Stochastic heating of cooling flows

It is generally accepted that the heating of gas in clusters of galaxies by active galactic nuclei (AGN) is a form of feedback. Feedback is required to ensure a long term, sustainable balance between heating and cooling. This work investigates the impact of proportional stochastic feedback on the energy balance in the intracluster medium. Using a generalised analytical model for a cluster atmosphere, it is shown that an energy equilibrium can be reached exponentially quickly. Applying the tools of stochastic calculus it is demonstrated that the result is robust with regard to the model parameters, even though they affect the amount of variability in the system.Comment: 7 pages, 6 figures, accepted by MNRAS, http://www.astro.soton.ac.uk/~gbp/pub/pavlovski_stochh.pd

Interactions between magnetohydrodynamic shear instabilities and convective flows in the solar interior

Motivated by the interface model for the solar dynamo, this paper explores the complex magnetohydrodynamic interactions between convective flows and shear-driven instabilities. Initially, we consider the dynamics of a forced shear flow across a convectively-stable polytropic layer, in the presence of a vertical magnetic field. When the imposed magnetic field is weak, the dynamics are dominated by a shear flow (Kelvin-Helmholtz type) instability. For stronger fields, a magnetic buoyancy instability is preferred. If this stably stratified shear layer lies below a convectively unstable region, these two regions can interact. Once again, when the imposed field is very weak, the dynamical effects of the magnetic field are negligible and the interactions between the shear layer and the convective layer are relatively minor. However, if the magnetic field is strong enough to favour magnetic buoyancy instabilities in the shear layer, extended magnetic flux concentrations form and rise into the convective layer. These magnetic structures have a highly disruptive effect upon the convective motions in the upper layer.Comment: 11 pages, 10 figures, accepted for publication in MNRA

Reconstruction of Radio Relics and X-Ray Tails in an Off-axis Cluster Merger: Hydrodynamical Simulations of A115

Although a merging galaxy cluster is a useful laboratory to study many important astrophysical processes and fundamental physics, only limited interpretations are possible without careful analysis of the merger scenario. However, the study is demanding because a thorough comparison of multiwavelength observations with detailed numerical simulations is required. In this paper, we present such a study for the off-axis binary merger A115. The system possesses a number of remarkable observational features, but no convincing merger scenario explaining the shape and location of the radio relic in harmony with the orientation of the cometary X-ray tails has been presented. Our hydrodynamical simulation with adaptive mesh refinement suggests that the cometary X-ray tail of A115 might be a slingshot tail that can arise similar to 0.3 Gyr after the impact, before the two subclusters reach their apocenters. This scenario can predict the location and orientation of the giant radio relic, which is parallel to the northern X-ray tail. In addition, our study indicates that diffusive shock acceleration alone cannot generate the observed radio power unless aided by stronger magnetic fields and/or more significant presence of fossil electrons

Evolution of Primordial Magnetic Fields during Large-scale Structure Formation

Primordial magnetic fields (PMFs) could explain the large-scale magnetic fields present in the universe. Inflation and phase transitions in the early universe could give rise to such fields with unique characteristics. We investigate the magnetohydrodynamic evolution of these magnetogenesis scenarios with cosmological simulations. We evolve inflation-generated magnetic fields either as (i) uniform (homogeneous) or as (ii) scale-invariant stochastic fields, and phase-transition-generated ones either as (iii) helical or as (iv) nonhelical fields from the radiation-dominated epoch. We find that the final distribution of magnetic fields in the simulated cosmic web shows a dependence on the initial strength and the topology of the seed field. Thus, the observed field configuration retains information on the initial conditions at the moment of the field generation. If detected, PMF observations would open a new window for indirect probes of the early universe. The differences between the competing models are revealed on the scale of galaxy clusters, bridges, as well as filaments and voids. The distinctive spectral evolution of different seed fields produces imprints on the correlation length today. We discuss how the differences between rotation measures from highly ionized regions can potentially be probed with forthcoming surveys

Magnetogenesis and the cosmic web: A joint challenge for radio observations and numerical simulations

The detection of the radio signal from filaments in the cosmic web is crucial to distinguish possible magnetogenesis scenarios. We review the status of the different attempts to detect the cosmic web at radio wavelengths. This is put into the context of the advanced simulations of cosmic magnetism carried out in the last few years by our MAGCOW project. While first attempts of imaging the cosmic web with the MWA and LOFAR have been encouraging and could discard some magnetogenesis models, the complexity behind such observations makes a definitive answer still uncertain. A combination of total intensity and polarimetric data at low radio frequencies that the SKA and LOFAR2.0 will achieve is key to removing the existing uncertainties related to the contribution of many possible sources of signal along deep lines of sight. This will make it possible to isolate the contribution from filaments, and expose its deep physical connection with the origin of extragalactic magnetism. ?? 2021 by the authors. Licensee MDPI, Basel, Switzerland

Magnetogenesis and the cosmic web: A joint challenge for radio observations and numerical simulations

The detection of the radio signal from filaments in the cosmic web is crucial to distinguish possible magnetogenesis scenarios. We review the status of the different attempts to detect the cosmic web at radio wavelengths. This is put into the context of the advanced simulations of cosmic magnetism carried out in the last few years by our MAGCOW project. While first attempts of imaging the cosmic web with the MWA and LOFAR have been encouraging and could discard some magnetogenesis models, the complexity behind such observations makes a definitive answer still uncertain. A combination of total intensity and polarimetric data at low radio frequencies that the SKA and LOFAR2.0 will achieve is key to removing the existing uncertainties related to the contribution of many possible sources of signal along deep lines of sight. This will make it possible to isolate the contribution from filaments, and expose its deep physical connection with the origin of extragalactic magnetism. ?? 2021 by the authors. Licensee MDPI, Basel, Switzerland

Chandra Observations of the Spectacular A3411???12 Merger Event

We present deep Chandra observations of A3411???12, a remarkable merging cluster that hosts the most compelling evidence for electron reacceleration at cluster shocks to date. Using the {Y}_{{\rm{X}}}\mbox{--}M scaling relation, we find r 500 ~ 1.3 Mpc, ${M}_{500}=(7.1\pm 0.7)\times {10}^{14}\ {M}_{\odot }$, ${kT}=6.5\pm 0.1\,\mathrm{keV}$, and a gas mass of ${M}_{{\rm{g}},500}=(9.7\pm 0.1)\times {10}^{13}{M}_{\odot }$. The gas mass fraction within r 500 is ${f}_{{\rm{g}}}=0.14\pm 0.01$. We compute the shock strength using density jumps to conclude that the Mach number of the merging subcluster is small ($M\leqslant {1.15}_{-0.09}^{+0.14}$). We also present density, temperature, pseudo-pressure, and pseudo-entropy maps. Based on the pseudo-entropy map, we conclude that the cluster is undergoing a mild merger, consistent with the small Mach number. On the other hand, radio relics extend over Mpc scale in the A3411???12 system, which strongly suggests that a population of energetic electrons already existed over extended regions of the cluster