arXiv subjects

Thomas Berlok

Publications and source records attributed to Thomas Berlok.

18 recordsLinked to original sources

arXiv2026

Active galactic nucleus driven jet feedback in cosmologically forming cool-core galaxy clusters I: The effect of hierarchical assembly on intra-cluster medium properties

The atmospheres of cool-core galaxy clusters are excellent probes of astrophysical plasmas. However, how the interplay between assembly and active galactic nucleus (AGN) feedback leads to the observed gas profiles remains uncertain. We study the impact of hierarchical assembly on the intra-cluster medium (ICM) in cool-core galaxy clusters using hydrodynamic simulations as part of the PICO-Cluster project. We compare cosmological zoom simulations employing an explicit AGN jet model against PICO-Cluster simulations with IllustrisTNG kinetic AGN feedback, as well as against isolated galaxy cluster simulations using jet feedback. The stellar and gas fractions of our cosmological galaxy cluster simulations with jet feedback are in excellent agreement with observed galaxy clusters, and the ICM thermodynamic profiles resemble those of local cool-core galaxy clusters while those run with IllustrisTNG kinetic AGN feedback do not match these observations. In all simulations, cosmological and isolated, the AGN heating roughly balances the cooling losses, with star formation being significantly suppressed. The most notable differences between the cosmological and isolated simulations are the resulting velocity and multi-phase structure: gas at radii $> 50$ kpc is shaped by satellite galaxies rather than jet feedback originating form the central galaxy. This leads to significant differences in non-thermal pressure support, with only the cosmological simulations being consistent with recent observations. A second notable difference is the abundance of warm ($<10^5$ K) gas beyond the core region, which is absent in our isolated simulation. Our results highlight the need for taking cosmological assembly into account in comparisons of the ICM dynamics and its multi-phase nature, while self-regulation is altered by hierarchical assembly via merger-driven growth of the central supermassive black hole.

astro-ph.GA↗

arXiv2026

The PICO-Cluster Project: presenting the galaxy cluster sample and studying magnetic field growth, Faraday rotation and Braginskii heating

Galaxy clusters constitute a microcosm of the Universe and offer a unique laboratory for studying plasma astrophysics, encompassing processes such as cosmic-ray acceleration and non-thermal radio emission, turbulence, weakly collisional plasma physics, and transformative mechanisms in galaxy evolution. To investigate these phenomena, we introduce the PICO-Cluster project, studying 'Plasmas In COsmological Clusters' using a suite of high-resolution cosmological zoom-in simulations of massive galaxy clusters with masses $\gtrsim10^{15}$M$_\odot$ selected from a parent simulation box with a comoving side length of 1 $h^{-1}$Gpc. In this work, we present 24 baseline simulations performed with the moving-mesh AREPO code and the IllustrisTNG galaxy formation model, achieving a baryonic mass resolution of up to $1.4\times10^{6}\mathrm{M}_\odot$. The initial conditions are carefully designed to exclude low-resolution particle contamination within the high-resolution region; as a result, all clusters remain free of such contamination out to at least 2.7 $R_{200}$ at all times. Our galaxy and cluster properties agree with recent simulations and many observational constraints, including scaling relations and thermodynamic profiles. The magnetic energy within the cluster is numerically converged once the small-scale dynamo has saturated, yielding a remarkably tight volume-averaged plasma-beta of $\beta\approx100$ inside $R_{200}$ across our sample after redshift $z\sim1.2$. Faraday rotation measure profiles, which trace the line-of-sight magnetic field and electron density, decline with cylindrical radius; notably, the mean decreases more rapidly than the root-mean-square due to the increasing relative contribution of galaxies at larger radii. Finally, viscous heating rates in Braginskii theory are highly intermittent and, on average, approach radiative cooling rates in the cluster outskirts.

astro-ph.GA↗

arXiv2026

Identifying heating processes in simulations with an entropy-based scheme: A single jet episode in a galaxy cluster

Understanding heating processes in galaxy clusters is essential for predicting the regulation of radiative cooling and star formation, and for clarifying the mechanisms underlying active galactic nucleus (AGN) feedback in cool-core clusters. We investigate the processes through which AGN jets deposit heat into the intracluster medium (ICM) by tracking passive entropy scalars in magneto-hydrodynamic (MHD) simulations. This enables us to systematically disentangle the contributions from different heating channels. We successfully validate this method with several idealized tests, including turbulent heating, heating by anisotropic Braginskii viscosity, dissipative and adiabatic heating by shocks using in-situ shock-detection methods, and cosmic ray (CR) heating through the excitations and damping of Alfv\'en waves. Using this methodology, we simulate single-epoch outbursts of high-power jets with varying densities in a cluster environment. Light jets produce wider bubbles, displacing a larger fraction of the gas in the cluster's core, whereas comparatively denser jets propagate more efficiently to larger distances without significantly disturbing the central region. During early evolution, shock heating dominates for the jets irrespective of their densities. At later times, light jets primarily heat the ICM through turbulent dissipation, while the denser jets continue to dissipate most of their energy via shocks. Turbulent and/or mixing-driven heating prevails inside the cocoon, whereas shock and acoustic compressions dominate outside. In light jets, the forward shock weakens rapidly, whereas dense jets can sustain strong bow shocks to large distances. This heating estimator allows us to identify the dominant heating mechanism responsible for resolving the cooling flow problem in future self-regulated AGN jet simulations.

astro-ph.HE↗

arXiv2026

Characterizing turbulence in galaxy clusters: defining turbulent energies and assessing multi-scale versus fixed-scale filters

Disentangling turbulence and bulk motions in the intracluster medium (ICM) of galaxy clusters is inherently ambiguous, as the plasma is continuously stirred by different processes on disparate scales. This poses a serious problem in the interpretation of both observations and numerical simulations. In this paper, we use filtering operators in real space to separate bulk motion from turbulence at different scales. We show how filters can be used to define consistent kinetic and magnetic energies for the bulk and turbulent component. We apply our GPU-accelerated filtering pipeline to a simulation of a major galaxy cluster merger, which is part of the PICO-Clusters suite of zoom-in cosmological simulations of massive clusters using the moving mesh code Arepo and the IllustrisTNG galaxy formation model. We find that during the merger the turbulent pressure fraction on physical scales $\lesssim$160 kpc reaches a maximum of 5%, before decreasing to 2% after $\sim$1.3 Gyr from the core passage. These low values are consistent with recent observations of clusters with XRISM, and suggest that unless a cluster was recently perturbed by a major merger, turbulence levels are low. We then re-examine the popular multiscale iterative filter method. In our tests, we find that its use can introduce artifacts, and that it does not reliably disentangle fluctuations living on widely separated length scales. Rather, we believe it is more fruitful to use fixed-scale filters and turbulent energies to compare between simulations and observations. This work significantly improves our understanding of turbulence generation by major mergers in galaxy clusters, which can be probed by XRISM and next-generation X-ray telescopes, allowing us to connect high-resolution cosmological simulations to observations.

astro-ph.GA↗

arXiv2025

Anisotropic Velocity Fluctuations in Galaxy Mergers: A Probe of the Magnetic Field

Magnetic fields and turbulence are fundamental to the evolution of galaxies, yet their precise measurement and analysis present significant challenges. The recently developed Velocity Gradient Technique (VGT), which capitalizes on the anisotropy inherent in magnetohydrodynamic (MHD) turbulence, represents a new method for mapping magnetic fields in galaxies using spectroscopic observations. Most validations of VGT thus far, however, have relied upon idealized MHD turbulence simulations, which lack the more complex dynamics found in galaxies and galaxy mergers. In this study, we scrutinize VGT using an AREPO-based cosmological galaxy merger simulation, testing its effectiveness across pre-merger, merging, and post-merger stages. We examine the underlying assumptions of VGT and probe the statistics of gas density, velocity, and magnetic fields over time. We find that velocity fluctuations are indeed anisotropic at each stage, being larger in the direction perpendicular to the local magnetic field, as required by VGT. We find, additionally, that galaxy mergers substantially intensify velocity and density fluctuations and amplify magnetic fields at all scales. The observed scaling of the velocity fluctuations shows a steeper trend than $r^{1/2}$ between 0.6 and 3~kpc and a shallower trend at larger scales. The scaling of the magnetic field and density fluctuations at scales $\lesssim$ 1.0 kpc also predominantly aligns with $r^{1/2}$. Finally, we compare results from VGT to those derived from polarization-like mock magnetic field measurements, finding consistent and statistically significant global agreement in all cases.

astro-ph.GA↗

arXiv2025

Does the magneto-thermal instability survive whistler-suppression of thermal conductivity in galaxy clusters?

The hot and dilute intracluster medium (ICM) plays a central role in many key processes that shape galaxy clusters.Nevertheless, the nature of plasma turbulence and particle transport in the ICM remain poorly understood, and quantifying the effect of kinetic plasma instabilities on the macroscopic dynamics represents an outstanding problem. Here we focus on the impact of whistler-wave suppression of the heat flux on the magnetothermal instability (MTI), which is expected to drive significant turbulent motions in the periphery of galaxy clusters. We perform small-scale Boussinesq simulations with a sub-grid closure for the thermal diffusivity in the regime of whistler-wave suppression. Our model is characterized by a single parameter that quantifies the collisionality of the ICM on the astrophysical scales of interest that we tune to explore a range appropriate for the periphery of galaxy clusters. We find that the MTI is qualitatively unchanged for weak whistler suppression. Conversely, with strong suppression the magnetic dynamo is interrupted and MTI turbulence dies out. In the astrophysically relevant limit, however, the MTI is likely to be supplemented by additional sources of turbulence. Investigating this scenario, we show that the inclusion of external forcing has a beneficial impact and revives even MTI simulations with strong whistler suppression. As a result, the plasma remains buoyantly unstable, with important consequences for turbulent mixing in the ICM.

astro-ph.CO↗

arXiv2025

Thermal conductivity with bells and whistlers: suppression of the magnetothermal instability in galaxy clusters

In the hot intracluster medium (ICM) in galaxy clusters, plasma microinstabilities may play an important role in the transport of heat and momentum on the large scales. In this paper, we continue our investigation of the effect of whistler suppression of thermal conductivity on the magneto-thermal instability (MTI), which may be active in the periphery of galaxy clusters and contribute to the observed turbulence. We use a closure for the heat flux inspired by kinetic simulations and show that MTI turbulence with whistler suppression exhibits a critical transition: for modest suppression of the conductivity, the MTI turbulent velocities decrease in agreement with previous MTI scaling laws. However, for suppression above a critical threshold, the MTI loses its ability to maintain equipartition-level magnetic fields through a small-scale dynamo, and the system enters a ``death-spiral''. We propose a model to explain this critical transition, and speculate that conditions in the hot ICM are favourable to the upkeep of the dynamo. Additionaly, with whistler suppression high-$β$ regions are brought out of thermal equilibrium while the efficiency of MTI turbulent driving is reduced. Finally, we show that external turbulence interferes with the MTI and leads to lower levels of turbulence. While individually both external turbulence and whistler suppression weaken the MTI, we find that they can exhibit a complex interplay when acting in conjunction, with external turbulence boosting the whistler-suppressed thermal conductivity and even reviving a ``dead'' MTI. Our study illustrates how extending magnetohydrodynamics with a simple prescription for microscale plasma physics can lead to the formation of a complicated dynamical system and demonstrates that further work is needed in order to bridge the gap between micro- and macro scales in galaxy clusters.

astro-ph.CO↗

arXiv2024

Magnetic dynamos in galaxy clusters: the crucial role of galaxy formation physics at high redshifts

Observations of Faraday rotation and synchrotron emission in galaxy clusters imply large-scale magnetic fields with $μ\mathrm{G}$ strengths possibly extending back to $z=4$. Non-radiative cosmological simulations of galaxy clusters show a comparably slow magnetic field growth that only saturates at late times. We include galaxy formation physics and find a significantly accelerated magnetic field growth. We identify three crucial stages in the magnetic field evolution. 1) At high redshift, the central dominant galaxy serves as the prime agent that magnetizes not only its immediate vicinity but also most of the forming protocluster through a combination of a small-scale dynamo induced by gravitationally driven, compressive turbulence and stellar and active galactic nuclei feedback that distribute the magnetic field via outflows. 2) This process continues as other galaxies merge into the forming cluster in subsequent epochs, thereby transporting their previously amplified magnetic field to the intracluster medium (ICM) through ram pressure stripping and galactic winds. 3) At lower redshift, gas accretion and frequent cluster mergers trigger additional small-scale dynamo processes, thereby preventing the decay of the magnetic field and fostering the increase of the magnetic coherence scale. We show that the magnetic field observed today in the weakly collisional ICM is consistently amplified on collisional scales. Initially, this occurs in the collisional interstellar medium during protocluster assembly, and later in the ICM on the magnetic coherence scale, which always exceeds the particle mean free path, supporting the use of magneto-hydrodynamics for studying the cluster dynamo. We generate synthetic Faraday rotation measure observations of protoclusters, highlighting the potential for studying magnetic field growth during the onset of cluster formation at cosmic dawn.

astro-ph.GA↗

arXiv2024

Paicos: A Python package for analysis of (cosmological) simulations performed with Arepo

We present Paicos, a new object-oriented Python package for analyzing simulations performed with Arepo. Paicos strives to reduce the learning curve for students and researchers getting started with Arepo simulations. As such, Paicos includes many examples in the form of Python scripts and Jupyter notebooks as well as an online documentation describing the installation procedure and recommended first steps. Paicos' main features are automatic handling of cosmological and physical units, computation of derived variables, 2D visualization (slices and projections), 1D and 2D histograms, and easy saving and loading of derived data including units and all the relevant metadata.

astro-ph.IM↗

arXiv2022

Hydromagnetic waves in an expanding universe -- cosmological MHD code tests using analytic solutions

We describe how analytic solutions for linear hydromagnetic waves can be used for testing cosmological magnetohydrodynamic (MHD) codes. We start from the comoving MHD equations and derive analytic solutions for the amplitude evolution of linear hydromagnetic waves in a matter-dominated, flat Einstein-de-Sitter (EdS) universe. The waves considered are comoving, linearly polarized Alfvén waves and comoving, magnetosonic (fast) waves modified by self-gravity. The solution for compressible waves is found for a general adiabatic index and we consider the limits of hydrodynamics without self-gravity in addition to the full solution. In addition to these analytic solutions, the linearized equations are solved numerically for a $Λ$CDM cosmology. We use the analytic and numeric solutions to compare with results obtained using the cosmological MHD code AREPO and find good agreement when using a sufficient number of grid points. We interpret the numerical damping clearly evident in simulations with few grid points by further deriving the Alfvén wave solution including physical Navier-Stokes viscosity. A comparison between Alfvén wave simulations and theory reveals that the dissipation can be described by a numerical viscosity coefficient $η_\mathrm{num} \propto a^{-5/2}$ where $a$ is the scale factor. We envision that our examples could be useful when developing a new cosmological MHD code or for regression testing of existing codes.

astro-ph.CO↗

arXiv2021

The in situ formation of molecular and warm ionised gas triggered by hot outflows

Molecular outflows contributing to the matter cycle of star forming galaxies are now observed in small and large systems at low and high redshift. Their physical origin is still unclear. In most theoretical studies only warm ionised/neutral and hot gas outflowing from the interstellar medium is generated by star formation. We investigate an in-situ H$_2$ formation scenario in the outflow using high-resolution simulations, including non-equilibrium chemistry and self-gravity, of turbulent, warm, and atomic clouds with densities 0.1, 0.5 and $1\,\mathrm{cm}^{-3}$ exposed to a magnetised hot wind. For cloud densities $\gtrsim 0.5\,\mathrm{cm}^{-3}$ a magnetised wind triggers H$_2$ formation before cloud dispersal. Up to 3 per cent of the initial cloud mass can become molecular on $\sim 10\,\mathrm{Myr}$ time scales. The effect is stronger for winds with perpendicular $B$-fields and intermediate density clouds ($n_\mathrm{c}\sim 0.5\,\mathrm{cm}^{-3}$). Here H$_2$ formation can be boosted by up to one order of magnitude compared to isolated cooling clouds independent of self-gravity. Self-gravity preserves the densest clouds way past their $\sim 15\,\mathrm{Myr}$ cloud crushing time scales. This model could provides a plausible in-situ origin for the observed molecular gas. Warm ionised gas is also generated, almost independent of the cloud density. The amount solely depend on the magnetic field configuration in the wind. For low density clouds ($0.1\,\mathrm{cm}^{-3}$), the forming warm ionised gas can be as much as 60 per cent of the initially atomic cloud mass. This could contribute to observations of outflows with ionised gas sensitive tracers.

astro-ph.GA↗

arXiv2021

Suppressed heat conductivity in the intracluster medium: implications for the magneto-thermal instability

In the outskirts of the intracluster medium (ICM) in galaxy clusters, the temperature decreases with radius. Due to the weakly collisional nature of the plasma, these regions are susceptible to the magneto-thermal instability (MTI), which can sustain turbulence and provide turbulent pressure support in the ICM. This instability arises due to heat conduction directed along the magnetic field, with a heat conductivity which is normally assumed to be given by the Spitzer value. Recent numerical studies of the ion mirror and the electron whistler instability using particle-in-cell codes have shown that microscale instabilities can lead to a reduced value for the heat conductivity in the ICM. This could in turn influence the efficiency with which the MTI drives turbulence. In this paper we investigate the influence of reduced heat transport on the nonlinear evolution of the MTI. We study plane-parallel, initially static atmospheres and employ a subgrid model that mimics the influence of the mirror instability on the heat conductivity. We use this subgrid model to assess the effect of microscales on the large scale dynamics of the ICM. We find that the nonlinear saturation of the MTI is surprisingly robust in our simulations. Over a factor of $\sim 10^3$ in the thermal-to-magnetic pressure ratio and collisionality we find at most modest changes to the saturation of the MTI with respect to reference simulations where heat transport is unsuppressed.

astro-ph.CO↗

arXiv2020

Local Simulations of Instabilities Driven by Composition Gradients in the ICM

The distribution of Helium in the intracluster medium (ICM) permeating galaxy clusters is not well constrained due to the very high plasma temperature. Therefore, the plasma is often assumed to be homogeneous. A non-uniform Helium distribution can however lead to biases when measuring key cluster parameters. This has motivated one-dimensional models that evolve the ICM composition assuming that the effects of magnetic fields can be parameterized or ignored. Such models for non-isothermal clusters show that Helium can sediment in the cluster core leading to a peak in concentration offset from the cluster center. The resulting profiles have recently been shown to be linearly unstable when the weakly-collisional character of the magnetized plasma is considered. In this paper, we present a modified version of the MHD code Athena, which makes it possible to evolve a weakly-collisional plasma subject to a gravitational field and stratified in both temperature and composition. We thoroughly test our implementation and confirm excellent agreement against several analytical results. In order to isolate the effects of composition, in this initial study we focus our attention on isothermal plasmas. We show that plasma instabilities, feeding off gradients in composition, can induce turbulent mixing and saturate by re-arranging magnetic field lines and alleviating the composition gradient. Composition profiles that increase with radius lead to instabilities that saturate by driving the average magnetic field inclination to roughly $45^{\circ}$. We speculate that this effect may alleviate the core insulation observed in homogeneous settings, with potential consequences for the associated cooling flow problem.

astro-ph.HE↗

arXiv2020

On Helium Mixing in Quasi-global Simulations of the Intracluster Medium

The assumption of a spatially uniform helium distribution in the intracluster medium can lead to biases in the estimates of key cluster parameters if composition gradients are present. The helium concentration profile in galaxy clusters is unfortunately not directly observable. Current models addressing the putative sedimentation are one-dimensional and parametrize the presence of magnetic fields in a crude way, ignoring the weakly-collisional, magnetized nature of the medium. When these effects are considered, a wide variety of instabilities can play an important role in the plasma dynamics. In a series of recent papers, we have developed the local, linear theory of these instabilities and addressed their non-linear development with a modified version of Athena. Here, we extend our study by developing a quasi-global approach that we use to simulate the mixing of helium as induced by generalizations of the heat-flux-driven buoyancy instability (HBI) and the magneto-thermal instability (MTI), which feed off thermal and composition gradients. In the inner region of the ICM, mixing can occur on few Gyrs, after which the average magnetic field inclination angle is $\sim 45^{\circ}$ resulting in an averaged Spitzer parameter higher by about 20 % than the value obtained in homogeneous simulations. In the cluster outskirts the instabilities are rather inefficient, due to the shallow gradients. This suggests that compositions gradients in cluster cores might be shallower than one-dimensional models predict. More quantitative statements demand more refined models that can incorporate the physics driving the sedimentation process and simultaneously account for the weakly-collisional nature of the plasma.

astro-ph.HE↗

arXiv2020

The impact of magnetic fields on cold streams feeding galaxies

High redshift, massive halos are observed to have sustained, high star formation rates, which require that the amount of cold gas in the halo is continuously replenished. The cooling time scale for the hot virialized halo gas is too long to provide the source of cold gas. Supersonic, cold streams have been invoked as a mechanism for feeding massive halos at high redshift and deliver the cold gas required for continued star formation at the rates observed. This mechanism for replenishing the cold gas reservoir is motivated by some cosmological simulations. However, the cold streams are likely to be subject to the supersonic version of the Kelvin-Helmholtz instability (KHI), which eventually leads to stream disruption. Cosmological simulations have yet to obtain the spatial resolution required for understanding the detailed stability properties of cold streams. In this paper, we consider instead an idealized model of magnetized cold streams that we spatially resolve. Using linear theory we show how magnetic fields with dynamically important field strengths do not inhibit the KHI but rather enhance its growth rate. We perform nonlinear simulations of magnetized stream disruption and find that magnetic fields can nevertheless increase stream survival times by suppressing the mixing rate of cold gas with the circumgalactic medium. We find that magnetic fields can allow streams to survive $\sim 2-8$ times longer and, consequently, that streams $\sim 2-8$ times thinner can reach the central galaxy if the magnetic field strength is $\sim 0.3-0.8 μ$G.

astro-ph.GA↗

arXiv2019

Braginskii viscosity on an unstructured, moving mesh accelerated with super-time-stepping

We present a method for efficiently modelling Braginskii viscosity on an unstructured, moving mesh. Braginskii viscosity, i.e., anisotropic transport of momentum with respect to the direction of the magnetic field, is thought to be of prime importance for studies of the weakly collisional plasma that comprises the intracluster medium (ICM) of galaxy clusters. Here anisotropic transport of heat and momentum has been shown to have profound consequences for the stability properties of the ICM. Our new method for modelling Braginskii viscosity has been implemented in the moving mesh code Arepo. We present a number of examples that serve to test the implementation and illustrate the modified dynamics found when including Braginskii viscosity in simulations. These include (but are not limited to) damping of fast magneto-sonic waves, interruption of linearly polarized Alfvén waves by the firehose instability and the inhibition of the Kelvin-Helmholtz instability by Braginskii viscosity. An explicit update of Braginskii viscosity is associated with a severe time step constraint that scales with $(Δx)^2$ where $Δx$ is the grid size. In our implementation, this restrictive time step constraint is alleviated by employing 2nd order accurate Runge-Kutta-Legendre super-time-stepping. We envision including Braginskii viscosity in future large-scale simulations of Kelvin-Helmholtz unstable cold fronts in cluster mergers and AGN-generated bubbles in central cluster regions.

astro-ph.GA↗

arXiv2019

On the Kelvin-Helmholtz instability with smooth initial conditions -- Linear theory and simulations

The Kelvin-Helmholtz instability (KHI) is a standard test of hydrodynamic and magnetohydrodynamic (MHD) simulation codes and finds many applications in astrophysics. The classic linear theory considers a discontinuity in density and velocity at the interface of two fluids. However, for numerical simulations of the KHI such initial conditions do not yield converged results even at the linear stage of the instability. Instead, smooth profiles of velocity and density are required for convergence. This renders analytical theory to be only approximately valid and hinders quantitative comparisons between the classical theory and simulations. In this paper we derive a linear theory for the KHI with smooth profiles and illustrate code testing with the MHD code Athena. We provide the linear solution for the KHI with smooth initial conditions in three different limits: inviscid hydrodynamics, ideal MHD and Braginskii-MHD. These linear solutions are obtained numerically with the framework Psecas (Pseudo-Spectral Eigenvalue Calculator with an Automated Solver), which generates and solves numerical eigenvalue problems using an equation-parser and pseudo-spectral methods. The Athena simulations are carried out on a periodic, Cartesian domain which is useful for code testing purposes. Using Psecas and analytic theory, we outline the differences between this artificial numerical setup and the KHI on an infinite Cartesian domain and the KHI in cylindrical geometry. We discuss several astrophysical applications, such as cold flows in galaxy formation and cold fronts in galaxy cluster mergers. Psecas, and the linear solutions used for code testing, are publicly available and can be downloaded from the web.

astro-ph.GA↗

arXiv2015

Plasma Instabilities in the Context of Current Helium Sedimentation Models: Dynamical Implications for the ICM in Galaxy Clusters

Understanding whether Helium can sediment to the core of galaxy clusters is important for a number of problems in cosmology and astrophysics. All current models addressing this question are one-dimensional and do not account for the fact that magnetic fields can effectively channel ions and electrons, leading to anisotropic transport of momentum, heat, and particle diffusion in the weakly collisional intracluster medium (ICM). This anisotropy can lead to a wide variety of instabilities, which could be relevant for understanding the dynamics of heterogeneous media. In this paper, we consider the radial temperature and composition profiles as obtained from a state-of-the-art Helium sedimentation model and analyze its stability properties. We find that the associated radial profiles are unstable, to different kinds of instabilities depending on the magnetic field orientation, at all radii. The fastest growing modes are usually related to generalizations of the Magnetothermal Instability (MTI) and the Heat-flux-driven Buoyancy Instability (HBI) which operate in heterogeneous media. We find that the effect of sedimentation is to increase (decrease) the predicted growth rates in the inner (outer) cluster region. The unstable modes grow fast compared to the sedimentation timescale. This suggests that the composition gradients as inferred from sedimentation models, which do not fully account for the anisotropic character of the weakly collisional environment, might not be very robust. Our results emphasize the subtleties involved in understanding the gas dynamics of the ICM and argue for the need of a comprehensive approach to address the issue of Helium sedimentation beyond current models.

astro-ph.CO↗