Physics of the Intracluster Medium: Theory and Computation 2016
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Item Cluster kinematics in the post-Hitomi era(2016-08-22) Heinz, SebastianItem Viscosity, Conduction, Dynamo, and Convection in a Weakly Collisional Magnetized Plasma(2016-08-22) Kunz, MatthewI will report on recent advances in our understanding of stability, transport, and magnetic-field amplification in weakly collisional magnetized plasmas, such as the intracluster medium of galaxy clusters. These include: (1) the linear properties of the magnetothermal instability in a collisionless plasma; (2) the regulation of viscous transport by ion-Larmor-scale kinetic instabilities; (3) the efficacy of heat conduction in the presence of ion-Larmor-scale magnetic mirrors; and (4) turbulent dynamo in a collisionless plasma. All of these subjects are connected by a deceivingly simple question: what is the evolution of a plasma in which the long-wavelength (“fluid”) and small-wavelength (“kinetic”) scales are simultaneously driven unstable by the same free-energy source?Item Microphysics of ICM from X-ray images: effective equation of state, turbulence, heating(2016-08-22) Zhuravleva, IrinaThe only Hitomi direct velocity measurement in the core of the Perseus cluster showed a surprising consistency with the indirect velocity measurements from the analysis of gas fluctuations imprinted in the high-resolution X-ray images provided by Chandra. This result motivates to extend the analysis of fluctuations on a sample of galaxy clusters, which I will present during the meeting. In particular, I will discuss the effective equation of state of ICM fluctuations, velocity measurements on a range of spatial scales, down to small, microphysical scales, and will address the role of gas motions in the ICM heating. I will emphasize the main assumptions of the analysis, which should be studied numerically in future.Item High-resolution micro-calorimeter X-ray spectroscopy of the core of the Perseus Cluster of galaxies(2016-08-22) Hitomi CollaborationItem Multiphase Gas and Star Formation in Cluster Cores(2016-08-22) Donahue, MeganWe will present new Hubble Space Telescope COS and STIS spectroscopy of the central knots in the Brightest Cluster Galaxy in the CLASH cluster RXJ1532.9+3021. X-ray observations show this cluster hosts a radio AGN that has excavated large holes in the X-ray intracluster medium. The X-ray gas in the core of this cluster is extremely luminous. Although the star formation visible in the cluster is higher than that of many starburst galaxies (> 100 solar masses per year, estimated from FIR and UV -based SFRs), the AGN appears to preventing a far higher rate of star formation. We place very strong limits on the rate of gas cooling from intermediate gas temperatures (~1 million K) using UV spectroscopy. The UV spectra from the center of this BCG bear a strong resemblance to that of the most extreme Lyman Break galaxies at z~3, including significant UV continuum and absorption line features from recently formed hot stars and Lyman alpha emission and Lyman beta absorption consistent with winds driven by hot stars. We will discuss the implications of the state of this system in context with the handful of other active BCGs that have spectroscopy at similar rest wavelengths.Item New Kid on the Block: Kinematic Sunyaev-Zel’dovich Effect(2016-08-22) Nagai, DaisukeIn recent years, the kinematic Sunyaev-Zel’dovich (kSZ) effect measurements of galaxy cluster peculiar velocities have emerged as a new promising probe of cosmology and fundamental physics. However, the power of kSZ cosmology is fundamentally limited by an uncertainty the optical depth of galaxy clusters, which depends sensitively on still poorly understood astrophysical processes, such as star formation and feedback from supernovae and active galactic nuclei on the hot X-ray emitting intracluster medium. I will describe the current state of affairs on our understanding of the optical depth of galaxy clusters from both simulations and observations and discuss a roadmap for the use of kSZ effect (and other cluster-based techniques) as a precision cosmological probe in the era of large-scale CMB and galaxy surveys.Item Witnessing the growth of the nearest galaxy cluster(2016-08-22) Simionescu, AuroraWe present results from the last Suzaku Key Project dedicated to cluster outskirts studies, namely a mosaic of 60 pointings exploring the Virgo Cluster from its centre out to beyond the virial radius along 4 different directions. We discuss the thermodynamical properties in the intracluster medium of this dynamically young system, focusing on departures from hydrodynamic equilibrium and from the expected baseline pressure and entropy profiles. Moreover, we present measurements of the chemical composition of the ICM at large radii and discuss the implication of these results for the chemical enrichment history in the Universe.Item New High-Order Methods using Gaussian Processes(2016-08-22) Lee, DongwookWe present an entirely new class of high-order numerical algorithms for computational fluid dynamics. The new method is based on the Gaussian Processes (GP) modeling that generalizes the Gaussian probability distribution. Our approach is to adapt the idea of the GP prediction technique which utilizes the covariance kernel functions, and use it to reconstruct a high-order approximations for computational simulations. We propose the GP high-order method as a new numerical high-order formulation, alternative to the conventional polynomial-based approaches. We will show that the GP method is shown to be much faster in both convergence and performance rates than the popular choices of polynomial-based high-order methods such as PPM, WENO-5, and WENO-Z.Item Deep Chandra observations as probes of detailed ICM physics(2016-08-22) Werner, NorbertThe spectacular, sub-arcsecond imaging capabilities of the Chandra X-ray Observatory will remain unsurpassed for at least the next 20 years, and deep observations with this instrument still provide opportunities for breakthroughs in our understanding of the microphysics of the hot plasma. I will present new results from deep Chandra observations of the Virgo cold front and the Ophiuchus Cluster, which allow us to place constraints on the effective conductivity and viscosity of the ICM. The deep 500 ks observation of the Virgo cold front reveals intriguing quasi-linear features that may be due to the amplification of magnetic fields by gas sloshing in wide layers below the cold front. The core of the Ophiuchus Cluster lacks strong AGN activity, which may be due to bulk of the cooling taking place offset from the central supermassive black hole. The observed unchecked cooling where a largely dormant AGN is offset from the cooling flow solidifies the idea that AGN play a key role in maintaining the cooling/heating balance in cluster cores.Item Fluid Dynamics in the ICM(2016-08-22) Roediger, ElkeGas stripping of elliptical galaxies in clusters is accompanied by a wealth of fluid dynamics features, including an upstream contact discontinuity between the galactic gas and the cluster ICM, the upstream bow shock, Kelvin-Helmholz-instabilities at the sides of the galactic atmosphere, the galaxy’s gas tail and its wake along its orbit through the ICM. The prominence of each of these features is modified by additional aspects or processes, e.g., the stage of the stripping process, the ellipticity of the galaxy’s potential, transport coefficients in the ICM, or simultaneous AGN activity. I will compare our recent simulations with deep X-ray observations of several galaxies.Item The Impact of Magnetic Fields on Thermal Instability(2016-08-22) Ji, SuoqingMultiphase gas structure is ubiquitous in our universe which exists in both galaxy clusters and galaxy halos. Recent results from COS (Cosmic Origins Spectrograph) suggest that large quantities of cold gas with temperature of a few 10^4 K are found in so called circumgalactic medium (CGM), which extends up to galactic virial radius (~ 100 kpc). However, the origin and stability of such cold gas still remains unclear. In this presented ongoing work, we explore thermal instability as a possible mechanism, and find that the existence of magnetic fields can significantly enhance thermal instability and modify the structure of multiphase gas (even for very weak fields), which is applicable for both ICM and CGM.Item How Turbulence Accelerates Cosmic Rays and Amplifies B-fields in the ICM(2016-08-23) Oh, PengI address the role of turbulence in accelerating cosmic rays in cluster radio halos, and whether matching the observations requires fine tuning. I also consider how turbulence induced by shock-clump interactions amplify B-fields, and implications for radio relics.Item Cluster outskirts as a gateway to the physics of particle acceleration and magnetogenesis(2016-08-23) Vazza, FrancoCluster outskirts are expected to be the region where there is maximum conversion of infall gas kinetic energy into the thermal/non-thermal energy components. of the intracluster medium. In this talk I will present recent studies targeting the interplay of turbulence, shocks, magnetic fields and particle acceleration in galaxy cluster outskirts. Using high-resolution grid simulations with the ENZO code, I will discuss how simulations and available observations of non-thermal emissions can constrain, both, the acceleration efficiency of particles at shocks and the origin of extragalactic magnetic fields.Item Non-thermal phenomena in the ICM: insights from radio observations(2016-08-23) Bonafede, AnnalisaRadio observations provide the best tool to understand the physics and the evolution of the non-thermal component n the Intra-cluster Medium. In particular, they probe(re)acceleration processes due to shocks and turbulence, allowing one to explore turbulence and low Mach number shock acceleration. In this talk, I will present the main discoveries done in the past years thanks to the advent of low frequency observatories (LOFAR and GMRT).Item Cosmic Ray Propaga/on in High β Plasmas(2016-08-23) Zweibel, EllenCosmic ray propagation in galaxy clusters is important for understanding nonthermal emission (or its absence) from these systems, and for understanding the energetics of AGN and starforming galaxies. We discuss work in progress on the driving and damping of the cosmic ray streaming instability under the high beta conditions prevailing in cluster plasmas, and the implications for cosmic ray propagation and confinement.Item Superdiffusion and Streaming of Cosmic Rays(2016-08-23) Lazarian, AlexI shall relate the modern understanding of MHD turbulence with the basic processes of cosmic ray propagation and acceleration. First of all, I shall show that on the scales less than the turbulence injection scale, which is approximately 100 pc for the Milky Way, cosmic rays undergo super diffusion in the direction perpendicular to the mean galactic magnetic field. Then I will show how turbulence damping of the streaming instability depends on the regime of turbulence, e.g. strong Alfvenic or weak Alfvenic turbulence, and what are the astrophysical consequences of this instability. In particular, I shall show that the in the Galactic halo the weak turbulence and not strong turbulence is responsible for the damping of the streaming instability. Finally, I shall discuss how these two processes affect cosmic ray acceleration in shocks.Item Shock Waves in the Outskirt of Galaxy Clusters(2016-08-23) Ryu, DongsuGalaxy clusters are continuously disturbed by major and minor mergers of clumps and gas infall along filaments of the WHIM. Such activities induce shock waves, which are observed as radio relics and X-ray shocks in cluster outskirts. In this talk, we discuss the properties of shocks that form in sample galaxy clusters from simulations of the large-scale structure of the universe, with a special emphasis on their temporal evolution. We present the radio and X-ray manifestations of simulated shocks, and compare them with observed characteristics of radio relics and X-ray shocks.Item Re‐acceleration Model for Radio Relics in Galaxy Clusters(2016-08-23) Kang, HyesungRadio relics are diffuse radio sources found in the outskirts of galaxy clusters and they are thought to trace synchrotron-emitting electrons accelerated via diffusive shock acceleration (DSA) at merger-driven shocks. For some radio relics, the shock Mach number inferred from the radio spectral index is often larger than that estimated from the discontinuities in X-ray observations. Here we study the re-acceleration model in which a weak shock encounters the intracluster plasma with a pre-existing population of relativistic electrons. Toward this end, we perform DSA simulations of spherical shocks with the parameters relevant for cluster radio relics and calculate the ensuing radio synchrotron emission from re-accelerated electrons. We demonstrate that the re-accelerated electron spectrum depends on the shape of the preshock electron spectrum, so the shock Mach number cannot be estimated directly from radio spectral index. Moreover, this scenario alleviates the problems regarding low acceleration efficiency of weak shocks and low frequency of merging clusters with detected radio relics.Item Is cosmic ray heating relevant in cool core clusters?(2016-08-23) Pfrommer, ChristophThe absence of large cooling flows in cool core clusters appears to require self-regulated energy feedback by active galactic nuclei (AGNs) but the exact heating mechanism has not yet been identified. Here, we analyse whether a combination of cosmic-ray (CR) heating that is provided by the AGN and thermal conduction can offset radiative cooling. Using a large sample of about 40 cool core clusters, we determine steady state solutions of the hydrodynamic equations that are coupled to an evolution equation for the CRs. We find stable solutions that match the observed density and temperature profiles for all our clusters well. Radiative cooling is balanced by CR heating in the cluster centres and by thermal conduction on larger scales, thus demonstrating the relevance of both heating mechanisms. Our mass deposition rates vary by three orders of magnitude and are linearly correlated to the observed star formation rates. Clusters with large mass deposition rates show larger cooling radii and require a larger radial extent of the CR injection function. However, the resulting radio-synchrotron and gamma-ray emission of clusters that are hosting radio mini halos is in conflict with observational data suggesting that these clusters cannot be stably heated by CRs. Most interestingly, this inability of supplying sufficient heat to the cooling cluster gas is reflected in the thermodynamic profiles of these cool cores: clusters with radio mini halos are characterised by the largest cooling radii, star formation and mass deposition rates in our sample and thus signal the presence of a higher cooling activity. On the contrary, clusters without radio mini halos show little signs of cooling and appear to be stably heated. This diversity of cool cores for the first time demonstrates evidence for a heating-cooling cycle in cool cores with potentially CR heating from AGNs as the underlying physical source of heat. Contributors:C.Pfrommer, S. Jacob (HITS Heidelberg)C. Pfrommer, HITS Heidelberg, christoph.pfrommer@h-its.org S. Jacob, HITS Heidelberg, svenja.jacob@h-its.orgItem The challenge of turbulence in galaxy clusters: physics and particle acceleration(2016-08-23) Brunetti, GianfrancoNon-thermal phenomena in galaxy clusters suggest that a fraction of the energy that is associated with large-scale motions in the intra-cluster-medium (ICM) is channeled into electromagnetic fluctuations on small scales. This poses fundamental questions about the microphysics of the ICM: presumably a plasma that is weakly collisional, stirred and unstable. I will discuss relevant points of the physics of turbulence in the ICM and focus on the mechanisms of particle acceleration mediated by turbulence in the ICM. Observational consequences on cluster-scale radio emission and on the high energy emission from galaxy clusters are discussed.