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

Azadeh Fattahi, Associate Professor at OKC and leader of the research collective, highlights the sheer scale and ambition of this computational endeavor. She explains that their work has produced the most extensive sample of faint galaxies simulated at resolutions never before achieved. Modeling these ultra-faint systems is exceptionally challenging due to their minuscule mass—often a million times less than that of the Milky Way—and their extreme fragility in the face of cosmic forces. These galaxies form within small dark matter halos, predicted by the prevailing cosmological model, situating them at the frontier where our understanding of galaxy formation meets the mysteries of dark matter.

Dwarf galaxies, often described metaphorically as the small cousins of the Milky Way, serve as critical laboratories for studying the Universe’s past. The ultra-faint category represents the lower bounds of this spectrum, composed of tiny clusters of stars and gas hosted within diminutive dark matter halos. Due to their faint luminosity and sparse populations, they have until now evaded thorough exploration in simulations, limiting insights into their evolution. With this new suite of simulations, scientists can now systematically explore the interplay between these galaxies and their primordial environment, providing an unprecedentedly deep glimpse into cosmic history.

To elucidate the significance of their findings, co-author Shaun Brown draws an evocative parallel: much like how a farmer can infer spring weather conditions by observing summer crop yields, astronomers can deduce the early Universe’s “climate” by analyzing the properties of ultra-faint dwarf galaxies today. This analogy underscores how the state of small galaxies acts as a cosmic recorder, encapsulating episodes that occurred tens of billions of years ago during epochs less accessible through traditional observations.

One of the most compelling aspects of this research lies in its exploration of different hypothetical scenarios regarding the early cosmic radiation environment, specifically within the Universe’s first 500 million years. The team discovered a remarkable sensitivity of the smallest dark matter halos to variations in this early “background radiation,” particularly in the Lyman-Werner band, which can dissociate molecular hydrogen and thus impede star formation. While massive galaxies such as the Milky Way appear largely impervious to these fluctuations, the faintest dwarf galaxies demonstrate a pronounced responsiveness, dramatically altering their star formation trajectories depending on the early radiation milieu.

This sensitivity implies a pivotal role for these minuscule galaxies as probes of early cosmic physics. The simulations reveal that certain dark matter halos either successfully form stars and become visible galaxies or remain barren, starless repositories of dark matter, contingent on the radiation conditions of the infant Universe. This dichotomy offers critical clues about the environmental thresholds required for galaxy formation in the earliest epochs and challenges existing paradigms that have largely focused on larger galactic systems.

The implications of this research are far-reaching, notably with the imminent commissioning of the Vera C. Rubin Observatory. Anticipated to conduct the most comprehensive survey of the Milky Way’s satellite galaxies, the observatory’s data could significantly enhance our census of these elusive ultra-faint dwarfs. According to Fattahi, the new simulations provide a theoretical framework for interpreting future observational catalogs in the context of early Universe conditions, effectively transforming local dwarf galaxies into a cosmic archaeological record that complements direct high-redshift studies.

Moreover, the findings hold particular resonance amid recent surprising discoveries by the James Webb Space Telescope (JWST), which has detected unexpectedly massive and luminous galaxies at very early cosmic times. These revelations suggest the early Universe’s landscape may be more complex and dynamic than previously assumed. Ultra-faint dwarf galaxies, as studied through these new simulations, offer a complementary route to unraveling this complexity by linking local relics to distant phenomena across space-time.

Behind these advancements lies the monumental computational challenge of running such extensive and high-resolution simulations. Fattahi emphasizes that generating this rich data set required over six months of continuous processing on the COSMA 8 supercomputer, a facility specifically geared toward simulation-driven astrophysical research and hosted by Durham University. The sheer volume of generated data, reaching approximately 300 terabytes, necessitated substantial improvements to existing data management and analysis algorithms to ensure efficient and thorough scientific exploration.

Looking forward, the research team intends to leverage this sophisticated simulation suite to dive deeper into unresolved questions in cosmology and galaxy formation. Among their ambitions are pinpointing the locations of the Universe’s first generation of stars and probing how the properties of ultra-faint dwarf galaxies can inform theoretical models of dark matter. The ability to integrate these faint cosmic structures into our broader understanding could catalyze transformative insights into the fundamental building blocks of the Universe.

Ultimately, this work exemplifies the power of combining cutting-edge computational astrophysics with observational prospects to decode the origins and evolution of the cosmos. By treating ultra-faint dwarf galaxies not merely as local curiosities but as sensitive indicators of primordial conditions, this research opens a new frontier for studying how the faintest glimmers in the night sky record the Universe’s formative “weather,” bridging the gap between the past and present in astrophysical inquiry.

Subject of Research: Formation and evolution of ultra-faint dwarf galaxies as probes of early Universe conditions.

Article Title: LYRA ultra-faints: The emergence of faint dwarf galaxies in the presence of an early Lyman-Werner background

News Publication Date: 24-Apr-2026

Web References:

• Article DOI: 10.1093/mnras/stag439

References:

• Azadeh Fattahi et al., Monthly Notices of the Royal Astronomical Society, 2026.

Image Credits:
J Sureda, A Fattahi, S Brown, S Avraham

Keywords

Ultra-faint dwarf galaxies, early Universe, galaxy formation, dark matter halos, cosmological simulations, Lyman-Werner background, Vera C. Rubin Observatory, James Webb Space Telescope, cosmic archaeology, dark matter, COSMA 8 supercomputer, cosmic radiation environment