ modes, with an inward drift of the activity's epicenter. This behavior reflects a combination of local swing amplification, which explains the initial growth of short-wavelength modes, and interference between coexisting long-lived spiral modes, which accounts for the recurrent short-timescale amplitude modulations. In contrast, models with a fixed halo potential or coarse halo resolution ($N_{\rm DM}=1.14\times10^6$ and $m_{\rm DM}/m_\star=100$) show strong early spirals but lack this coherent cascading behavior, owing to excessive shot noise and insufficient halo responsiveness. The $m=3$ mode plays a transitional role, marking the onset of angular-momentum transport in the inner disk that precedes bar formation, a process absent in fixed-potential models. Our results show that a live halo with appropriate mass resolution provides the gravitational response needed to sustain and regenerate multi-mode spiral structure, even though the total angular-momentum exchange remains small.

astro-ph.GA↗