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Weak gravitational lensing is a powerful tool for precision tests of cosmology. As the expected deflection angles are small, predictions based on non-linear N -body simulations are commonly computed with the Born approximation. Here, we examine this assumption using DORIAN, a newly developed full-sky ray-tracing scheme applied to high-resolution mass-shell outputs of the two largest simulations in the MillenniumTNG suite, each with a 3000 Mpc box containing almost 1.1 trillion cold dark matter particles in addition to 16.7 billion particles representing massive neutrinos. We examine simple two-point statistics like the angular power spectrum of the convergence field, as well as statistics sensitive to higher order correlations such as peak and minimum statistics, void statistics, and Minkowski functionals of the convergence maps. Overall, we find only small differences between the Born approximation and a full ray-tracing treatment. While these are negligibly small at power-spectrum level, some higher order statistics show more sizeable effects; ray-tracing is necessary to achieve per cent level precision. At the resolution reached here, full-sky maps with 0.8 billion pixels and an angular resolution of 0.43 arcmin, we find that interpolation accuracy can introduce appreciable errors in ray-tracing results. We therefore implemented an interpolation method based on non-uniform fast Fourier transforms (NUFFT) along with more traditional methods. Bilinear interpolation introduces significant smoothing, while nearest grid point sampling agrees well with NUFFT, at least for our fiducial source redshift, |$z_s=1.0$|⁠ , and for the 1 arcmin smoothing we use for higher order statistics. [ABSTRACT FROM AUTHOR]

For decades, the boundary of cosmic filaments has been a subject of debate. In this work, we determine the physically motivated radii of filaments by constructing stacked galaxy number density profiles around the filament spines. We find that the slope of the profile changes with distance to the filament spine, reaching its minimum at approximately 1 Mpc at |$z=0$| in both state-of-the-art hydrodynamical simulations and observational data. This can be taken as the average value of the filament radius. Furthermore, we note that the average filament radius rapidly decreases from |$z=4$| to 1, and then slightly increases. Moreover, we find that the radius of the filament depends on the length of the filament, the distance from the connected clusters, and the masses of the clusters. These results suggest a two-phase formation scenario of cosmic filaments. The filaments experienced rapid contraction before |$z=1$|⁠ , but their density distribution has remained roughly stable since then. The subsequent mass transport along the filaments to the connected clusters is likely to have contributed to the formation of the clusters themselves. [ABSTRACT FROM AUTHOR]

The Voids-within-Voids-within-Voids project used dark-matter-only (DMO) simulations to study the abundance and structure of dark matter (DM) haloes over the full mass range populated in the standard Lambda cold dark matter cosmology. Here, we explore how baryonic effects modify these results for |$z=0$| halo masses in the range |$10^4$| – |$10^7~\mathrm{M_\odot }$|⁠ , below the threshold for galaxy formation. Our main study focuses on three simulations from identical initial conditions at |$z=127$|⁠ , one following DMO, one including non-radiative gas, and one additionally including the baryonic physics relevant in this halo mass range (cooling and photoheating). In the non-radiative simulation, above |$10^{5.5}~\mathrm{M_\odot }$|⁠ , halo abundance and internal structure are very similar to the DMO simulation, and the baryon to DM ratio is everywhere close to the cosmic value. At lower mass, this ratio drops and haloes are less concentrated and less massive in the non-radiative case. Test simulations at higher resolution show this to be mainly a resolution effect; the expected drop in baryon content due to residual pressure effects only becomes substantial for |$z=0$| haloes below |${\sim}10^{2.7}~\mathrm{M_\odot }$|⁠. However, gas is heated by reionization at |$z=6$| in our 'full physics' run, and this results in almost complete expulsion of gas from all haloes in our simulated mass range. This suppresses the halo mass function by |${\sim}30{{\ \rm per\ cent}}$|⁠ , lowers halo concentration, and consequently weakens the DM annihilation signal by |${\sim}40{-}60{{\ \rm per\ cent}}$|⁠. [ABSTRACT FROM AUTHOR]

We present an extended suite of the Auriga cosmological gravo-magnetohydrodynamical 'zoom-in' simulations of 40 Milky Way-mass haloes and 26 dwarf galaxy–mass haloes run with the moving-mesh code arepo. Auriga adopts the Lambda cold dark matter cosmogony and includes a comprehensive galaxy formation physics model following the coupled cosmic evolution of dark matter, gas, stars, and supermassive black holes which has been shown to produce numerically well-converged galaxy properties for Milky Way-mass systems. We describe the first public data release of this augmented suite of Auriga simulations, which includes raw snapshots, group catalogues, merger trees, initial conditions, and supplementary data, as well as public analysis tools with worked examples of how to use the data. To demonstrate the value and robustness of the simulation predictions, we analyse a series of low-redshift global properties that compare well with many observed scaling relations, such as the Tully–Fisher relation, the star-forming (SF) main sequence, and H  i gas fraction/disc thickness. Finally, we show that SF gas discs appear to build rotation and velocity dispersion rapidly for |$z\gtrsim 3$| before they 'settle' into ever-increasing rotation-dispersion ratios (⁠|$V/\sigma$|⁠). This evolution appears to be in rough agreement with some kinematic measurements from H |$\alpha$| observations, and demonstrates an application of how to utilize the released data. [ABSTRACT FROM AUTHOR]

We present a machine-learning-based model for the total density profiles of subhaloes with masses |$M \gtrsim 7\times 10^8\, h^{-1}{\rm M}_\odot$| in the IllustrisTNG100 simulation. The model is based on an interpretable variational encoder (IVE) which returns the independent factors of variation in the density profiles within a low-dimensional representation, as well as the predictions for the density profiles themselves. The IVE returns accurate and unbiased predictions on all radial ranges, including the outer region profile where the subhaloes experience tidal stripping; here its fit accuracy exceeds that of the commonly used Einasto profile. The IVE discovers three independent degrees of freedom in the profiles, which can be interpreted in terms of the formation history of the subhaloes. In addition to the two parameters controlling the normalization and inner shape of the profile, the IVE discovers a third parameter that accounts for the impact of tidal stripping on to the subhalo outer profile; this parameter is sensitive to the mass loss experienced by the subhalo after its infall on to its parent halo. Baryonic physics in the IllustrisTNG galaxy formation model does not impact the number of degrees of freedom identified in the profile compared to the pure dark matter expectations, nor their physical interpretation. Our newly proposed profile fit can be used in strong lensing analyses or other observational studies which aim to constrain cosmology from small-scale structures. [ABSTRACT FROM AUTHOR]

Approximate N -body methods, such as fastpm and cola , have been successful in modelling halo and galaxy clustering statistics, but their low resolution on small scales is a limitation for applications that require high precision. Full N -body simulations can provide better accuracy but are too computationally expensive for a quick exploration of cosmological parameters. This paper presents a method for correcting distinct haloes identified in fast N -body simulations, so that various halo statistics improve to a percent level accuracy. The scheme seeks to find empirical corrections to halo properties such that the virial mass is the same as that of a corresponding halo in a full N -body simulation. The modified outer density contour of the corrected halo is determined on the basis of the fastpm settings and the number of particles inside the halo. This method only changes some parameters of the halo finder, and does not require any extra CPU-cost. We demonstrate that the adjusted halo catalogues of fastpm simulations significantly improve the precision of halo mass-based statistics from redshifts |$z=0.0$| to 1.0, and that our calibration can be applied to different cosmologies without needing to be recalibrated. [ABSTRACT FROM AUTHOR]

An important characteristic of cosmic hydrogen reionization is the growth of ionized gas bubbles surrounding early luminous objects. Ionized bubble sizes are beginning to be probed using Lyman α emission from high-redshift galaxies, and will also be probed by upcoming 21 cm maps. We present results from a study of bubble sizes using the state-of-the-art thesan radiation-hydrodynamics simulation suite, which self-consistently models radiation transport and realistic galaxy formation. We employ the mean free path method and track the evolution of the effective ionized bubble size at each point (R eff) throughout the Epoch of Reionization. We show that there is a slow growth period for regions ionized early, but a rapid 'flash ionization' process for regions ionized later as they immediately enter a large, pre-existing bubble. We also find that bright sources are preferentially in larger bubbles, and find consistency with recent observational constraints at z ≳ 9, but tension with idealized Lyman α damping-wing models at z ≈ 7. We find that high-overdensity regions have larger characteristic bubble sizes, but the correlation decreases as reionization progresses, likely due to runaway formation of large percolated bubbles. Finally, we compare the redshift at which a region transitions from neutral to ionized (z reion) with the time it takes to reach a given bubble size and conclude that z reion is a reasonable local probe of small-scale bubble size statistics (⁠|$R_\text{eff} \lesssim 1\, \rm {cMpc}$|⁠). However, for larger bubbles, the correspondence between z reion and size statistics weakens due to the time delay between the onset of reionization and the expansion of large bubbles, particularly at high redshifts. [ABSTRACT FROM AUTHOR]

Cosmological simulations serve as invaluable tools for understanding the Universe. However, the technical complexity and substantial computational resources required to generate such simulations often limit their accessibility within the broader research community. Notable exceptions exist, but most are not suited for simultaneously studying the physics of galaxy formation and cosmic reionization during the first billion years of cosmic history. This is especially relevant now that a fleet of advanced observatories (e.g. James Webb Space Telescope, Nancy Grace Roman Space Telescope , SPHEREx, ELT, SKA) will soon provide an holistic picture of this defining epoch. To bridge this gap, we publicly release all simulation outputs and post-processing products generated within the thesan simulation project at www.thesan-project.com. This project focuses on the z ≥ 5.5 Universe, combining a radiation-hydrodynamics solver (arepo-rt), a well-tested galaxy formation model (IllustrisTNG) and cosmic dust physics to provide a comprehensive view of the Epoch of Reionization. The thesan suite includes 16 distinct simulations, each varying in volume, resolution, and underlying physical models. This paper outlines the unique features of these new simulations, the production and detailed format of the wide range of derived data products, and the process for data retrieval. Finally, as a case study, we compare our simulation data with a number of recent observations from the James Webb Space Telescope , affirming the accuracy and applicability of thesan. The examples also serve as prototypes for how to utilize the released data set to perform comparisons between predictions and observations. [ABSTRACT FROM AUTHOR]

The first magnetic field in a star other than the Sun was detected in 1947 in the star 78 Vir. Today, we know that about 10% of these intermediate-mass and high-mass stars have strong, large-scale surface magnetic fields whose origin has remained a mystery till today. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields. The massive star τ Sco is a well-known member of the group of magnetic stars and is a blue straggler given its apparently young age compared to that of other members of the Upper Scorpius association. Here, we present 3D magnetohydrodynamic simulations of the coalescence of two massive main-sequence stars and 1D stellar evolution computations of the subsequent evolution of the merger product that can explain τ Sco's magnetic field, apparent youth and other observed characteristics. We argue that field amplification in stellar mergers is a general mechanism to form strongly-magnetised massive stars. Such stars are promising progenitors of magnetars, which may give rise to some of the enigmatic fast radio bursts, and their supernova explosions may be affected by the strong magnetic fields. [ABSTRACT FROM AUTHOR]

Dwarf satellites in galaxy groups are distributed in an anisotropic and asymmetric manner, which is called the 'lopsided satellite distribution'. This lopsided signal has been observed not only in galaxy pairs but also in isolated systems. However, the physical origin of the lopsided signal in isolated systems is still unknown. In this work, we investigate this in the state-of-the-art hydrodynamical simulation of the MillenniumTNG Project by tracing each system back to high redshift. We find that the lopsided signal is dominated by satellites located in the outer regions of the halo and is also dominated by recently accreted satellites. The lopsided signal originates from the anisotropic accretion of galaxies from the surrounding large-scale structure and that, after accretion, the non-linear evolution of satellites inside the dark matter halo weakens the lopsidedness. The signal decreases as cosmic time passes because of a competition between anisotropic accretion and internal evolution within dark matter haloes. Our findings provide a useful perspective for the study of galaxy evolution, especially for the origin of the spatial satellite galaxy distributions. [ABSTRACT FROM AUTHOR]

The properties of galaxies in low-density regions of the universe suggest an interplay between galaxy formation and environment. However, the specific reason why this particular large-scale environment influences the evolution of galaxies remains unclear. This paper examines the properties and evolutionary paths of galaxies within cosmic voids using the Illustris TNG300 simulation. The population of void galaxies at z = 0 has a higher star formation rate, a smaller stellar-to-halo-mass ratio, higher gas metallicity, and lower stellar metallicity in comparison with non-void galaxies at fixed stellar mass. Our analysis shows that these differences are mainly due to the characteristics of galaxies classified as satellites, for which the largest differences between void and non-void samples are found. Although the mean number of mergers is similar between void and non-void samples at a fixed stellar mass, void galaxies tend to experience mergers at later times, resulting in a more recent accumulation of accreted stellar mass. While the mean net accreted mass is comparable for high-mass galaxies, low-mass void galaxies tend to exhibit higher fractions of accreted stars than non-void galaxies. This finding challenges the common notion that void galaxies predominantly experience growth with infrequent mergers or interactions. With this paper, we also publicly release our void catalogue as part of the IllustrisTNG project. [ABSTRACT FROM AUTHOR]

Magnetic fields are ubiquitous in the Universe. Recently, cosmological simulations of galaxies have successfully begun to incorporate magnetic fields and their evolution in galaxies and their haloes. However, so far they have mostly focused on Milky Way-like galaxies. Here, we analyse a sample of high-resolution cosmological zoom simulations of disc galaxies in haloes with mass |${M}_\rm {200c}$| from |$10^{10}$| to |$10^{13}\, \rm {M}_\odot$|⁠ , simulated with the Auriga galaxy formation model. We show that with sufficient numerical resolution the magnetic field amplification and saturation is converged. The magnetic field strength reaches equipartition with turbulent energy density for galaxies in haloes with |${M}_\rm {200c}\gtrsim 10^{11.5}\, \mathrm{M_\odot }$|⁠. For galaxies in less massive haloes, the magnetic field strength saturates at a fraction of equipartition that decreases with decreasing halo mass. For our lowest mass haloes, the magnetic field saturates significantly below 10  per cent of equipartition. We quantify the resolution we need to obtain converged magnetic field strengths and discuss our resolution requirements also in the context of the IllustrisTNG cosmological box simulations. We show that, at z  = 0, rotation-dominated galaxies in our sample exhibit for the most part an ordered large-scale magnetic field, with fewer field reversals in more massive galaxies. Finally, we compare the magnetic fields in our cosmological galaxies at z  = 0 with simulations of isolated galaxies in a collapsing halo set-up. Our results pave the way for detailed studies of cosmic rays and other physical processes in similar cosmological galaxy simulations that crucially depend on the strength and structure of magnetic fields. [ABSTRACT FROM AUTHOR]

We investigate the growth of stellar discs in Milky Way-mass galaxies using the magnetohydrodynamical simulations of the Auriga Project in a full cosmological context. We focus on the gas accretion process along the discs, calculating the net, infall and outflow rates as a function of galactocentric distance, and investigate the relation between them and the star formation activity. The stellar distributions of around 70 per cent of the simulated galaxies exhibit an 'inside–out' pattern, with older (younger) stellar populations preferentially located in the inner (outer) disc regions. In all cases, we find a very tight correlation between the infall, outflow, and net accretion rates, as well as between these three quantities and the star formation rate. This is because the amount of gas which is ultimately available for star formation in each radial ring depends not only on the infall rates, but also on the amount of gas leaving the disc in outflows, which directly relates to the local star formation level. Therefore, any of these rates can be used to identify galaxies with inside–out growth. For these galaxies, the correlation between the dominant times of accretion/star formation and disc radius is well fitted by a linear function. We also find that, when averaged over galaxies with formation histories similar to the Milky Way, the simulated accretion rates show a similar evolution (both temporally and radially integrated) to the usual accretion prescriptions used in chemical evolution models, although some major differences arise at early times and in the inner disc regions. [ABSTRACT FROM AUTHOR]

We implement general relativistic hydrodynamics in the moving-mesh code arepo. We also couple a solver for the Einstein field equations employing the conformal flatness approximation. The implementation is validated by evolving isolated static neutron stars using a fixed metric or a dynamical space–time. In both tests, the frequencies of the radial oscillation mode match those of independent calculations. We run the first moving-mesh simulation of a neutron star merger. The simulation includes a scheme to adaptively refine or derefine cells and thereby adjusting the local resolution dynamically. The general dynamics are in agreement with independent smoothed particle hydrodynamics and static-mesh simulations of neutron star mergers. Coarsely comparing, we find that dynamical features like the post-merger double-core structure or the quasi-radial oscillation mode persist on longer time scales, possibly reflecting a low numerical diffusivity of our method. Similarly, the post-merger gravitational wave emission shows the same features as observed in simulations with other codes. In particular, the main frequency of the post-merger phase is found to be in good agreement with independent results for the same binary system, while, in comparison, the amplitude of the post-merger gravitational wave signal falls off slower, i.e. the post-merger oscillations are less damped. The successful implementation of general relativistic hydrodynamics in the moving-mesh arepo code, including a dynamical space–time evolution, provides a fundamentally new tool to simulate general relativistic problems in astrophysics. [ABSTRACT FROM AUTHOR]

We take advantage of the unprecedented dynamical range provided by the 'Cosmic-Zoom' project to study the mass accretion history (MAH) of present-day dark matter haloes over the entire mass range present in the Lambda cold dark matter paradigm when the dark matter is made of weakly interacting massive particles of mass 100 GeV. In particular, we complement previous studies by exploring the MAHs of haloes with mass from |$10^8\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$| down to Earth mass, |$10^{-6}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$|⁠. The formation redshift of low-mass haloes anticorrelates weakly with mass, peaking at z = 3 for haloes of mass |$10^{-4}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$|⁠. Even lower masses are affected by the free-streaming cut-off in the primordial spectrum of density fluctuations and form at lower redshift. We compare MAHs in our simulations with predictions from two analytical models based on the extended Press–Schechter theory (EPS), and three empirical models derived by fitting and extrapolating either results from cosmological N -body simulations or Monte Carlo realizations of halo growth. All models fit our simulations reasonably well over the mass range for which they were calibrated. While the empirical models match better for more massive haloes, |$M\gt 10^{10}\ h^{-1}\,\mathrm{{\rm M}_{\odot }}$|⁠ , the analytical models do better when extrapolated down to Earth mass. At the higher masses, we explore the correlation between local environment density and MAH, finding that biases are relatively weak, with typical MAHs for haloes in extremely low-density and in typical regions differing by less than 20 per cent at high redshift. If this result can be extrapolated to lower halo masses, we conclude that EPS theory predicts the hierarchical build up of dark matter haloes quite well over the entire halo mass range. [ABSTRACT FROM AUTHOR]

We analyse the internal structure and dynamics of cosmic-web filaments connecting massive high- z haloes. Our analysis is based on a high-resolution arepo cosmological simulation zooming-in on three Mpc-scale filaments feeding three massive haloes of |$\sim 10^{12}\, \text{M}_\odot$| at z ∼ 4, embedded in a large-scale sheet. Each filament is surrounded by a cylindrical accretion shock of radius |$r_{\rm shock} \sim 50 \, {\rm kpc}$|⁠. The post-shock gas is in virial equilibrium within the potential well set by an isothermal dark-matter filament. The filament line-mass is |$\sim 9\times 10^8\, \text{M}_\odot \, {\rm kpc}^{-1}$|⁠ , the gas fraction within r shock is the universal baryon fraction, and the virial temperature is ∼7 × 105 K. These all match expectations from analytical models for filament properties as a function of halo mass and redshift. The filament cross-section has three radial zones. In the outer 'thermal' (T) zone, |$r \ge 0.65 \, r_{\rm shock}$|⁠ , inward gravity, and ram-pressure forces are overbalanced by outward thermal pressure forces, decelerating the inflowing gas and expanding the shock outwards. In the intermediate 'vortex' (V) zone, 0.25 ≤ r / r shock ≤ 0.65, the velocity field is dominated by a quadrupolar vortex structure due to offset inflow along the sheet through the post-shock gas. The outward force is dominated by centrifugal forces associated with these vortices, with additional contributions from global rotation and thermal pressure. Shear and turbulent forces associated with the vortices act inwards. The inner 'stream' (S) zone, |$r \lt 0.25 \, r_{\rm shock}$|⁠ , is a dense isothermal core, |$T\sim 3 \times 10^4 \, {\rm K}$| and |$n_{\rm H}\sim 0.01 \, {\rm cm^{-3}}$|⁠ , defining the cold streams that feed galaxies. The core is formed by an isobaric cooling flow and is associated with a decrease in outward forces, though exhibiting both inflows and outflows. [ABSTRACT FROM AUTHOR]

Cosmic rays (CRs) are dynamically important in the evolution of galaxies by regulating star formation and powering galactic outflows. However, to what extent CRs regulate galaxy formation depends on the coupling strength of CRs with the ambient plasma and the effective CR transport speed. Moreover, both properties sensitively depend on the CR momentum, which is largely unexplored in three-dimensional hydrodynamical simulations. We perform magnetohydrodynamical simulations of entire galaxies with masses ranging from 1010 to |$10^{12}\, \mathrm{M}_\odot$| and compare dynamically coupled CRs in the grey approximation with a spectrally resolved model that includes CR momenta from |$0.1\, \mathrm{GeV} c^{-1}$| to |$100\, \mathrm{TeV} c^{-1}$|⁠. We find that hadronic cooling of CRs dominates over Alfvén cooling, with the latter emulating CR losses as a result of streaming of CRs down their pressure gradient. While star formation rates and galaxy morphologies are only mildly affected by the spectral CR modelling, mass loading factors of galactic outflows can differ by up to a factor of 4 in dwarf galaxies. All simulated low-mass haloes (M  = 1010, 1011, and |$3\times 10^{11}\, \mathrm{M}_\odot$|⁠) drive strong outflows, where CR transport is temporally dominated by advection. In contrast, the Milky Way-mass galaxy with |$M=10^{12}\, \mathrm{M}_\odot$| does not drive sustained outflows, so that CR transport is entirely dominated by diffusion. The effective energy weighted diffusion coefficients vary by two orders of magnitude from the canonical energy-weighted values of |$\langle {D}\rangle _{e_\mathrm{cr}}\sim 10^{28}\, \mathrm{cm^2\, s^{-1}}$| in the disc up to |$3\times 10^{29}\, \mathrm{cm^2\, s^{-1}}$| in the circumgalactic medium, where we observe substantial temperature and CR pressure differences between our grey and spectral CR models. [ABSTRACT FROM AUTHOR]

In this work, we extend the non-ideal magnetohydrodynamics (MHD) solver in the moving-mesh code arepo to include the Hall effect. The core of our algorithm is based on an estimation of the magnetic field gradients by a least-squares reconstruction on the unstructured mesh, which we also used in the companion paper for Ohmic and ambipolar diffusion. In an extensive study of simulations of a magnetic shock, we show that without additional magnetic diffusion our algorithm for the Hall effect becomes unstable at high resolution. We can however stabilize it by artificially increasing the Ohmic resistivity, ηOR, so that it satisfies the condition ηOR ≥ ηH/5, where ηH is the Hall diffusion coefficient. Adopting this solution, we find second-order convergence for the C-shock and are also able to accurately reproduce the dispersion relation of the whistler waves. As a first application of the new scheme, we simulate the collapse of a magnetized cloud with constant Hall parameter ηH and show that, depending on the sign of ηH, the magnetic braking can either be weakened or strengthened by the Hall effect. The quasi-Lagrangian nature of the moving-mesh method used here automatically increases the resolution in the forming core, making it well suited for more realistic studies with non-constant magnetic diffusivities in the future. [ABSTRACT FROM AUTHOR]

Multibody dynamical interactions of binaries with other objects are one of the main driving mechanisms for the evolution of star clusters. It is thus important to bring our understanding of three-body interactions beyond the commonly employed point-particle approximation. To this end, we here investigate the hydrodynamics of three-body encounters between star–black hole (BH) binaries and single stars, focusing on the identification of final outcomes and their long-term evolution and observational properties, using the moving-mesh hydrodynamics code AREPO. This type of encounter produces five types of outcomes: stellar disruption, stellar collision, weak perturbation of the original binary, binary member exchange, and triple formation. The two decisive parameters are the binary phase angle, which determines which two objects meet at the first closest approach, and the impact parameter, which sets the boundary between violent and non-violent interactions. When the impact parameter is smaller than the semimajor axis of the binary, tidal disruptions and star-BH collisions frequently occur when the BH and the incoming star first meet, while the two stars mostly merge when the two stars meet first instead. In both cases, the BHs accrete from an accretion disc at super-Eddington rates, possibly generating flares luminous enough to be observed. The stellar collision products either form a binary with the BH or remain unbound to the BH. Upon collision, the merged stars are hotter and larger than the main sequence stars of the same mass at similar age. Even after recovering their thermal equilibrium state, stellar collision products, if isolated, would remain hotter and brighter than main sequence stars until becoming giants. [ABSTRACT FROM AUTHOR]

Especially in cold and high-density regions, the assumptions of ideal magnetohydrodynamics (MHD) can break down, making first order non-ideal terms such as ohmic and ambipolar diffusion as well as the Hall effect important. In this study, we present a new numerical scheme for the first two resistive terms, which we implement in the moving-mesh code arepo using the single-fluid approximation combined with a new gradient estimation technique based on a least-squares fit per interface. Through various test calculations including the diffusion of a magnetic peak, the structure of a magnetic C-shock, and the damping of an Alfvén wave, we show that we can achieve an accuracy comparable to the state-of-the-art code athena++  . We apply the scheme to the linear growth of the magnetorotational instability and find good agreement with the analytical growth rates. By simulating the collapse of a magnetized cloud with constant magnetic diffusion, we show that the new scheme is stable even for large contrasts in the spatial resolution. Thanks to the Lagrangian nature of the moving mesh method the new scheme is thus well suited for intended future applications where a high resolution in the dense cores of collapsing protostellar clouds needs to be achieved. In a forthcoming work, we will extend the scheme to include the Hall effect. [ABSTRACT FROM AUTHOR]