arXiv subjects

Christoph Federrath

Publications and source records attributed to Christoph Federrath.

At least 55 records · Page 3Linked to original sources

arXiv2025

A comparison of the turbulent dynamo in weakly-collisional and collisional plasmas: from subsonic to supersonic turbulence

Weakly-collisional plasmas, such as the solar wind or the intra-cluster medium (ICM) of galaxy clusters, evolve in the presence of dynamically strong magnetic fields. The turbulent dynamo can amplify magnetic fields to such levels by converting turbulent kinetic energy into magnetic energy. While extensively studied in collisional magnetohydrodynamic (MHD) simulations, the weakly-collisional regime has only been explored recently. Here, we determine the properties of the weakly-collisional turbulent dynamo in the exponential ``kinematic" growth phase in both the subsonic and the previously unexplored supersonic regime of turbulence, using hybrid particle-in-cell (HPIC) and MHD simulations. We conduct a large parameter study, fixing the magnetic Reynolds number, Rm = 500, and the initial ratio of the magnetic to kinetic energy, $(E_{\rm{mag}}/E_{\rm{kin}})_{0} = 10^{-10}$, and then vary the kinetic Reynolds number, Re = 500, 50, and 5, for the MHD simulations. In the HPIC runs, only Rm = 500 is controlled, while Re emerges self-consistently from wave-particle interactions. We find that the velocity and magnetic field structures, probability distribution functions, and power spectra of the HPIC runs are similar to that of the MHD dynamo with Re ~ 50-500 and Re ~ 500 in the subsonic and supersonic regimes, respectively. Using MHD scaling relations, we infer $\text{Re}_{\rm inferred}=480^{+170}_{-250}$ and $690^{+360}_{-360}$ in the subsonic and supersonic weakly-collisional plasma, respectively. Overall, we find that the turbulent dynamo shares similar physical properties in both weakly-collisional and collisional plasmas. Our results of the weakly-collisional turbulent dynamo may have relevant applications to the solar wind, weakly-collisional shocks, and the hot ICM.

astro-ph.GA↗

arXiv2025

Numerical viscosity and resistivity in MHD turbulence simulations

Accurate magnetohydrodynamical (MHD) turbulence simulations require understanding numerical dissipation. We quantify numerical viscosity and resistivity in subsonic (M=0.1) and supersonic (M=10) turbulence regimes. The hydrodynamic (Re) and magnetic Reynolds numbers (Rm) on the turbulence driving scale l_turb in a cubic domain of side length L with N^3 resolution elements are well-described by Re=[2(N/N_Re)(l_turb/L)]^p_Re and Rm=[2(N/N_Rm)(l_turb/L)]^p_Rm. We provide two sets of fit values of (N_Re,p_Re,N_Rm,p_Rm): one with p_Re & p_Rm fixed at their theoretical values, and the other one allowing all 4 parameters to vary. The sets for M=0.1 are (1.57_{-0.12}^{+0.10},4/3,1.55_{-0.14}^{+0.45},4/3) and (0.83_{-0.08}^{+0.09},1.20_{-0.02}^{+0.02},4.19_{-4.05}^{+2.95},1.60_{-0.33}^{+0.18}), respectively. For M=10, they are (3.55_{-0.56}^{+0.78},3/2,1.03_{-0.11}^{+0.12},3/2) and (10.46_{-0.85}^{+0.96},1.90_{-0.04}^{+0.04},0.44_{-0.23}^{+0.61},1.32_{-0.09}^{+0.17}). The resulting magnetic Prandtl numbers (Pm=Rm/Re) are consistent with constant values of 1.0_{-0.2}^{+0.3} for M=0.1, and 6.2_{-4.8}^{+5.6} for M=10. These apply when the magnetic energy (E_mag) is <10% of the kinetic energy (E_kin). When E_mag/E_kin~0.1-1, Rm is reduced by a factor~3 (increase in N_Rm by a factor~2) for M=0.1, while Rm for M=10 and Re (for any M) remain largely unaffected. We compare our Re-N relation with 14 other simulations from the literature, employing various numerical methods (with & without Riemann solvers, different reconstruction schemes & orders, and smoothed particle hydrodynamics), and find agreement within a factor of 3. Additionally, we compare these results to target Re and Rm values from simulations with explicit dissipation. These comparisons and our relations help users determine the Re and Rm achievable at a given N, ensuring physical dissipation dominates over numerical dissipation.

astro-ph.SR↗

arXiv2025

The influence of the cloud virial parameter on the initial mass function

Crucial for star formation is the interplay between gravity and turbulence. The observed cloud virial parameter, $α_{\mathrm{vir}}$, which is the ratio of twice the turbulent kinetic energy to the gravitational energy, is found to vary significantly in different environments, where the scatter among individual star-forming clouds can exceed an order of magnitude. Therefore, a strong dependence of the initial mass function (IMF) on $α_{\mathrm{vir}}$ may challenge the notion of a universal IMF. To determine the role of $α_{\mathrm{vir}}$ on the IMF, we compare the star-particle mass functions obtained in high-resolution magnetohydrodynamical simulations including jet and heating feedback, with $α_{\mathrm{vir}}=0.0625$, $0.125$, and $0.5$. We find that varying $α_{\mathrm{vir}}$ from $α_{\mathrm{vir}}\sim0.5$ to $α_{\mathrm{vir}}<0.1$ shifts the peak of the IMF to lower masses by a factor of $\sim2$ and increases the star formation rate by a similar factor. The dependence of the IMF and star formation rate on $α_{\mathrm{vir}}$ is non-linear, with the dependence subsiding at $α_{\mathrm{vir}}<0.1$. Our study shows a systematic dependence of the IMF on $α_{\mathrm{vir}}$. Yet, it may not be measurable easily in observations, considering the uncertainties, and the relatively weak dependence found in this study.

astro-ph.GA↗

arXiv2024

Nexus: A framework for conrolled simulations of idealised galaxies

Motivated by the need for realistic, dynamically self-consistent, evolving galaxy models that avoid the complexity of full, and zoom-in, cosmological simulations, we have developed NEXUS, an integral framework to create and evolve synthetic galaxies made of collisionless and gaseous components. NEXUS leverages the power of publicly available, tried-and-tested packages: the stellar-dynamics, action-based library AGAMA; and the adaptive mesh refinement, N-body/hydrodynamical code RAMSES, modified to meet our needs. In addition, we make use of a proprietary module to account for galaxy formation physics, including gas cooling and heating, star formation, stellar feedback, and chemical enrichment. NEXUS' basic functionality consists in the generation of bespoke initial conditions (ICs) for a diversity of galaxy models, which are advanced in time to simulate the galaxy's evolution. The fully self-consistent ICs are generated with a distribution-function based approach, as implemented in the galaxy modelling module of AGAMA -- up to now restricted to collisionless components, extended in this work to treat two types of gaseous configurations: hot halos and gas discs. NEXUS allows constructing equilibrium models with disc gas fractions $0~\leq~f_{\rm gas}~\leq~1$, appropriate to model both low- and high-redshift galaxies. Similarly, the framework is ideally suited to the study of galactic ecology, i.e. the dynamical interplay between stars and gas over billions of years. As a validation and illustration of our framework, we reproduce several isolated galaxy model setups reported in earlier studies, and present a new, `nested bar' galaxy simulation. Future upgrades of NEXUS will include magneto-hydrodynamics and highly energetic particle (`cosmic ray') heating.

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arXiv2024

On the limitations of H alpha luminosity as a star formation tracer in spatially resolved observations

This study examines the limitations of H$α$ luminosity as a tracer of star formation rates (SFR) in spatially resolved observations. We carry out high-resolution simulations of a Milky Way-like galaxy including both supernova and photoionization feedback, and from these we generate synthetic H$α$ emission maps that we compare to maps of the true distribution of young stellar objects (YSOs) on scales from whole-galaxy to individual molecular clouds ($\lesssim 100$ pc). Our results reveal significant spatial mismatches between H$α$ and true YSO maps on sub-100 pc scales, primarily due to ionizing photon leakage, with a secondary contribution from young stars drifting away from their parent molecular clouds. On small scales these effect contribute significantly to the observed anti-correlation between gas and star formation, such that there is noticeably less anti-correlation if we replace an H$α$-based star formation map with a YSO-based one; this in turn implies that previous studies have underestimated the time it takes for young stars to disperse their parent molecular clouds. However, these effects are limited in dense regions with hydrogen columns $N_\mathrm{H} > 3 \times 10^{21}$ cm$^{-2}$, where the H$α$- and YSO-based SFR maps show better agreement. Based on this finding we propose a calibration model that can precisely measure the SFR of large molecular clouds (mean radius > 100 pc) with a combination of H$α$ and CO observations, which provides a foundation for future study of star formation processes in extragalactic molecular clouds.

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arXiv2024

Cosmic evolution of the Faraday rotation measure in the intracluster medium of galaxy clusters

Radio observations have revealed magnetic fields in the intracluster medium (ICM) of galaxy clusters nearly in equipartition with turbulence. This suggests magnetic field amplification by dynamo processes. However, observations are limited to redshifts z <~ 0.7, and the weakly collisional nature of the ICM complicates studying magnetic field evolution at higher redshifts through theoretical models and simulations. Using a model of the weakly collisional dynamo, we modelled the evolution of the Faraday rotation measure (RM) in clusters of different masses, up to z <~ 1.5, and investigated its properties. We compared our results with radio observations of various galaxy clusters. We used merger trees generated by the modified GALFORM algorithm to track the evolution of plasma quantities during galaxy cluster formation. Assuming the magnetic field remains in equipartition with the turbulent velocity field, we generated RM maps to study their properties. We find that both the standard deviation of RM, sigma_RM, and the absolute average |mu_RM| increase with cluster mass. Due to redshift dilution, RM values for a fixed cluster mass remain nearly constant between z=0 and z=1.5. For r/r200 >~ 0.4, sigma_RM does not vary significantly with L/r200, with L being the size of the observed RM patch. Below this limit, sigma_RM increases as L decreases. We find that radial RM profiles have a consistent shape, proportional to 10^{-1.2(r/r200)}, and are nearly independent of redshift. Our z~0 profiles for M_clust = 10^15 Msol match RM observations in the Coma cluster but show discrepancies with Perseus, possibly due to high gas mixing. Models for clusters with M_clust = 10^13 and M_clust = 10^15 Msol at z=0 and z = 0.174 align well with Fornax and A2345 data for r/r200 <~ 0.4. Our model can be useful for generating mock polarization observations for radio telescopes.

astro-ph.CO↗

arXiv2024

Pulling back the curtain on shocks and star-formation in NGC 1266 with Gemini-NIFS

We present Gemini near-infrared integral field spectrograph (NIFS) K-band observations of the central 400 pc of NGC 1266, a nearby (D$\approx$30 Mpc) post-starburst galaxy with a powerful multi-phase outflow and a shocked ISM. We detect 7 H$_2$ ro-vibrational emission lines excited thermally to $T$$\sim$2000 K, and weak Br$γ$ emission, consistent with a fast C-shock. With these bright H$_2$ lines, we observe the spatial structure of the shock with an unambiguous tracer for the first time. The Br$γ$ emission is concentrated in the central $\lesssim$100 pc, indicating that any remaining star-formation in NGC 1266 is in the nucleus while the surrounding cold molecular gas has little on-going star-formation. Though it is unclear what fraction of this Br$γ$ emission is from star-formation or the AGN, assuming it is entirely due to star-formation we measure an instantaneous star-formation rate of 0.7 M$_\odot$ yr$^{-1}$, though the star-formation rate may be significantly higher in the presence of additional extinction. NGC 1266 provides a unique laboratory to study the complex interactions between AGN, outflows, shocks, and star-formation, all of which are necessary to unravel the evolution of the post-starburst phase.

astro-ph.GA↗

arXiv2024

AHKASH: a new Hybrid particle-in-cell code for simulations of astrophysical collisionless plasma

We introduce $\texttt{A}$strophysical $\texttt{H}$ybrid-$\texttt{K}$inetic simulations with the $\texttt{flASH}$ code ($\texttt{AHKASH}$) -- a new Hybrid particle-in-cell (PIC) code developed within the framework of the multi-physics code $\texttt{FLASH}$. The new code uses a second-order accurate Boris integrator and a predictor-predictor-corrector algorithm for advancing the Hybrid-kinetic equations, using the constraint transport method to ensure that magnetic fields are divergence-free. The code supports various interpolation schemes between the particles and grid cells, with post-interpolation smoothing to reduce finite particle noise. We further implement a $δf$ method to study instabilities in weakly collisional plasmas. The new code is tested on standard physical problems such as the motion of charged particles in uniform and spatially varying magnetic fields, the propagation of Alfvén and whistler waves, and Landau damping of ion acoustic waves. We test different interpolation kernels and demonstrate the necessity of performing post-interpolation smoothing. We couple the $\texttt{TurbGen}$ turbulence driving module to the new Hybrid PIC code, allowing us to test the code on the highly complex physical problem of the turbulent dynamo. To investigate steady-state turbulence with a fixed sonic Mach number, it is important to maintain isothermal plasma conditions. Therefore, we introduce a novel cooling method for Hybrid PIC codes and provide tests and calibrations of this method to keep the plasma isothermal. We describe and test the `hybrid precision' method, which significantly reduces (by a factor $\sim1.5$) the computational cost, without compromising the accuracy of the numerical solutions. Finally, we test the parallel scalability of the new code, showing excellent scaling up to 10,000~cores.

astro-ph.IM↗

arXiv2024

Star Formation

In this chapter, we will cover how stars form from the stellar nurseries that are giant molecular clouds. We will first review the physical processes that compete to regulate star formation. We then review star formation in turbulent, magnetized molecular clouds and the associated statistics giving rise to the star formation rate and the initial mass function of stars. We then present the protostellar stages in detail from an observational perspective. We will primarily discuss low-mass ($<1.5\msun$) stars. Finally, we examine how multiplicity complicates the single-star formation picture. This chapter will focus on star formation at redshift~0

astro-ph.SR↗

arXiv2024

Protostellar spin-up and fast rotator formation through binary star formation

(Edited) Many fast rotator stars (rotation periods of < 2 days) are found in unresolved binaries with separations of tens of au. This correlation leads to the question of whether the formation of binary stars inherently produces fast rotators. We aim to understand whether the formation of companions plays a role in spinning up stars. We use magneto-hydrodynamical simulations to study the formation of multiple star systems from turbulent and non-turbulent protostellar cores. We track the angular momentum accreted by individual star and inner disc systems by using a sink particle technique. We run a resolution study to extrapolate protostellar properties. We find in all simulations that the primary star can experience spin-up events that are correlated with the formation of companions. The primary star can spin up by up to 84% of its pre-fragmentation angular momentum and by up to 18% of its pre-fragmentation mass-specific angular momentum. The mechanism for the spin-up is gravitational disc instabilities in the circumstellar disc around the primary star, leading to the accretion of material with high specific angular momentum. The simulations that experience the strongest disc instabilities fragment to form companions. Simulations with weaker spin-up events experience disc instabilities triggered by a companion flyby, and the disc instability in these cases does not produce further fragments. We conclude that the primary star in multiple star systems may end up with a higher spin than single stars. This is because gravitational instabilities in the circumstellar disc around the primary star can trigger a spin-up event. In the strongest spin-up events, the instability is likely to cause disc fragmentation and the formation of companions. This companion formation coupled with shorter disc lifetimes, because the companion truncates the circumstellar disc, can help produce fast rotators.

astro-ph.SR↗

arXiv2024

Structure functions with higher-order stencils as a probe to separate small- and large-scale magnetic fields

Magnetic fields are an energetically important component of star-formation galaxies, but it is often difficult to measure their properties from observations. One of the complexities stems from the fact that the magnetic fields, especially in spiral galaxies, have a two-scale nature: a large-scale field, coherent over ${\rm kpc}$ scales and a small-scale, random field with a scale of $\lesssim$ $100~{\rm pc}$. Moreover, it is known that the strength of small- and large-scale fields are comparable and this makes it even harder to find their imprints in radio polarisation observations such as the Faraday rotation measure, ${\rm RM}$, which is the integral over the path length of the product of the thermal electron density and the parallel component of the magnetic field to the line of sight. Here, we propose and demonstrate the use of second-order structure functions of ${\rm RM}$ computed with multiple higher-order stencils as a powerful analysis to separate the small- and large-scale magnetic field components. In particular, we provide new methods and calibrations to compute the scale and the strength of the large-scale magnetic field in the presence of small-scale magnetic fluctuations. We then apply the method to find the scale of large-scale magnetic fields in the nearby galaxies M51 and NGC 6946, using archival data and further discuss the need for computing the ${\rm RM}$ structure functions with higher-order stencils. With multiple modern radio polarisation observatories and eventually the Square Kilometre Array, ${\rm RM}$ observations will significantly improve in quantity and quality, and the higher-order stencil structure function techniques developed here can be used to extract information about multiscale magnetic fields in galaxies.

astro-ph.GA↗

arXiv2024

Wide-binary eccentricity distribution in young star clusters: dependence on the binary separation and mass

We study the wide-binary eccentricity ($e$) distribution in young star clusters and the role of turbulence in setting the form of the $e$ distribution using magnetohydrodynamical (MHD) simulations of star cluster formation. The simulations incorporate gravity, turbulence, magnetic fields, protostellar heating, and jets/outflows. We find that (1) simulations that employ purely compressive turbulence driving produce binaries with a superthermal $e$ distribution ($α>1$ in $p(e) \propto e^α$), while simulations with purely solenoidal driving or natural mixture of driving modes produce subthermal/thermal distributions ($α\leq$ 1), (2) the $e$ distribution over the full range of binary separations in our simulations is set at the early stages of the star cluster formation process, (3) while binaries (separation of $r_{\mathrm{pair}} \leq 1000\, \mathrm{AU}$) have subthermal to thermal $e$ distributions ($α\sim 0.8$), wide binaries ($r_{\mathrm{pair}} > 1000\, \mathrm{AU}$) have a superthermal distribution ($α\sim 1.8$), and (4) low-mass binary systems (system masses of $M_{\mathrm{sys}} \leq 0.8\, \mathrm{M_\odot}$) have a highly superthermal distribution ($α\sim 2.4$), whereas high-mass systems ($M_{\mathrm{sys}} > 0.8\, \mathrm{M_\odot}$) exhibit a subthermal/thermal distribution ($α\sim 0.8$). The binary eccentricity distribution is often modelled as a thermal distribution. However, our results suggest that the $e$ distribution depends on the range of separation of the sampled binaries, which agrees with the findings from recent Gaia observations. We conclude that the dependence of the $e$ distribution on the binary separation and mass is linked to the binary formation mechanism governed by the turbulent properties of the parent cloud.

astro-ph.GA↗

arXiv2024

Magnetized compressible turbulence with a fluctuation dynamo and Reynolds numbers over a million

Supersonic magnetohydrodynamic (MHD) turbulence is a ubiquitous state for many astrophysical plasmas. However, even the basic statistics for this type of turbulence remains uncertain. We present results from supersonic MHD turbulence simulations at unparalleled resolutions, with plasma Reynolds numbers of over a million. In the kinetic energy spectrum we find a break between the scales that are dominated by kinetic energy, with spectral index $-2$, and those that become strongly magnetized, with spectral index $-3/2$. By analyzing the Helmholtz decomposed kinetic energy spectrum, we find that the compressible modes are not passively mixed through the cascade of the incompressible modes. At high magnetic Reynolds number, above $10^5$, we find a power law in the magnetic energy spectrum with spectral index $-9/5$. On the strongly magnetized, subsonic scales the plasma tends to self-organize into locally relaxed regions, where there is strong alignment between the current density, magnetic field, velocity field and vorticity field, depleting both the nonlinearities and magnetic terms in the MHD equations, which we attribute to plasma relaxation on scales where the magnetic fluctuations evolve on shorter timescales than the velocity fluctuations. This process constrains the cascade to inhomogenous, volume-poor, fractal surfaces between relaxed regions, which has significant repercussions for understanding the nature of magnetized turbulence in astrophysical plasmas and the saturation of the fluctuation dynamo.

astro-ph.GA↗

arXiv2024

Probing the role of self-gravity in clouds impacted by AGN-driven winds

The impact of winds and jet-inflated bubbles driven by active galactic nuclei (AGN) are believed to significantly affect the host galaxy's interstellar medium (ISM) and regulate star formation. To explore this scenario, we perform a suite of hydrodynamic simulations to model the interaction between turbulent star-forming clouds and highly pressurised AGN-driven outflows, focusing on the effects of self-gravity. Our results demonstrate that the cloudlets fragmented by the wind can become gravitationally bound, significantly increasing their survival time. While external pressurisation leads to a global collapse of the clouds in cases of weaker winds ($10^{42}-10^{43}~{\rm erg~s^{-1}}$), higher-power winds ($10^{44}-10^{45}~{\rm erg~s^{-1}}$) disperse the gas and cause localised collapse of the cloudlets. We also demonstrate that a kinetic energy-dominated wind is more efficient in accelerating and dispersing the gas than a thermal wind with the same power. The interaction can give rise to multi-phase outflows with velocities ranging from a few 100 to several 1000~${\rm km\,s^{-1}}$. The mass outflow rates are tightly correlated with the wind power, which we explain by an ablation-based mass-loss model. Moreover, the velocity dispersion and the virial parameter of the cloud material can increase by up to one order of magnitude through the effect of the wind. Even though the wind can suppress or quench star formation for about 1 Myr during the initial interaction, a substantial number of gravitationally bound dense cloudlets manage to shield themselves from the wind's influence and subsequently undergo rapid gravitational collapse, leading to an enhanced star formation rate (SFR).

astro-ph.GA↗

arXiv2024

Turbulent gas-rich disks at high redshift: bars & bulges in a radial shear flow

Recent observations of high-redshift galaxies ($z \lesssim 7$) reveal that a substantial fraction have turbulent, gas-rich disks with well-ordered rotation and elevated levels of star formation. In some instances, disks show evidence of spiral arms, with bar-like structures. These remarkable observations have encouraged us to explore a new class of dynamically self-consistent models using our hydrodynamic N-body simulation framework that mimic a plausible progenitor of the Milky Way at high redshift. We explore disk gas fractions of $f_{\rm gas} = 0, 20, 40, 60, 80, 100\%$ and track the creation of stars and metals. The high gas surface densities encourage vigorous star formation, which in turn couples with the gas to drive turbulence. We explore three distinct histories: (i) there is no ongoing accretion and the gas is used up by the star formation; (ii) the star-forming gas is replenished by cooling in the hot halo gas; (iii) in a companion paper, we revisit the models in the presence of a strong perturbing force. At low $f_{\rm disk}$ ($<0.3$), where $f_{\rm disk}$ is the mass fraction of stars relative to dark matter within 2.2 $R_{\rm disk}$, a bar does not form in a stellar disk; this remains true even when gas dominates the inner disk potential. For a dominant baryon disk ($f_{\rm disk} \gtrsim 0.5$) at all gas fractions, the turbulent gas forms a strong "radial shear flow" that leads to an intermittent star-forming bar within about 500 Myr; turbulent gas speeds up the formation of bars compared to gas-free models. For $f_{\rm gas} \lesssim 60\%$, all bars survive, but for higher gas fractions, the bar devolves into a central bulge after 1 Gyr. The star-forming bars are reminiscent of recent discoveries in high-redshift ALMA observations of gaseous disks.

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arXiv2024

Turbulence statistics of HI clouds entrained in the Milky Way's nuclear wind

The interstellar medium (ISM) is ubiquitously turbulent across many physically distinct environments within the Galaxy. Turbulence is key in controlling the structure and dynamics of the ISM, regulating star formation, and transporting metals within the Galaxy. We present the first observational measurements of turbulence in neutral hydrogen entrained in the hot nuclear wind of the Milky Way. Using recent MeerKAT observations of two extra-planar HI clouds above (gal. lat.$\,\sim7.0^{\circ}$) and below (gal. lat.$\,\sim-3.9^{\circ}$) the Galactic disc, we analyse centroid velocity and column density maps to estimate the velocity dispersion ($σ_{v,\mathrm{3D}}$), the turbulent sonic Mach number ($\mathcal{M}$), the volume density dispersion ($σ_{ρ/ρ_0}$), and the turbulence driving parameter ($b$). We also present a new prescription for estimating the spatial temperature variations of HI in the presence of related molecular gas. We measure these turbulence quantities on the global scale of each cloud, but also spatially map their variation across the plane-of-sky extent of each cloud by using a roving kernel method. We find that the two clouds share very similar characteristics of their internal turbulence, despite their varying latitudes. Both clouds are in the sub-to-trans-sonic Mach regime, and have primarily compressively-driven ($b\sim1$) turbulence. Given that there is no known active star-formation present in these clouds, this may be indicative of the way the cloud-wind interaction injects energy into the entrained atomic material on parsec scales.

astro-ph.GA↗

arXiv2024

Amplification and saturation of turbulent magnetic field in collapsing primordial gas clouds

Recent numerical studies suggest that magnetic fields play an important role in primordial star formation in the early universe. However, the detailed evolution of the magnetic field in the collapse phase still has uncertainties because of the complicated physics associated with turbulence in a collapsing magnetized system. Here, we perform a suite of numerical MHD simulations that follow the collapse of magnetized, turbulent primordial gas clouds to investigate the evolution of the magnetic field associated with the turbulence, assuming a polytropic equation of state with exponent $γ_{\rm eff}$ and with various numerical resolutions. In addition, we generalize the analytic theory of magnetic field growth/saturation so that it can deal with various exponents $γ_{\rm eff}$ and turbulence energy spectra. We find that the numerical results are well reproduced by the theory for various $γ_{\rm eff}$ through the collapse phase during the formation of the first stars. The magnetic field is eventually amplified by a factor of $10^{12}$ -- $10^{15}$ due to kinematic and non-linear turbulent dynamo effects and reaches 3% -- 100% of the equipartition level, depending on $γ_{\rm eff}$. We also find that the transition between the kinematic and non-linear stages can be analytically estimated. These results indicate that the strong magnetic field accompanied by supersonic turbulence is a general property and suggest that it can play a crucial role in the formation of the first stars.

astro-ph.GA↗

arXiv2024

Critical magnetic Reynolds number of the turbulent dynamo in collisionless plasmas

The intracluster medium of galaxy clusters is an extremely hot and diffuse, nearly collisionless plasma, which hosts dynamically important magnetic fields of $\sim μ{\rm G}$ strength. Seed magnetic fields of much weaker strength of astrophysical or primordial origin can be present in the intracluster medium. In collisional plasmas, which can be approximated in the magneto-hydrodynamical (MHD) limit, the turbulent dynamo mechanism can amplify weak seed fields to strong dynamical levels efficiently by converting turbulent kinetic energy into magnetic energy. However, the viability of this mechanism in weakly collisional or completely collisionless plasma is much less understood. In this study, we explore the properties of the collisionless turbulent dynamo by using three-dimensional hybrid-kinetic particle-in-cell simulations. We explore the properties of the collisionless turbulent dynamo in the kinematic regime for different values of the magnetic Reynolds number, ${\rm Rm}$, initial magnetic-to-kinetic energy ratio, $(E_{\rm{mag}}/E_{\rm{kin}})_{\rm{i}}$, and initial Larmor ratio, $(r_{\rm Larmor}/L_{\rm box})_{\rm i}$, i.e., the ratio of the Larmor radius to the size of the turbulent system. We find that in the `un-magnetised' regime, $(r_{\rm Larmor}/L_{\rm box})_{\rm i} > 1$, the critical magnetic Reynolds number for the dynamo action ${\rm Rm}_{\rm crit} \approx 107 \pm 3$. In the `magnetised' regime, $(r_{\rm Larmor}/L_{\rm box})_{\rm i} \lesssim 1$, we find a marginally higher ${\rm Rm}_{\rm crit} = 124 \pm 8$. We find that the growth rate of the magnetic energy does not depend on the strength of the seed magnetic field when the initial magnetisation is fixed. We also study the distribution and evolution of the pressure anisotropy in the collisionless plasma and compare our results with the MHD turbulent dynamo.

astro-ph.CO↗