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Jellyfish galaxies have long tails of gas that is stripped from the disk by ram pressure due to the motion of galaxies in the intracluster medium in galaxy clusters. Here, we present the magnetic field strength and orientation within the disk and the (90-kpc-long) Hα-emitting tail of the jellyfish galaxy JO206. The tail has a large-scale magnetic field (>4.1 μG), a steep radio spectral index (α ≈ −2.0), indicating an ageing of the electrons propagating away from the star-forming regions, and extremely high fractional polarization (>50 %), indicating low turbulent motions. The magnetic field vectors are aligned with (parallel to) the direction of the ionized-gas tail and stripping direction. High-resolution simulations of a large, cold gas cloud that is exposed to a hot, magnetized turbulent wind show that the fractional polarization and ordered magnetic field can be explained by accretion of draped magnetized plasma from the hot wind that condenses onto the external layers of the tail, where it is adiabatically compressed and sheared. The ordered magnetic field, preventing heat and momentum exchange, may be a key factor in allowing in situ star formation in the tail. Observations of the jellyfish galaxy JO206 reveal an ordered, large-scale magnetic field and extremely high polarization, which can be explained by the accretion of magnetized plasma from the intergalactic medium that condenses onto the external layers of the tail.

Radio relics are arc-like synchrotron sources at the periphery of galaxy clusters, produced by cosmic-ray electrons in a μG magnetic field, which are believed to have been (re-)accelerated by merger shock fronts. However, not all relics appear at the same location as shocks as seen in the X-ray. In a previous work, we suggested that the shape of some relics may result from the pre-existing spatial distribution of cosmic-ray electrons, and tested this hypothesis using simulations by launching AGN jets into a cluster atmosphere with sloshing gas motions generated by a previous merger event. We showed that these motions could transport the cosmic ray-enriched material of the AGN bubbles to large radii and stretch it in a tangential direction, producing a filamentary shape resembling a radio relic. In this work, we improve our physical description for the cosmic rays by modeling them as a separate fluid which undergoes diffusion and Alfvén losses. We find that, including this additional cosmic ray physics significantly diminishes the appearance of these filamentary features, showing that our original hypothesis is sensitive to the modeling of cosmic ray physics in the intracluster medium.

Understanding the physical mechanisms that control galaxy formation is a fundamental challenge in contemporary astrophysics. Recent advances in the field of astrophysical feedback strongly suggest that cosmic rays (CRs) may be crucially important for our understanding of cosmological galaxy formation and evolution. The appealing features of CRs are their relatively long cooling times and relatively strong dynamical coupling to the gas. In galaxies, CRs can be close to equipartition with the thermal, magnetic, and turbulent energy density in the interstellar medium, and can be dynamically very important in driving large-scale galactic winds. Similarly, CRs may provide a significant contribution to the pressure in the circumgalactic medium. In galaxy clusters, CRs may play a key role in addressing the classic cooling flow problem by facilitating efficient heating of the intracluster medium and preventing excessive star formation. Overall, the underlying physics of CR interactions with plasmas exhibit broad parallels across the entire range of scales characteristic of the interstellar, circumgalactic, and intracluster media. Here we present a review of the state-of-the-art of this field and provide a pedagogical introduction to cosmic ray plasma physics, including the physics of wave–particle interactions, acceleration processes, CR spatial and spectral transport, and important cooling processes. The field is ripe for discovery and will remain the subject of intense theoretical, computational, and observational research over the next decade with profound implications for the interpretation of the observations of stellar and supermassive black hole feedback spanning the entire width of the electromagnetic spectrum and multi-messenger data.

In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts a radio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination of XMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopic information, and 1.16 GHz MeerKAT data to study the cluster properties. Various X-ray and optical morphology parameters are calculated to investigate the level of disturbance. We find disturbances in two X-ray parameters and the optical density map shows elongated and axisymmetric structures with the main cluster component southeast of the cluster centre and another component northwest of the cluster centre. We also find a BCG offset of ∼950 km/s from the mean velocity of the cluster, and a discrepancy between the SZ mass, X-ray mass, and dynamical mass (MX,500 and MSZ,500 lies >3σ away from Mdyn,500), showing that J0019 is a merging cluster and probably in a post-merging phase.

We study the longitudinal stability of beam–plasma systems in the presence of a density inhomogeneity in the background plasma. Previous works have focused on the non-relativistic regime where hydrodynamical models are used to evolve pre-existing Langmuir waves within inhomogeneous background plasmas. Here, for the first time we study the problem with kinetic equations in a fully relativistic way. We do not assume the existence of Langmuir waves, and we focus on the rate and the mechanism by which waves are excited in such systems from an initial perturbation. We derive the structure of the unstable modes and compute an analytical approximation for their growth rates. Our computation is limited to dilute and cold beams, and shows an excellent agreement with particle-in-cell simulations performed using the SHARP code. We show that, due to such an inhomogeneity, the virulent beam–plasma instabilities in the intergalactic medium are not suppressed but their counterparts in the solar wind can be suppressed as evidenced by propagating type-III solar radio bursts.

Clusters of galaxies, filled with hot magnetized plasma, are the largest bound objects in existence and an important touchstone in understanding the formation of structures in our Universe. In such clusters, thermal conduction follows field lines, so magnetic fields strongly shape the cluster's thermal history; that some have not since cooled and collapsed is a mystery. In a seemingly unrelated puzzle, recent observations of Virgo cluster spiral galaxies imply ridges of strong, coherent magnetic fields offset from their centre. Here we demonstrate, using three-dimensional magnetohydrodynamical simulations, that such ridges are easily explained by galaxies sweeping up field lines as they orbit inside the cluster. This magnetic drape is then lit up with cosmic rays from the galaxies' stars, generating coherent polarized emission at the galaxies' leading edges. This immediately presents a technique for probing local orientations and characteristic length scales of cluster magnetic fields. The first application of this technique, mapping the field of the Virgo cluster, gives a startling result: outside a central region, the magnetic field is preferentially oriented radially as predicted by the magnetothermal instability. Our results strongly suggest a mechanism for maintaining some clusters in a \"non-cooling-core\" state. [PUBLICAITON ABSTRACT]

Understanding the physical mechanisms that control galaxy formation is a fundamental challenge in contemporary astrophysics. Recent advances in the field of astrophysical feedback strongly suggest that cosmic rays (CRs) may be crucially important for our understanding of cosmological galaxy formation and evolution. The appealing features of CRs are their relatively long cooling times and relatively strong dynamical coupling to the gas. In galaxies, CRs can be close to equipartition with the thermal, magnetic, and turbulent energy density in the interstellar medium, and can be dynamically very important in driving large-scale galactic winds. Similarly, CRs may provide a significant contribution to the pressure in the circumgalactic medium. In galaxy clusters, CRs may play a key role in addressing the classic cooling flow problem by facilitating efficient heating of the intracluster medium and preventing excessive star formation. Overall, the underlying physics of CR interactions with plasmas exhibit broad parallels across the entire range of scales characteristic of the interstellar, circumgalactic, and intracluster media. Here we present a review of the state-of-the-art of this field and provide a pedagogical introduction to cosmic ray plasma physics, including the physics of wave-particle interactions, acceleration processes, CR spatial and spectral transport, and important cooling processes. The field is ripe for discovery and will remain the subject of intense theoretical, computational, and observational research over the next decade with profound implications for the interpretation of the observations of stellar and supermassive black hole feedback spanning the entire width of the electromagnetic spectrum and multi-messenger data.

In our study, we show a multiwavelength view of ACT-CL J0019.6+0336 (which hosts aradio halo), to investigate the cluster dynamics, morphology, and ICM. We use a combination ofXMM-Newton images, Dark Energy Survey (DES) imaging and photometry, SDSS spectroscopicinformation, and 1.16 GHz MeerKAT data to study the cluster properties. Various X-ray and opticalmorphology parameters are calculated to investigate the level of disturbance. We find disturbancesin two X-ray parameters and the optical density map shows elongated and axisymmetric structureswith the main cluster component southeast of the cluster centre and another component northwest ofthe cluster centre. We also find a BCG offset of∼950 km/s from the mean velocity of the cluster, anda discrepancy between the SZ mass, X-ray mass, and dynamical mass (MX,500andMSZ,500lies>3σaway fromMdyn,500), showing that J0019 is a merging cluster and probably in a post-merging phase.

Cosmic ray (CR) hydrodynamics is a (re-)emerging field of high interest due to the importance of CRs for the dynamical evolution of the interstellar, the circumgalactic, and the intracluster medium. In these environments, CRs with GeV energies can influence large-scale dynamics by regulating star formation, driving galactic winds or by altering the pressure balance of galactic halos. Recent efforts have moved the focus of the community from a one-moment description of CR transport towards a two-moment model as this allows for a more accurate description of the microphysics impacting the CR population. Like all hydrodynamical theories, these two-moment methods require a closure relation for a consistent and closed set of evolution equations. The goal of this paper is to quantify the impact of different closure relations on the resulting solutions. To this end, we review the common P1 and M1 closure relations, derive a new four-moment H1 description for CR transport and describe how to incorporate CR scattering by Alfvén waves into these three hydrodynamical models. While there are significant differences in the transport properties of radiation in the P1 and M1 approximations in comparison to more accurate radiative transfer simulations using the discrete ordinates approximation, we only find small differences between the three hydrodynamical CR transport models in the free streaming limit when we neglect CR scattering. Most importantly, for realistic applications in the interstellar, circumgalactic or intracluster medium where CR scattering is frequent, these differences vanish and all presented hydrodynamical models produce the same results.

Galactic outflows influence the evolution of galaxies not only by expelling gas from their disks but also by injecting energy into the circumgalactic medium (CGM). This alters or even prevents the inflow of fresh gas onto the disk and thus reduces the star formation rate. Supernovae (SNe) are the engines of galactic winds as they release thermal and kinetic energy into the interstellar medium (ISM). Cosmic rays (CRs) are accelerated at the shocks of SN remnants and only constitute a small fraction of the overall SN energy budget. However, their long live-times allow them to act far away from the original injection site and thereby to participate in the galactic wind launching process. Using high-resolution simulations of an isolated Milky Way-type galaxy with the moving-mesh code Arepo and the new multi-phase ISM model Crisp (Cosmic Rays and InterStellar Physics), we investigate how SNe and CRs launch galactic outflows and how the inclusion of CR-mediated feedback boosts the energy and mass entrained in the galactic wind. We find that the majority of thermal SN energy and momentum is used for stirring turbulence either directly or indirectly by causing fountain flows, thereby self-regulating the ISM and not for efficiently driving outflows to large heights. A simulation without CRs only launches a weak galactic outflow at uniformly high temperatures and low densities by means of the thermal pressure gradient. By contrast, most of the CR energy accelerated at SN remnants (\\(\\sim80\\%\\)) escapes the ISM and moves into the CGM. In the inner CGM, CRs dominate the overall pressure and are able to accelerate a large mass fraction in a galactic wind. This wind is turbulent and multi-phase with cold cloudlets embedded in dilute gas at intermediate temperatures (\\(\\sim10^5\\) K) and the CGM shows enhanced OVI and CIV absorption in comparison to a simulation without CRs.