Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters

Transport equations for electron thermal energy in the high-β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te /β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L c > L/β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.

One Source, Two Source(s): Ribs and Tethers

We present the unique and challenging case of a radio galaxy in Abell 3266 observed as part of the MeerKAT Galaxy Cluster Legacy Survey. It has quasi-periodic bright patches along the tail which connect to never-before-seen thin transverse extensions, which we call “ribs”, reaching up to ∼50 kpc from the central axis of the tail. At a distance of ∼400 kpc from the host (assuming the z=0.0594 redshift of Abell 3266) we found what appears to be a triple source with its own apparent host at a photometric redshift of 0.78. Mysteriously, the part of the tail far from the host and the triple are connected by a series of thin filaments, which we call “tethers”. The far tail, tethers and triple also have similar spectra and Faraday rotation measures, suggesting that there is only one—quite complicated—source, with a serendipitous background AGN in the triple. We look at possible causes for the “rib” and “tether” structures, and the emerging phenomena of intracluster medium filaments associated with radio galaxies.

Shocks in the stacked Sunyaev–Zel’dovich profiles of clusters – I. Analysis with the Three Hundred simulations

ABSTRACT Gas infalling into the gravitational potential wells of massive galaxy clusters is expected to experience one or more shocks on its journey to becoming part of the intracluster medium (ICM). These shocks are important for setting the thermodynamic properties of the ICM and can therefore impact cluster observables such as X-ray emission and the Sunyaev–Zel’dovich (SZ) effect. We investigate the possibility of detecting signals from cluster shocks in the averaged thermal SZ profiles of galaxy clusters. Using zoom-in hydrodynamic simulations of massive clusters from the Three Hundred Project, we show that if cluster SZ profiles are stacked as a function of R/R200m, shock-induced features appear in the averaged SZ profile. These features are not accounted for in standard fitting formulae for the SZ profiles of galaxy clusters. We show that the shock features should be detectable with samples of clusters from ongoing and future SZ surveys. We also demonstrate that the location of these features is correlated with the cluster accretion rate, as well as the location of the cluster splashback radius. Analyses of ongoing and future surveys, such as SPT-3g, AdvACT, Simons Observatory, and CMB-S4, which include gas shocks will gain a new handle on the properties and dynamics of the outskirts of massive haloes, both in gas and in mass.

Properties of Polarized Synchrotron Emission from Fluctuation Dynamo Action—II. Effects of Turbulence Driving in the ICM and Beam Smoothing

Polarized synchrotron emission from the radio halos of diffuse intracluster medium (ICM) in galaxy clusters are yet to be observed. To investigate the expected polarization in the ICM, we use high resolution (1 kpc) magnetohydrodynamic simulations of fluctuation dynamos, which produces intermittent magnetic field structures, for varying scales of turbulent driving (lf) to generate synthetic observations of the polarized emission. We focus on how the inferred diffuse polarized emission for different lf is affected due to smoothing by a finite telescope resolution. The mean fractional polarization ⟨p⟩ vary as ⟨p⟩∝lf1/2 with ⟨p⟩>20% for lf≳60 kpc, at frequencies ν>4GHz. Faraday depolarization at ν<3 GHz leads to deviation from this relation, and in combination with beam depolarization, filamentary polarized structures are completely erased, reducing ⟨p⟩ to below 5% level at ν≲1 GHz. Smoothing on scales up to 30 kpc reduces ⟨p⟩ above 4 GHz by at most a factor of 2 compared to that expected at 1 kpc resolution of the simulations, especially for lf≳100 kpc, while at ν<3 GHz, ⟨p⟩ is reduced by a factor of more than 5 for lf≳100 kpc, and by more than 10 for lf≲100 kpc. Our results suggest that observational estimates of, or constrain on, ⟨p⟩ at ν≳4 GHz could be used as an indicator of the turbulent driving scale in the ICM.

Troubled cosmic flows: turbulence, enstrophy, and helicity from the assembly history of the intracluster medium

ABSTRACT Both simulations and observations have shown that turbulence is a pervasive phenomenon in cosmic scenarios, yet it is particularly difficult to model numerically due to its intrinsically multiscale character which demands high resolutions. Additionally, turbulence is tightly connected to the dynamical state and the formation history of galaxies and galaxy clusters, producing a diverse phenomenology which requires large samples of such structures to attain robust conclusions. In this work, we use an adaptive mesh refinement (AMR) cosmological simulation to explore the generation and dissipation of turbulence in galaxy clusters, in connection to its assembly history. We find that major mergers, and more generally accretion of gas, is the main process driving turbulence in the intracluster medium. We have especially focused on solenoidal turbulence, which can be quantified through enstrophy. Our results seem to confirm a scenario for its generation which involves baroclinicity and compression at the external (accretion) and internal (merger) shocks, followed by vortex stretching downstream of them. We have also looked at the infall of mass to the cluster beyond its virial boundary, finding that gas follows trajectories with some degree of helicity, as it has already developed some vorticity in the external shocks.

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 ∼103 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.