Volker Springel

It is often understood that supernova (SN) feedback in galaxies is responsible for regulating star formation (SF) and generating gaseous outflows. However, a detailed look at the small-scale effects of SNe on the interstellar medium (ISM) in simulations shows that the macroscopic processes of SF suppression and outflow generation proceed in distinct channels. We demonstrate this finding in two independent simulations of isolated dwarf galaxies with very high (m_gas ~ Msun) numerical resolution, LYRA and RIGEL. Our findings suggest that the macroscopic effect of a given SN on the galaxy is best predicted by its local density. Outflows are primarily linked to SNe in diffuse regions expanding to their cooling radii on large (~kpc) scales, while dense SF regions are disrupted in a localized (~pc) manner. However, these separate feedback channels are only distinguishable at very high resolutions capable of following mass scales ^2 . When averaging on coarser scales, ISM densities are greatly mis-estimated, and variations between different SF and SNe-affected regions are severely washed out. It therefore cannot be self-consistently determined, from coarse-resolution information alone , (1) whether a SN tends to contribute to outflows or direct SF suppression, and (2) the rate of SF in a given region. In particular, commonly used parameters in coarse-resolution (subgrid) models, such as the SN cooling radius and SF density threshold, may require more detailed treatments informed by high-resolution studies.

We used 50 Sobol-sampled VariableTNG (VTNG) simulations, varying eight subgrid parameters at a fixed cosmology and initial conditions, to determine which baryonic processes regulate galaxies and black holes at z = 6 and z = 8. We compared the simulations with recent high-redshift measurements of the stellar mass function (SMF), star-forming main sequence, stellar and gas-phase mass–metallicity relations, stellar mass–size relation, and black-hole host and accretion properties. The simulations reproduced several broad trends in these observables, although differences remain for the gas-phase metallicity, galaxy size, and the most extreme black-hole populations. Given the substantial uncertainties in both the physical modelling and the observational inference of high-redshift galaxy properties, we regard these differences as diagnostic tensions rather than definitive model failures. Random-forest analyses reveal a clear hierarchy in parameter sensitivity. The abundance of low-mass galaxies is primarily regulated by stellar feedback, particularly the supernova temperature T _SN and thermal wind fraction τ _w , whereas the sensitivity of the scatter in the star-forming main sequence is weaker and redshift dependent. The high-accretion tail of black-hole growth depends on black-hole seeding and feedback parameters, but also on T _SN , suggesting that stellar feedback indirectly regulates rapid black-hole growth through its impact on the available gas supply. For an independent VTNG-sized volume, the estimated field-to-field scatter in the integrated SMF abundance exceeds the parameter-driven scatter by factors of approximately 2.3 and 1.6 at z = 6 and z = 8, respectively. This volume-dependent scatter limits the detectability of the subgrid response in individual small fields, but it does not alter the intrinsic response isolated by the fixed-initial-condition experiment. Our controlled experiment identifies stellar feedback as a common physical link between early low-mass galaxy formation and rapid black-hole growth.

The dispersion measures (DMs) of fast radio bursts (FRBs) arise predominantly from free electrons in the large-scale structure of the Universe. The increasing number of FRB observations has started to empirically constrain the distribution of cosmic baryons, making it crucial to accurately forward model their propagation within cosmological simulations. In this work, we present a method for measuring FRB DMs in IllustrisTNG that continuously traces rays through the simulation while reconstructing all traversed line segments within the underlying Voronoi mesh. Leveraging this technique, we create over 20 publicly available DM catalogs, including full-sky DM maps observed from Milky Way–like environments. Our method addresses a problem in previous TNG-based studies, in which a sparse snapshot sampling in the path integral leads to a misestimation of the standard deviation and higher moments of the DM distribution p (DM∣ z ) by over 50%. We show that our results are consistent with the most recent observational data from the DSA-110, ASKAP, and CHIME. We offer a functional form for p (DM∣ z ) that provides very good fits across all redshifts from 0 to 5.5, showing that the previously proposed log-normal distribution is not well matched to the data. We find that using simulation box sizes of or below 35 h ⁻¹ Mpc, or low resolutions with baryonic masses exceeding 5 × 10 ⁸ h ⁻¹ M _⊙ , can distort the derived DM signal by more than 8%. Our findings provide new insights into how the cosmic web shapes FRB signals and highlight the importance of accurate methodology when comparing cosmological simulations against observations.

X-ray observations are a primary probe of the intracluster medium, and are widely used to infer galaxy cluster masses and scaling relations. We present and validate a pipeline to generate X-ray emission maps of galaxy groups and clusters from large cosmological simulations, and use it to study the origin of deviations from self-similarity across the group-to-cluster transition. We applied this pipeline to the Illustris-TNG300 and MillenniumTNG simulations, constructing X-ray emission maps and spectra based on APEC cooling functions. For simulations that do not explicitly track individual chemical abundances, we introduced a metallicity-based prescription that accurately reproduces the full spectral emission. We derived the L _X – M ₅₀₀ , M _gas – M ₅₀₀ , and T – M ₅₀₀ scaling relations over 10 ^12.5 ≤ M ₅₀₀ ≤ 10 ^15.5 M _⊙ , compared them with observational data, and quantified the hydrostatic equilibrium and spectroscopic temperature biases through synthetic X-ray analyses. The simulated scaling relations are in good overall agreement with observations and are best described by broken power laws with a pivot at M ₅₀₀ = 10 ^13.67 M _⊙ . At high masses, the slopes are close to self-similar expectations; at lower masses the relations steepen significantly, reflecting the growing importance of active galactic nuclei (AGN) feedback. X-ray hydrostatic masses are systematically underestimated by 15%, independently of cluster mass. When spectroscopic effects are included, the bias becomes mass-dependent, ranging from 15% at low masses to 21% at high masses. The recovered X-ray luminosity, measured in the 0.15–1 R ₅₀₀ aperture, is also mass-dependent: high-mass clusters are underestimated by 18%, while low-mass systems show discrepancies of up to 33%, driven by single-temperature spectral modelling of the gas outside the core. Despite these biases, the slope and normalisation of the M ₅₀₀ – L ₅₀₀ scaling relation are only weakly affected, because the spectroscopic pipeline shifts both mass and luminosity simultaneously, absorbing a fraction of the bias along the intrinsic scaling relation rather than perpendicular to it.

Radio continuum observations are widely used to study cosmic ray (CR) electron populations and transport processes in star-forming galaxies, but their interpretation relies on several simplifying assumptions. Here, we revisit three common assumptions: that some vertical radio profiles can be explained by CR advection alone, that radio spectra directly trace the galaxy-wide CR electron spectrum, and that bremsstrahlung and Coulomb losses are negligible for radio-emitting electrons. We model radio emission using time-dependent CR electron evolution in a magnetohydrodynamical simulation of an isolated Milky Way-mass galaxy. CR electron spectra are evolved self-consistently along Lagrangian tracer particles with the Crest framework, including injection at supernova remnants, advection with the gas, and spatially and temporally varying radiative losses. We compare these results to commonly adopted steady-state models. We find that advection-only transport in self-consistently driven galactic winds fails to reproduce the extended vertical radio intensity profiles observed in edge-on galaxies, despite reproducing the observed steepening of spectral indices with height. This is because slowly accelerating winds keep electrons in strong cooling environments for too long. Matching observed radio haloes with advection alone requires unrealistically high mid-plane wind velocities, implying that additional transport or re-acceleration processes are required. Although galaxy-integrated CR electron spectra at radio-emitting energies are similar across models, the resulting synchrotron spectra differ systematically because radio emission is biased towards young electrons in dense, strongly magnetized regions. Finally, we show that bremsstrahlung and Coulomb losses significantly shape radio spectra even when their loss rate is subdominant and therefore cannot be neglected.

Recent pulsar timing array (PTA) results provide strong evidence of a stochastic gravitational wave background (sGWB), consistent with a population of merging massive black holes (MBHs) at z < 1. Meanwhile, JWST observations at z > 5 suggest a higher number density of accreting MBHs than previously estimated. Together with constraints from local MBHs and high- z quasars, these findings offer a unique opportunity to test MBH seeding and early growth models. We explored this using L-Galaxies BH , a new extension of the galaxy formation model L-Galaxies , developed to explicitly model all stages of MBH evolution, including seeding, accretion, and binary dynamics. To take advantage of both the high resolution of the MillenniumII and the large volume of the Millennium simulations, we ran L-Galaxies BH on the former and used its outputs as initial conditions for the latter, via our “grafting” method. We find that reproducing the number density of high- z active MBHs observed by JWST requires either a heavy seed formation rate that is significantly higher than that predicted by current theoretical models (≳0.01 Mpc ⁻³ at z ∼ 10) or a widespread formation of light seeds undergoing multiple phases of super-Eddington accretion. Furthermore, matching the amplitude of the PTA sGWB signal requires nearly all galaxies with stellar masses M _∗ > 10 ⁹ M _⊙ to host central MBHs by z ∼ 0. Given the extreme heavy seed densities required to satisfy both PTA and JWST constraints, our results favor a scenario in which MBHs originate from light seeds that grow rapidly and efficiently in the early Universe. This work demonstrates the power of combining multimessenger data with physical models to probe the origins and evolution of MBHs across cosmic time.

The origin of supermassive black holes remains a long-standing problem in astrophysics. Recent James Webb Space Telescope (JWST) observations reveal an unexpectedly abundant population of overmassive black holes at z > 4–6, at which the black hole masses lie far above local scaling relations and are not reproduced by present cosmological models ^1–5 . How such overmassive black holes form and rapidly grow within young galaxies has remained unclear. Here we present fully cosmological radiation-hydrodynamic simulations that self-consistently follow the birth, early growth and emergent observable signatures of supermassive black holes in protocluster environments. We find that heavy seeds on the order 10 ⁶ M _⊙ naturally form, exceeding typical theoretical expectations by an order of magnitude. These seeds rapidly develop dense, optically thick disks whose strong electron scattering produces broad Hα emission comparable to that seen in little red dots ^6–10 . Sustained super-Eddington accretion then drives fast growth to about 3 × 10 ⁷ M _⊙ by z ≃ 8. To our knowledge, this is the first demonstration that unifies little red dots to a short-lived, enshrouded phase of heavy-seed formation, which naturally evolve into the overmassive quasars detected by the JWST and ultimately the progenitors of today’s supermassive black holes.

We present the public release of OpenGadget3 (OG3), a substantially extended and re-organised version of the widely used Gadget-2/3 family of cosmological simulation codes. Since G2 was made publicly available, the absence of continued official development led to the creation of numerous, mutually incompatible developer versions of G3 across the community. This made it effectively impossible to track bug fixes, reproduce published results, or consistently credit the many individual contributors. OG3 addresses this by consolidating these disparate developments into a single, documented, and continuously maintained code base, released under the GNU GPLv3 through a GitLab repository (with an accompanying wiki), regularly synchronised with the active developer branch, and supported by a dedicated continuous-integration (CI) pipeline.OG3 retains the core algorithmic backbone of its predecessors, a Barnes\&Hut oct-tree combined with a Particle-Mesh method for gravity, and a choice of Smoothed Particle Hydrodynamics or Meshless-Finite-Mass solvers for gas dynamics, within a hybrid MPI/OpenMP parallelisation framework, extended with GPU offloading via OpenACC and OpenMP. The code integrates an extensive suite of sub-resolution and physical modules, including primordial chemistry, radiative cooling, star formation, stellar feedback and chemical enrichment, dust formation and evolution, black hole growth and AGN feedback, MHD, thermal conduction, physical viscosity, SIDM, and massive neutrinos.OG3 introduces new features improving robustness, reproducibility, and usability: a flexible mixed-precision framework spanning 16- to 128-bit representations; expert-level tagged parameters with restart-time change log; extensive consistency checks against invalid configurations; and FAIR-compliant reporting of all settings, parameters, and specific publications.(shortened and abridged)

Magnetic fields are found in many astrophysical objects, ranging from galaxy clusters to the interstellar medium of galaxies and neutron stars. Strong surface magnetic fields are also observed in about 7% of OBA-type stars, and stellar mergers are the likely origin of at least some of them. We investigated magnetic-field amplification during the merger of a 9 and an 8 M _⊙ main-sequence star using 3D magnetohydrodynamic simulations from our previous work. We focused on the magnetic-field amplification mechanisms, field geometry, and the structure and properties of the resulting merger, in particular its rotational configuration. The merger produces a star–torus structure in which the core of the initially less massive star is surrounded by material from the primary. The torus contains ≈3 M _⊙ and stores about 60% of the initial orbital angular momentum, and dynamic mass loss is negligible. Initially, turbulent motions driven by Kelvin–Helmholtz and magneto-rotational instabilities generate small-scale (≈0.1 R _⊙ ) magnetic fields. Subsequently, large-scale ordered azimuthal flows drive a larger-scale dynamo that amplifies and redistributes the magnetic energy to larger spatial scales (≈5 R _⊙ ), producing a remnant threaded by a strong large-scale magnetic field. The final magnetic configuration consists of intertwined poloidal and toroidal components, with a residual small-scale structure that resembles previously identified stable magnetic field equilibria. The amplification process is largely insensitive to the initial binary separation, numerical resolution, and seed magnetic-field strength, indicating that the final field loses memory of the initial magnetic conditions. The central regions of the merger remnant rapidly approach solid-body rotation, transitioning to a Keplerian-like profile within the surrounding torus. Our results support stellar mergers as a viable pathway for the formation of strongly magnetic massive stars and potentially highly magnetized compact remnants, such as magnetic white dwarfs and magnetars.