arXiv subjects

Mark R. Krumholz

Publications and source records attributed to Mark R. Krumholz.

At least 19 recordsLinked to original sources

arXiv2026

Magnetising the quokka code with fast, second-order accurate, error-correcting schemes for magnetohydrodynamics on GPUs

We implement a second-order accurate constrained transport (CT) magnetohydrodynamics (MHD) module with first-order flux correction in the open-source, adaptive mesh refinement (AMR), GPU-accelerated code quokka, supporting both ideal and resistive (constant Ohmic) regimes. For computing and averaging the electromotive forces (EMFs) at cell edges, the basis of any CT-MHD method, we experiment with a wide range of recent, state-of-the-art schemes, together with different reconstruction schemes. Alongside these, we develop a new EMF compute scheme that requires fewer reconstruction steps than existing approaches. We evaluate these scheme combinations on the basis of accuracy, stability, and GPU throughput, and demonstrate that our new scheme achieves accuracy comparable to the best-in-class existing methods, together with greater stability in reconnection-dominated flows and roughly 25--60% higher GPU throughput. Using this scheme, quokka achieves excellent results across a wide range of MHD flow regimes, reaching >50 million cell updates per GPU per second, with >70% parallel efficiency out to >500 GPUs. Finally, we confirm that our CT implementation preserves divergence-free magnetic fields (to machine precision) under AMR.

astro-ph.IM↗

arXiv2026

Ripples of Stellar Enrichment (RoSE) - simulating element production and mixing in a Milky Way-mass galactic disc star-by-star

We present the Ripples of Stellar Enrichment (RoSE) simulations, which follow an isolated Milky Way-mass disc galaxy with star-by-star feedback and nucleosynthesis from five physical enrichment channels -- Wolf-Rayet stars, type II supernovae, type Ia supernovae, asymptotic giant branch stars, and neutron star mergers. We use these simulations to test how elements' diverse nucleosynthetic origins imprint spatial, temporal, and inter-element abundance correlations in gas and newly formed stars. We find that nucleosynthetic source composition is the primary organising principle of elemental structure: elements sharing a dominant production channel exhibit similar spatial and temporal statistics and their abundances are strongly correlated with one another, while mixed-source pairs are much more weakly correlated. We show that a simple linear regression model based only on how element pairs differ in their nucleosynthetic origin is able to predict, with high fidelity, how strongly their abundances correlate, in both interstellar medium gas and coeval stars. Comparison with Milky Way stellar abundance data shows encouraging qualitative agreement, with differences between simulations and observations comparable to the scatter between independent observational datasets. Together, these results show that the covariance of galactic abundances is largely predictable from the mixture of nucleosynthetic sources supplying each element: same-source elements remain tightly correlated in gas and young stars, whereas differing source mixtures produce systematically weaker correlations.

astro-ph.GA↗

arXiv2026

Magnetic destabilisation in disc galaxies: Filament/feather formation

Gas gravitational instability plays a crucial role in secular galactic evolution, but the role played by magnetic fields in mediating this instability - despite their dynamical relevance and ubiquity - is still not understood. To investigate this question we conduct a parameter study using numerical simulations of 3D isolated disc galaxies that are initialized in equilibrium, but have a range of initial magnetisation, quantified by $β\in \{0.1, 0.5, 1, 10, 100, \infty\}$, where $β$ is the ratio of thermal to magnetic pressure. We analyse how magnetic field strength influences the formation of dense filaments and feathers driven by gravitational instability. The simulations show that filament growth rate and spacing are significantly altered by dynamically strong fields ($β\lesssim 10$), and that these effects depend on the value of $β$ and strength of shear in the disc. Magnetic fields either stabilise or destabilise, with destabilisation dominating in regions with low $β$, and low shear. This makes the destabilisation particularly important in dwarf galaxies with low-shear rotation curves. Filament spacings are similarly affected differently in different galactic regions, depending upon the local field strength and shear. Our results are in good agreement with predictions from the magneto-Jeans mechanism.

astro-ph.GA↗

arXiv2026

The Environmental Dependence of Star Cluster Demographics

Both the star cluster mass function and the lifetimes of clusters may vary with galactic environment, but measuring this variation is challenging because in observational surveys real features of cluster demographics are invariably entangled with catalogue incompleteness. Here we analyse $\approx$ 8300 star clusters in 12 galaxies drawn from the LEGUS survey using the slug Bayesian forward modelling framework coupled to our new c-4 neural network-based completeness estimator, which allows us to incorporate realistic catalogue-inclusion probabilities directly into the likelihood and compensate for these biases. We show that this approach allows us to fit observed cluster luminosity functions with excellent fidelity at both galactic and sub-galactic scales. We find that mass function slopes are relatively universal and broadly consistent with a power law $M^{-2}$ form, but high mass truncations vary by orders of magnitude both between and within galaxies. Our fits also strongly favour models where cluster disruption is mass-independent, but the time at which disruption begins again shows wide environmental variations. Our results demonstrate that young cluster demographics are environmentally dependent, with the clearest signal appearing at the upper end of the cluster mass function, but that these variations are not well-explained by any of the models currently in the literature, and do not correlate straightforwardly with properties such as star formation rate per unit area or strength of shear.

astro-ph.GA↗

arXiv2026

Updated Constraints on the Injection Energy of Positrons Generating the Galactic 511 keV $γ$-ray line

Even 50 years after the discovery of a positron annihilation line from the inner Galaxy, no class of astrophysical sources has emerged as a definitive explanation for both the emission morphology and flux. Positrons produced by dark matter annihilation or decay have been proposed, but the mass of any such candidate is constrained by continuum $γ$-ray emission at energies $>511$ keV. Earlier analyses have claimed that this emission requires that the positrons have kinetic energies less than a few MeV at injection, disfavoring both much of the dark matter parameter space and many potential compact astrophysical source classes such as pulsars. However, these constraints were not based on a full forward model of the absolute flux of the $γ$-ray line and continuum data, and did not marginalize over uncertainties about the relative angular distributions of the line and continuum. Here we describe an improved analysis that overcomes these limitations, and show that constraints on the injection energy are much weaker than previously claimed; even under conservative assumptions the data are consistent with initial energies up to $\sim 110$ MeV from INTEGRAL/SPI data alone, and up to $\sim 55$ MeV when including COMPTEL and EGRET data, subject to cross-normalization between them and INTEGRAL.

astro-ph.HE↗

arXiv2026

The HII Regions' Molecular Law of Star Formation

We combine imaging data from the HST, JWST, and ground-based millimeter facilities to investigate the correlation between star formation rate (SFR) and molecular gas at the ~100 pc scale of HII regions in three nearby galaxies: NGC628, NGC5194 and NGC5236. The JWST 21 micron maps of the three galaxies offer a unique insight into the dust-absorbed SFR at high resolution. We find that the relation between the surface densities of SFR and molecular gas has a slope of ~1.85, in log-log scale, significantly steeper than previous results for nearby galaxies but closer to the trends found for molecular clouds in the Milky Way. The steep relation also holds on larger, ~500 pc, scales, and results from the high-resolution imaging that cleanly isolates the star-forming region emission from the underlying galaxy's diffuse contribution. The diffuse emission at 21 micron is, in fact, found to correlate with the galaxy's stellar mass. Comparisons with physical models of star formation are inconclusive; they overlap with the locus of the 100 pc data, but have difficulties in reproducing the data scatter. Possible exceptions are models that add a power law tail to the gas density probability distribution, due to the large range of free parameters allowed. We find that local HII regions, high redshift star-forming clumps, and low and high redshift starburst galaxies form a single sequence of star formation over three orders of magnitude in gas surface density.

astro-ph.GA↗

arXiv2026

Spirited Away: Advective Loss of Cosmic Rays into the Milky Way's Circumgalactic Medium Explains the Large Magellanic Cloud's Low Gamma-ray Luminosity

Models of galactic-scale cosmic ray production and transport have successfully reproduced the radio and $γ$-ray spectra of many galaxies; however, one notable exception is the Large Magellanic Cloud (LMC), where models that successfully fit other galaxies consistently overestimate its $γ$-ray flux. Here we investigate this discrepancy by applying the CRIPTIC cosmic ray transport code to recent magnetohydrodynamic simulations of the LMC's interactions with the Milky Way circumgalactic medium that accurately reproduce observations of the LMC's magnetic field structure. We recover a simulated $γ$-ray luminosity that is close to the observed luminosity for a wide range of cosmic ray transport models, while our control simulations of an isolated LMC not interacting with the Milky Way show the same over-prediction problem as previous investigations. Simulations including the CGM interaction yield lower $γ$-ray luminosities because in them cosmic rays escape the galaxy primarily via advection, significantly decreasing the emission from collisional processes and changing the dominant $γ$-ray emission mechanism from pion decay to inverse Compton emission. Comparisons of the detailed spectral shape show that our interacting LMC models match the observed $γ$-ray spectrum at photon energies below $\sim 10$ GeV, but still slightly overestimate the flux at higher energies, suggesting either more strongly energy-dependent transport than the models we have explored, or a cosmic ray injection spectrum steeper than the $p^{-2.2}$ that we adopt.

astro-ph.HE↗

arXiv2026

Analysis and implications of the spatio-spectral morphology of the Fermi Bubbles

The Fermi Bubbles are gamma-ray structures extending from the center of the Milky Way to +/-50 degree Galactic latitude that were discovered in data obtained by the Fermi/LAT instrument. Their origin and power source remain uncertain. To help address this uncertainty, here we use a template-free reconstruction of ten years of all-sky Fermi/LAT data provided by Platz et al. (2023) to carry out a pixel-by-pixel spectral analysis of the Bubbles. We recover the position-dependent spectral shape and normalization that would be required for parent proton or electron cosmic ray populations to produce the Bubbles' observed gamma-ray spectra. We find that models in which the gamma-ray emission is driven by either hadronic or leptonic processes can explain the data equally well. The cosmic ray population driving the emission must have either broken power-law or exponentially cut-off spectra, with break or cutoff energies that are almost constant with latitude but spectral indices below the break that harden towards the Bubbles' southern tip. For the leptonic channel, reproducing the observed position-dependent gamma-ray spectrum also requires a cosmic ray electron energy density that grows with distance from the Galactic plane and increases towards the edges of the Bubbles. This finding disfavors scenarios for the origin of the Bubbles where a population of cosmic ray electrons is accelerated near the Milky Way center and subsequently advected out to the extremities of the Bubbles.

astro-ph.HE↗

arXiv2026

The Cluster Completeness Correction Calculator (C-4): A Neural-Network framework and pilot application to the LEGUS Survey of NGC 628

Integrated-light star cluster catalogues in external galaxies are subject to complex, often poorly-characterised selection effects that can bias inferred cluster demographics and introduce significant uncertainties, limiting the physical parameter space accessible to analysis. To mitigate this problem, here we introduce the Cluster Completeness Correction Calculator (C-4): a new software tool to quantify and predict these effects in both physical and photometric parameter spaces. C-4 adds artificial star clusters to observed galaxy images, processes these images through the same detection and filtering steps used to construct the original cluster catalogue, and then trains multilayer perceptron neural networks to learn the resulting selection function. The trained neural networks provide continuous, differentiable completeness functions that can be used for direct completeness corrections or incorporated into forward models. We present a pilot application of C-4 to NGC~628, demonstrating that the learned selection operator is highly accurate and successfully captures the strongly non-separable dependence of completeness on mass, age, and extinction. Applying the completeness correction to NGC 628 extends the range of cluster demographic analyses by roughly an order of magnitude in both mass and age, and removes artificial flattening in the observed cluster mass and age distributions. These results establish neural-network-based completeness modelling as a powerful and general approach for recovering intrinsic cluster populations, and provide a scalable framework for modelling high-dimensional selection functions in resolved stellar population studies.

astro-ph.IM↗

arXiv2026

Getting to know the Stellar Clusters in NGC 1569: Bayesian inference of stellar cluster properties in a dwarf starburst galaxy

We present a Bayesian analysis of star clusters in the dwarf starburst galaxy NGC 1569 based on high-resolution Hubble Space Telescope imaging combined with integral-field spectroscopy from the Keck Cosmic Web Imager, obtained as part of the DUVET survey. For each cluster identified, we infer posterior probability distributions for mass and age using a forward modelling method that properly accounts for uncertainties due to stochastic sampling of the IMF. We investigate how the inferred properties depend on photometric coverage by repeating the analysis with different filter combinations, including mock extensions to the ultraviolet and near-infrared that emulate the addition of HST UV bands and James Webb Space Telescope imaging. We find that, while inclusion of these wavelength regimes further breaks age and mass degeneracies, the currently available data yields reasonably strong constraints on cluster parameters. We compare inferred cluster properties to the conditions of the local interstellar medium, and find evidence for multiple interesting correlations. The truncation mass of the cluster mass function varies with galactocentric distance, particularly moving off the disk, consistent with a dependence on the density of the interstellar medium. Cluster mass positively correlates with metallicity, suggesting that massive clusters preferentially form in pre-enriched gas, and the ionisation state of the gas, reflecting the increased prevalence of high-mass stars in high-mass clusters. These results demonstrate the power of Bayesian, initial mass function-aware modelling for resolving cluster populations in nearby starburst dwarfs and provide new insight into how cluster formation and feedback respond to local galactic conditions.

astro-ph.GA↗

arXiv2026

Quokka-based understanding of outflows (QED) -- IV. Limitations of H$α$ as an outflow diagnostic

The presence of broad wings in the H$α$ line is commonly used as a diagnostic of the presence and properties of galactic winds from star-forming galaxies. However, the accuracy of this approach has not been subjected to extensive testing. In this paper, we use high-resolution simulations of galactic wind launching to calibrate the extent to which broad H$α$ wings can be used to infer the properties of galactic outflows. For this purpose, we analyse a series of high-resolution wind simulations from the QED suite spanning two orders of magnitude in star formation surface density ($Σ_\mathrm{SFR}$). We show that the broad component of H$α$ emission correlates well with the wind mass flux at heights $\sim1$ kpc above the galactic plane, but that the correlation is poor at larger distances from the plane, and that even at 1 kpc the relationship between mass flux and surface brightness of broad H$α$ is significantly sub-linear. The sub-linear scaling suggests that the electron column density in the wind increases systematically with outflow strength, and that the conventional assumption of constant electron density in the wind leads to a systematic overestimate of how steeply mass loading factors depend on $Σ_\mathrm{SFR}$. We provide empirical scaling relations that observers can apply to correct for this effect when converting H$α$ measurements to mass outflow rates. Finally, we use synthetic observations of the density-diagnostic $[\mathrm{S_{II}}]\,λ\lambda6716,6731$ doublet to show that using this diagnostic only slightly improves estimates of wind outflow rates compared to the naive assumption of constant electron density, and performs significantly worse than the empirical correlation we provide.

astro-ph.GA↗

arXiv2026

The emerging timescale of young star clusters regulated by cluster stellar mass

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in understanding the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet are difficult to constrain due to the complex interplay between stellar feedback and star formation across multiple physical scales. Here we present Hubble Space Telescope and James Webb Space Telescope observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628 and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging faster than their lower-mass counterparts. This is a critical constraint on star formation and stellar feedback simulations, which struggle to fully reproduce star clusters formation and emergence. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose time limitations for planet formation in massive cluster environments where disks get exposed to ultraviolet irradiation and further gas infall is halted.

astro-ph.GA↗

arXiv2026

The universal growth of magnetic energy during the nonlinear phase of subsonic and supersonic small-scale dynamos

Small-scale dynamos (SSDs) amplify magnetic fields in turbulent plasmas. Theory predicts nonlinear magnetic energy growth $E_\mathrm{mag} \propto t^{p_\mathrm{nl}}$, but this scaling has not been tested across flow regimes. Using a large ensemble of SSD simulations spanning subsonic to supersonic turbulence, we measure linear growth ($p_\mathrm{nl} = 1$) in subsonic flows and quadratic growth ($p_\mathrm{nl} = 2$) in supersonic flows. In all cases, the nonlinear dynamo converts a nearly constant fraction $\sim 1/100$ of the turbulent kinetic energy flux into magnetic energy, and the nonlinear phase has a characteristic duration $Δt \approx 20\,t_0$, where $t_0$ is the outer-scale turnover time. By isolating the onset of magnetic backreaction in SSDs, our statistical ensemble approach identifies a robust efficiency and duration for the nonlinear SSD that can be used to interpret more complex astrophysical and laboratory plasmas.

physics.plasm-ph↗

arXiv2026

Evolution of action-space coherence in a Milky Way-like simulation

Efforts to dynamically trace stars back to the now-dissolved clusters in which they formed rely implicitly on the assumption that stellar orbital actions are conserved. While this holds in a static, axisymmetric potential, it is unknown how strongly the time-varying, non-axisymmetric structure of a real galactic disk drives action drift that inhibits cluster reconstruction. We answer this question using a high-resolution magnetohydrodynamic simulation of a Milky Way-like spiral disc galaxy. We show that, while stars experience significant action evolution over $\lesssim 100$ Myr, they do so in a correlated fashion whereby stars born in close proximity maintain very similar actions for up to 0.5 Gyr. The degree of coherence shows no significant dependence on galactocentric radius, but varies between action components: vertical actions decohere for stars born more than a few hundred parsecs apart (likely due to giant molecular clouds), while radial and azimuthal actions remain correlated on kiloparsec scales (likely influenced by spiral arms). We use our measurements of the rate of action decoherence to develop a probabilistic framework that lets us infer the initial sizes of the star cluster progenitors of present-day stellar streams from their measured action distributions, which we apply to 438 known moving groups. Our results suggest that most of these streams likely originated from compact clusters, but that a significant minority are instead likely to be resonant or dynamically induced structures. This method of classifying streams complements existing methods, optimises the use of expensive spectroscopic abundance measurements, and will be enhanced by the more precise kinematic data that will soon become available from \textit{Gaia} DR4.

astro-ph.GA↗

arXiv2026

A novel algorithm for GPU-accelerated particle-mesh interactions implemented in the QUOKKA code

We present a novel, GPU-optimized algorithm for particle-mesh interactions in grid-based hydrodynamics simulations, designed for massively parallel architectures. This approach overcomes the inefficiency of particle neighbour searches or sorts across multiple GPU nodes by using a new "particle-mesh-particle" interaction scheme, which extends the particle-mesh method for self-gravity. The algorithm proceeds in two main stages: first, quantities exchanged between particles and the mesh -- such as mass, energy, and momentum added by stellar feedback or removed by accretion onto a sink -- are deposited into a buffer mesh equipped with ghost zones, where multiple contributions per cell are accumulated using atomic additions and then communicated across distributed memory ranks. In the second stage, the buffer states are applied to real mesh states, incorporating cell-wise limiters to enforce physical constraints such as positive density. We implement this scheme in the GPU-native radiation-magnetohydrodynamics code QUOKKA and validate it through a comprehensive suite of tests, including Bondi and Bondi-Hoyle accretion, and single and multiple supernova remnant evolution at varying spatial resolutions. We show that the algorithm achieves $\approx 50\%$ weak-scaling efficiency running on up to 8192 GPUs on the Frontier supercomputer. This scheme enables efficient, scalable particle-mesh coupling for GPU-optimized simulations of star formation and feedback in galaxies.

astro-ph.IM↗

arXiv2026

Not Where You Left Them: Displaced $γ$-Rays and X-Rays Reveal the Cosmic Ray Scattering Rate

Modern X-ray and $γ$-ray instruments are revealing a growing class of Galactic non-thermal sources whose emission centroids are measurably offset from the nearest plausible sites of cosmic ray (CR) acceleration. Such "displaced" sources are seen in keV X-rays and TeV-PeV $γ$-rays but not in GeV $γ$-rays, have hard spectra, and are not associated with gas clumps, suggesting a leptonic origin. We develop a general framework for understanding displacement, whereby relativistic CR electrons (CRe) injected into the interstellar medium (ISM) with a strongly anisotropic pitch-angle distribution propagate a finite distance from their acceleration site before scattering processes isotropise their directions sufficiently for the emission to become visible. We use CR transport simulations to investigate under what circumstances displacement is likely, finding that it requires an initial pitch angle distribution $\lesssim 45^\circ$ wide, a line of sight broadly edge-on to the magnetic field, and that the source be measured in a waveband where emission is dominated by CRe for which the radiative-loss and pitch-angle scattering timescales are comparable. For typical Galactic conditions the latter condition is satisfied only for CRe energies $\gtrsim$ 10 TeV, explaining why displaced sources appear at X-ray and TeV but not GeV energies. We further show that, when displacement is detected, it allows a direct inference of the CRe pitch-angle scattering rate.

astro-ph.HE↗

arXiv2026

ALMA Central molecular zone Exploration Survey (ACES) V: CS(2-1), SO(2_3-1_2), CH3CHO(5_1,4-4_1,3), HC3N(11-10), and H40a lines data

We present data from the ALMA Central Molecular Zone Exploration Survey (ACES) Large Program, which provides broad spectral-line and 3 mm continuum coverage of the Central Molecular Zone (CMZ) at a spatial resolution of 0.1 pc. The survey delivers homogeneous, wide-field mosaics that enable direct comparisons of the physical and chemical conditions across diverse environments in the Galactic center. In this data release paper, we present the CS(2-1), SO(2_3-1_2), CH3CHO(5_1,4-4_1,3), HC3N(11-10), and H40a lines observed simultaneously within two broad spectral windows. These lines reveal pronounced spatial and chemical variations across the CMZ, tracing distinct components of molecular gas, shock-affected regions, and ionized structures. The high angular resolution and multi-line capability of the ACES dataset make it a powerful resource for future studies of gas dynamics, star formation activity, and the physical connection between the CMZ and Sgr A*.

astro-ph.GA↗

arXiv2026

ALMA Central Molecular Zone Exploration Survey (ACES)-IV. Data of the two intermediate-width spectral windows

We release the intermediate-width spectral window data from the ALMA Central Molecular Zone Exploration Survey (ACES) Large Program, which covers SiO(2-1), SO(2_2-1_1), H13CO+(1-0), H13CN(1-0), HN13C(1-0), and HC15N (1-0), among other molecular line transitions, with an angular resolution of ~2 arcsec and a velocity resolution of 1.7 km s-1 . The full cubes of the two spectral windows as well as the key data products will be available to the community. We also present the integrated brightness, peak brightness, centroid velocity, and Galactic longitude-velocity maps of the six lines. We briefly discuss morphological correlations between the continuum and the molecular line emission, and brightness ratios between pairs of isotopologue or isotopomer lines. We highlight features and trends in the data that will be followed up in upcoming ACES science papers.

astro-ph.GA↗