Browsing by Subject "ICM"

Now showing 1 - 20 of 31
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    The Black-Hole Feedback Valve
    (2016-08-24) Voit, Mark
    Feedback from a central supermassive black hole is an essential component of galaxy evolution models. Without it, those models cannot obtain massive galaxies and galaxy clusters with the observed properties. However, the black-hole feedback mechanism remains mysterious. Somehow, accretion of matter onto the central black hole of a massive galaxy becomes precisely tuned so that it regulates radiative cooling and condensation of gas in a volume many orders of magnitude larger than the black-hole’s gravitational zone of influence. I will discuss how the required coupling can arise through condensation and precipitation of cold clouds out of a galaxy’s circumgalactic medium, through a mechanism in which the ambient entropy gradient acts as a valve that regulates black-hole feedback.
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    The challenge of turbulence in galaxy clusters: physics and particle acceleration
    (2016-08-23) Brunetti, Gianfranco
    Non-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.
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    Cluster kinematics in the post-Hitomi era
    (2016-08-22) Heinz, Sebastian
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    Cluster outskirts as a gateway to the physics of particle acceleration and magnetogenesis
    (2016-08-23) Vazza, Franco
    Cluster 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.
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    Cooling, Star Forma/on and AGN Hea/ng in Galaxy Clusters
    (2016-08-24) Li, Yuan
    AGN feedback is widely considered the major heating source to prevent a classical cooling flow in the center of galaxy clusters. Numerical simulations have shown that momentum-driven AGN feedback can suppress cooling successfully and reproduce cluster properties generally in good agreement with the observations. However, exactly how the jet is coupled to the intra-cluster medium is still unclear. In this work, we study how much heating comes from shock waves, and how much from turbulent dissipation in a simulation where cooling is balanced by AGN feedback. We also examine how the importance of different heating processes changes as a function of time, distance to the center, and the thermal properties of the gas.
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    Cosmic Ray Propaga/on in High β Plasmas
    (2016-08-23) Zweibel, Ellen
    Cosmic 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.
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    Deep Chandra observations as probes of detailed ICM physics
    (2016-08-22) Werner, Norbert
    The 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.
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    Fluid Dynamics in the ICM
    (2016-08-22) Roediger, Elke
    Gas 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.
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    How AGN Jets Heat the Intracluster Medium -- Insights from Simulations
    (2016-08-24) Yang, Hsiang-Yi Karen
    Feedback from active galactic nuclei (AGN) is believed to prevent catastrophic cooling in galaxy clusters. However, how the feedback energy is transformed into heat, and how the AGN jets heat the intracluster medium (ICM) isotropically, still remain elusive. Since recent simulations based on cold gas accretion and momentum-driven jet feedback have shown remarkable successes reproducing properties of the cool cores (CCs), we use such simulations to gain insights into the mechanisms and distributions of AGN heating. In this talk, I will present our findings on the relative importance of shock heating, mixing with the hot thermal gas within bubbles, and turbulent dissipation. I will also show how the thermal balance is established within the entire CC despite the fact that AGN jets preferentially dump the heat near the jet axis.
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    How Turbulence Accelerates Cosmic Rays and Amplifies B-fields in the ICM
    (2016-08-23) Oh, Peng
    I 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.
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    The Impact of Magnetic Fields on Thermal Instability
    (2016-08-22) Ji, Suoqing
    Multiphase 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.
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    Is cosmic ray heating relevant in cool core clusters?
    (2016-08-23) Pfrommer, Christoph
    The 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.org
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    Jet-Lobe Morphology Under Cluster Weather: Inferences on ICM Dynamics and Jet Composition
    (2016-08-24) Li, Hui
    Jets and lobes in the ICM are expected to be affected by their galaxy cluster environment and indeed observations often reveal distortions in jet-lobe morphologies. We present 3D MHD modeling of these systems and study the physical processes that might be occurring. We will discuss several findings: 1) jet survival is non-trivial and it can place strong constraints on the jet composition; 2) jet distortions can be used to infer the ICM conditions, its motion and energetics. Implications for inferring AGN feedback in the ICM will also be discussed.
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    Metal Mixing in the Presence of a Magnetic Field
    (2016-08-24) Scannapieco, Evan
    Random motions are essential to the mixing of entrained fluids, and they are also capable of amplifying weak initial magnetic fields by small-scale dynamo action. I will describe a systematic study of mixing in magnetized media as a function of magnetic Prandtl number and Mach number. Using three-dimensional magnetohydrodynamic simulations that include a scalar concentration field, I show that metallicity gradients are always strongly biased perpendicular to the direction of the magnetic field. This is true both early on, when the magnetic field strength is negligible, and at late times, when the field is strong enough to back-react on the flow. I describe the origin of this anticorrelation and its consequences for modeling metal mixing in the intracluster medium and in other environments.
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    Microphysics of ICM from X-ray images: effective equation of state, turbulence, heating
    (2016-08-22) Zhuravleva, Irina
    The 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.
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    Multiphase Gas and Star Formation in Cluster Cores
    (2016-08-22) Donahue, Megan
    We 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.
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    New High-Order Methods using Gaussian Processes
    (2016-08-22) Lee, Dongwook
    We 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.
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    New Kid on the Block: Kinematic Sunyaev-Zel’dovich Effect
    (2016-08-22) Nagai, Daisuke
    In 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.
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    Non-thermal phenomena in the ICM: insights from radio observations
    (2016-08-23) Bonafede, Annalisa
    Radio 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).