A major review published in Living Reviews in Computational Astrophysics by Maarit J. Korpi-Lagg, Mordecai-Mark Mac Low, and Frederick A. Gent surveys the state of the art in numerical modeling of galactic dynamos, the mechanisms by which conducting, turbulent plasma amplifies magnetic fields. The authors conclude that the evidence now strongly supports a two-stage picture. Small-scale dynamos, which amplify magnetic fluctuations on turbulent eddy timescales, operate during the formation of the first galaxies, growing fields faster than the galaxies accrete gas. Large-scale dynamos, driven by the differential rotation of galactic disks, subsequently build the organized fields observed in low-redshift spiral galaxies, bringing the field to equipartition with turbulence and giving it substantial power at the largest scales.

The theoretical framework rests on magnetohydrodynamics, which treats the magnetized interstellar plasma as a conducting continuum. The evolution of the magnetic field is governed by the induction equation, coupling the field to the velocity field, while the dynamics of the gas follow the Navier-Stokes equation including rotation, gravity, pressure, and Lorentz forces. In the interstellar medium, turbulence is driven by multiple sources: supernova explosions and their clustering into superbubbles, stellar winds and ionizing radiation, gravitational instability of the gas disk, and, especially in young galaxies, the turbulent accretion of fresh gas. Ionizing radiation delivers roughly an order of magnitude more energy than supernovae, though supernovae contribute more momentum, and the resulting multiphase structure of the gas strongly affects how dynamos operate.

Dynamo theory traditionally separates the magnetic field into mean and fluctuating components. In the kinematic regime, when the field is still too weak to influence the flow, turbulent motions correlated through rotation and shear generate a turbulent electromotive force whose leading terms are the alpha effect, describing the inductive twisting of field lines by helical turbulence, and turbulent diffusion. A long-standing concern was that this alpha effect could suffer catastrophic quenching as magnetic helicity conservation suppresses the inductive action. Modern mean-field models address this by including helicity fluxes, such as those carried by galactic fountains and winds, which allow the large-scale dynamo to saturate at the observed equipartition strengths rather than stalling far below them.

The small-scale, or fluctuation, dynamo operates differently. The interstellar medium is an extremely high magnetic Prandtl number fluid, with the ratio of magnetic to fluid resistivity estimated near ten billion billion, meaning magnetic fields dissipate on far smaller scales than the gas motions. Any flow whose magnetic Reynolds number exceeds a critical value of roughly 30 to 60 can exponentially amplify magnetic fluctuations on the turbulent eddy turnover time, far faster than the large-scale dynamo can act. Analytical theory predicts a characteristic Kazantsev spectrum for the growing field, and simulations confirm both this spectral shape and the rapid shift of magnetic power to larger scales as the dynamo saturates.

Perhaps the most striking recent result concerns seed fields. Kinetic simulations using hybrid and fully kinetic plasma models show that the Weibel instability can magnetize an initially unmagnetized turbulent plasma, and that small-scale dynamo action then amplifies those tiny fields to within a few percent of equipartition while increasing their characteristic length scale toward the turbulent driving scale. This process could operate behind the accretion shocks that form during the assembly of gas into galaxies, implying that galaxies may be born magnetized. The review’s authors argue that simulations of galaxy formation should therefore not begin with infinitesimal seed fields, since such a choice unphysically delays the onset of magnetohydrodynamic effects in galactic evolution.

At the kiloparsec scale, direct numerical experiments of supernova-driven, multiphase, rotating disks have now captured both dynamo modes simultaneously. In models resolved to parsec scales, the small-scale dynamo saturates within a few hundred megayears at only a few percent of equipartition with turbulence, a result that on its own conflicts with the observed strength of turbulent galactic fields. However, when differential rotation is included, the large-scale dynamo continues to grow after the small-scale dynamo saturates, and during the transition the turbulent field grows far more strongly than mean-field theory alone would predict, apparently through tangling of the emerging mean field. This tangling may explain why turbulent fields dominate the observed magnetic energy in real galaxies.

Global simulations of entire galaxies, including cosmological zoom-in models run with codes such as AREPO and RAMSES, reproduce the exponential growth of magnetic energy expected from the small-scale dynamo, confirmed both by growth rates and by the appearance of Kazantsev-like power spectra. In Milky Way-mass halos, magnetic pressure eventually rivals or exceeds the thermal pressure, while dwarf galaxies with shallower potentials saturate an order of magnitude lower. Synthetic observations of these models, including Faraday rotation maps and polarized synchrotron emission, now reproduce key features of the observed sky, particularly when sub-grid models of ionized regions around star clusters are included. The far-infrared radio correlation can likewise be reproduced by fully saturated small-scale dynamos coupled to cosmic ray transport.

Significant challenges remain. Numerical models still operate at magnetic Reynolds and Prandtl numbers many orders of magnitude below interstellar values, and only recently have local models reached resolutions where the small-scale dynamo growth rates converge. Measuring the turbulent transport coefficients that parameterize the large-scale dynamo, through methods such as the test-field technique, singular value decomposition, and the newer iterative removal of sources, reveals broadly consistent results but persistent discrepancies in turbulent resistivity. Whether helicity fluxes truly prevent catastrophic quenching awaits direct measurement in high Reynolds number experiments. Meanwhile, reconciling the ratio of mean to turbulent field strength between local models, which produce mean fields that are too strong, and global models, which often produce them too weak, will demand improved simulations, simulated observations, and ultimately global models that capture both dynamo modes in dwarf galaxies before scaling up to disks like our own.

Subject of Research: Numerical modeling of small-scale and large-scale dynamo mechanisms that generate and amplify magnetic fields in galaxies

Article Title: Computational approaches to modeling dynamos in galaxies

Article References: Korpi-Lagg, M. J., Mac Low, M.-M., & Gent, F. A. (2024). Computational approaches to modeling dynamos in galaxies. Living Reviews in Computational Astrophysics, 10(1), Article 3. https://doi.org/10.1007/s41115-024-00021-9

Image Credits: AI Generated

DOI: 10.1007/s41115-024-00021-9

Keywords: galactic dynamos, magnetic fields, magnetohydrodynamics, small-scale dynamo, large-scale dynamo, interstellar turbulence, supernova feedback, cosmic rays, Faraday rotation, numerical simulations, galaxy formation, magnetic helicity

The scale of the computational challenge is staggering. Temperature contrasts in astrophysical multiphase systems span roughly ten orders of magnitude, and the cold gas, while dense, can be scattered in tiny structures across enormous volumes. Observations suggest cold clouds in the circumgalactic medium may be as small as tens of parsecs. To fully resolve such structures across a Milky Way-sized halo would require more than a quadrillion resolution elements, a number so vast that no supercomputer on Earth comes close. As a result, cold gas quantities in typical cosmological simulations remain unconverged, and researchers have had to resort to a hierarchy of idealized experiments, from turbulent mixing layers to cloud-crushing simulations, each isolating one piece of the physics.

One of the central results the review highlights concerns the survival of cold clouds blasted by hot winds, the classic ‘cloud crushing’ problem. In the absence of cooling, a cold cloud embedded in a supersonic wind is shredded by Kelvin-Helmholtz and Rayleigh-Taylor instabilities on a characteristic crushing time. Worse, the drag time needed to accelerate the cloud is far longer than the destruction time, giving rise to the long-standing ‘entrainment problem’: how can cold gas be flung to hundreds or even thousands of kilometers per second in galactic winds if it should be destroyed before it can accelerate? The answer, it turns out, lies in radiative cooling. When mixed gas at the interface cools faster than the cloud is disrupted, hot gas condenses onto the cold phase, allowing clouds not only to survive but to grow in mass as they are carried along.

This insight has been crystallized into a simple survival criterion: clouds endure if the cooling time of the mixed gas is shorter than their destruction time, which can be recast as a minimum cloud size of roughly a few parsecs under typical wind conditions. Simulations show that surviving clouds grow continuously through cooling-driven mass transfer in their turbulent wakes, and that this same mixing efficiently transfers momentum, accelerating even dense molecular clouds entrained in galactic outflows. Magnetic fields, once heralded as a potential savior of the entrainment problem, help but are not sufficient on their own for the high density contrasts typical of real astrophysical clouds; combined with cooling, however, they shift the survival threshold by orders of magnitude.

The review also delves into thermal instability, the classic mechanism by which a hot medium can spontaneously fragment into a cold, clumpy phase. When radiative cooling increases as temperature drops, small density perturbations run away into dense clumps. In stratified halo atmospheres, precipitation occurs when the ratio of cooling time to free-fall time falls below a critical value of order ten, a criterion modified by turbulence, halo rotation, magnetic fields and cosmic rays. A related and still contentious question is whether cooling clouds ‘shatter’ into a characteristic scale of tiny fragments or instead undergo a violent pulsation dubbed ‘splattering’ before fragmenting, with recent three-dimensional simulations tending to favor the latter picture.

Scaling up, the review surveys supernova-driven bubbles, stratified ‘tall box’ simulations of galaxy disks, and fully global models of dwarf and Milky Way-mass galaxies. A consistent picture emerges: most outflowing mass travels in the warm phase at around 10,000 Kelvin, while most of the energy is carried by the hot, million-degree gas. Hot gas mass loading factors hover near 0.1 across a wide range of star formation rates, and warm outflows in massive galaxies tend to fall back as fountain flows rather than escaping. Including cosmic rays transforms these results, converting fountains into steady, cooler, denser winds that can double the outflowing mass and substantially reshape the circumgalactic medium.

At the largest scales, the review examines how simulations handle the circumgalactic and intracluster media. In cluster cores, jets from supermassive black holes stir turbulence that triggers local thermal instability, producing ‘chaotic cold accretion’ in which cold filaments rain onto the central galaxy and feed the black hole in a self-regulating cycle. In cosmological zoom-in simulations, a recent revolution has come from ‘super-Lagrangian’ refinement schemes that boost resolution specifically in the halo, reaching below 100 parsecs in the circumgalactic medium. These enhanced-resolution models consistently show more and smaller cool clouds, higher covering fractions of cool gas, and non-converged cloud mass functions, confirming that cold gas structure in halo simulations is far from fully resolved.

What emerges most clearly from this comprehensive synthesis is that the diverse simulation approaches, from idealized mixing layers to full cosmological models, are not competitors but complementary layers of a single framework. Small-scale experiments provide the physical intuition, survival criteria and subgrid prescriptions that large-scale simulations need; large-scale simulations in turn supply the realistic boundary conditions, pressures and turbulence levels under which the small-scale physics operates. The ultimate arbiter, the authors stress, is observation, and connecting simulations to real spectra, emission maps and absorption measurements through radiative transfer remains one of the field’s most demanding tasks.

The challenges ahead are formidable: achieving numerical convergence in multiphase diagnostics, capturing the interplay of magnetic fields, conduction, viscosity and cosmic rays, and resolving the critical scales that govern whether cold gas survives, grows or shatters. But the trajectory is clear. GPU-accelerated codes, adaptive refinement targeted at cooling lengths, and a maturing theoretical framework are converging on a unified picture of the multiphase Universe, one simulation at a time.

Subject of Research: Numerical simulations of multiphase gas dynamics in the interstellar, circumgalactic and intracluster media

Article Title: Simulations of multi-phase gas in and around galaxies

Article References: Gronke, M., & Schneider, E. E. (2026). Simulations of multi-phase gas in and around galaxies. Living Reviews in Computational Astrophysics, 12(1), Article 2. https://doi.org/10.1007/s41115-026-00025-7

Image Credits: AI Generated

DOI: 10.1007/s41115-026-00025-7

Keywords: multiphase gas, galaxy simulations, circumgalactic medium, thermal instability, galactic winds, cloud crushing, turbulent mixing layers, computational astrophysics, radiative cooling, cosmic rays, supernova feedback, Kelvin-Helmholtz instability