Rüdiger Pakmor

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.

Context. Lopsidedness is a common feature in disk galaxies, but its origin and evolution remain poorly understood. Previous works have generally focused on stellar or gas lopsidedness separately. However, a combined analysis can help identify the mechanisms driving lopsidedness and constrain recent galactic evolution and environmental interactions. Aims. We study lopsidedness in the density and kinematics of the stellar and atomic hydrogen (HI) components of nine Milky Way-type galaxies from the Auriga Superstars cosmological zoom-in simulations. The high stellar mass resolution (m _* = 800 M _⊙ ) of the simulations improves the visibility of features in the stellar disk and halo, reduces the effect of noise on the measurement of lopsidedness, and provides a refined sampling of the gravitational potential. Methods. We quantified morphological and kinematical lopsidedness through the amplitude of the first Fourier mode ( m = 1) of the face-on projected mass distribution and radial velocity maps consistently for stars and gas and over 0.5–1 R _opt , where R _opt is the stellar optical radius. Results. At z = 0, the morphological lopsidedness of the old stellar component (> 0.5 Gyr old) strongly correlates with that of HI, tracing distortions in the global gravitational potential, while the young stellar population (< 0.5 Gyr old) primarily traces asymmetric star formation along strong spiral arms. Morphological and kinematical lopsidedness are strongly correlated in stars but weakly correlated in HI, which typically exhibits stronger kinematical asymmetries. We also find that stellar disk lopsidedness is anticorrelated with bar strength, with strongly barred galaxies hosting more symmetric disks and higher central stellar mass densities. Based on our simulations of lopsidedness evolution over the past 7 Gyr, we observed that tidal interactions with massive satellites (mass ratio > 1:50) produce coherent lopsided responses in stars and HI. In contrast, smooth gas accretion primarily drives lopsidedness in HI and young stars, while the total stellar component remains largely symmetric. Individual galaxies often experience multiple mechanisms, producing complex coupling and decoupling among distinct tracers.

Previous radiative transfer calculations have shown that both multi-dimensionality and NLTE (non local thermodynamic equilibrium) effects impact the synthetic observables predicted for Type Ia supernova explosion models. Here we carry out a 2D NLTE radiative transfer simulation in the photospheric phase for a double detonation model with a 1 M⊙ carbon–oxygen core and 0.02 M⊙ helium shell. The predicted observables demonstrate that departures from both spherical symmetry and local thermodynamic equilibrium are significant on scales relevant to comparisons with observations. The NLTE treatment leads to a bluer spectral energy distribution, more slowly declining optical-band light curves, changes to the near-infrared light curve shapes, and differences in the evolution of key spectral features. Although substantial viewing-angle variation is predicted for the model, the scale of the variation is not significantly impacted by NLTE effects up to peak. However, after peak, our 2D NLTE simulation retains a strong viewing-angle dependence in its spectra whereas our simulation of the same ejecta model using an approximate NLTE treatment predicts significantly reduced spectroscopic viewing-angle variation. The NLTE treatment leads to improved agreement with normal Type Ia supernovae, primarily due to the increased ionisation state of the simulation, although the absence of a secondary near-infrared maximum suggests that the ejecta remain over-ionised relative to observations after peak. Despite the improved agreement, many viewing angles still show excessive line blanketing, suggesting that the 0.02 M⊙ helium-shell mass of the current model is too large to reproduce normal Type Ia supernovae over the majority of lines of sight.

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.

Type Ia supernovae are thermonuclear explosions of white dwarfs, yet the nature of their progenitor systems remains uncertain. Recent discoveries of hypervelocity stars provide unique constraints, as these stars likely represent the surviving companions of such explosions. Using detailed binary evolution models computed with MESA and population synthesis with MSE , we investigated the outcomes of hot subdwarf + white dwarf binaries undergoing helium accretion. We find that donors can nearly exhaust their helium and form compact, C/O (carbon-oxygen) cores before explosion. The predicted ejection velocities span a broad distribution, reaching up to ∼1000 km s ⁻¹ , with D6-2 representing the extreme high-velocity tail of this population. We estimate analytically that the thin residual helium envelope can be stripped by the supernova ejecta, which produces a C/O-rich surface composition. The Type Ia supernova rate from this channel is S ₈ = 0.811 ^+0.022 _−0.020 , i.e., it could be responsible for 1% of the observed Type Ia supernovae. Hot subdwarf + white dwarf binaries containing nearly exhausted He-star donors can therefore naturally explain the velocity and composition of D6-2 while providing a quantitatively consistent contribution to the observed Type Ia supernova rate. Our models predict a distribution of surviving donor remnants with various core He fractions and with ejection velocities extending down to ∼450 km s ⁻¹ . The orbital velocities of donor stars in this progenitor channel naturally yield orbital velocities consistent with US 708, LP 40–365 stars, and D6-2, indicating that a single class of thermonuclear supernova progenitors can account for their entire range of ejection velocities.

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.

Observations of binary black hole (BBH) mergers provide a unique window into the lives of massive stars across cosmic time. Connecting redshift-dependent merger properties to massive star progenitors requires accurate models of cosmic star formation and chemical enrichment histories. Analytical fits for the metallicity-specific cosmic star formation rate density S ( Z , z ) are commonly used as proxies for the complex underlying star formation history, yet they remain unconstrained. Using the IllustrisTNG cosmological simulations, we evaluate the accuracy of these analytical S ( Z , z ) prescriptions. By coupling the simulated and analytical S ( Z , z ) to the population synthesis code COMPAS , we investigate the resulting BBH merger rates and mass distributions. We find that analytical S ( Z , z ) prescriptions can overestimate BBH merger rates at high redshift ( z ≳ 6) by up to a factor of 10–10 ⁴ , depending on cosmological simulation resolution, and can introduce spurious features in the BBH mass distribution. For example, they can produce an artificial feature near 8 M _⊙ in the primary mass distribution at z ≲ 2, which is absent when using the full simulation-based S ( Z , z ) , while simultaneously suppressing high-mass features. These discrepancies arise because simple analytical models fail to capture a high-metallicity bump and a more flattened low-metallicity tail in the simulated S ( Z , z ) metallicity distribution. Our results highlight the importance of accurate star formation histories for modeling BBH populations, demonstrate the limitation of widely used analytical S ( Z , z ) fits, and underscore the need for careful integration of cosmological simulations, analytical fits, and population synthesis when interpreting gravitational-wave observations.

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.