Physics of the Intracluster Medium: Theory and Computation 2016

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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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    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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    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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    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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    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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    Witnessing the growth of the nearest galaxy cluster
    (2016-08-22) Simionescu, Aurora
    We 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.
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    Thermal Instability in Low Entropy Gas Lifted behind Buoyantly-Rising X-ray Bubbles
    (2016-08-24) McNamara, Brian
    Precipitation models indicate that cooling instabilities form when the ratio of cooling time to free-fall in hot atmospheres surrounding massive galaxies falls below ~10, i.e., t_c/t_ff < 10. We show that this criterion is rarely met in central galaxies and is a statistically weaker indicator of cooling instabilities than cooling time alone. Hot atmospheres apparently do not experience large swings in gas density and cooling time in response to powerful AGN outbursts, which is an essential aspect of precipitation models. Based primarily on new ALMA observations, we show instead that molecular gas likely forms from low entropy gas caught in the updraft of rising X-ray bubbles. We suggest that thermal instabilities ensue when the ratio of the cooling time to the infall time of gas lifted to high altitudes falls below unity, i.e., t_c/t_I ~ 1.
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    Shattering cold gas into tiny cloudlets
    (2016-08-24) McCourt, Mike
    Absorption studies of galaxies and galaxy clusters at high-redshift have unexpectedly found that their halos are also full of cold gas, in addition to the theoretically-predicted virialized plasma. These observations typically indicate a relatively modest total fraction of cold gas (~10^{-4}% by volume), yet find it in essentially every sightline through the galaxy. I will show that cold gas clouds are prone to “shattering” into tiny fragments, and that the resulting small clouds naturally reproduce the large area-covering fractions and small volume-filling fractions inferred from observations. This same effect enhances the drag force coupling the dynamics of cold and hot gasses; I will also discuss potential applications to entrainment of cold gas in galaxy winds, and the possibility of using cold gas to constrain the kinematics of the hot ICM.
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    Cluster kinematics in the post-Hitomi era
    (2016-08-22) Heinz, Sebastian
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    Shock Waves in the Outskirt of Galaxy Clusters
    (2016-08-23) Ryu, Dongsu
    Galaxy 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.
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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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    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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    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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    Simulations of AGN jet feedback in galaxy clusters: heating and cooling cycles
    (2016-08-24) Sharma, Prateek
    I will present results from hydrodynamic simulations of AGN jets interacting with the intracluster medium (ICM). Cold gas condensation (triggered by local thermal instability) and a sudden increase in feedback heating result in overheating of the ICM. Such overheating prevents further cooling and accretion of gas onto the central black hole. In absence of heating the core slowly cools again, restarting the heating cycle. Feedback heating driven by cold gas condensation leads to a large scatter between jet power, cold gas mass, core entropy, etc. I will highlight the importance of angular momentum of cold gas in closing the feedback heating loop. I will compare the kinematics of cold gas in our simulations with the ALMA observations of cold gas in cluster cores.
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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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    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, S. Jacob, HITS Heidelberg,
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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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    Triggering and delivery methods for AGN feedback
    (2016-08-24) O'Shea, Brian
    In this talk I present a comparison of several common sub-grid implementations of AGN feedback, focusing on the effects of different triggering mechanisms and the differences between thermal and kinetic feedback (although we also attempt to assess the importance of physical resolution and some AGN jet properties as well). Our main result is that pure thermal feedback that is centrally injected behaves differently from feedback with even a small kinetic component. We do not see large differences between implementations of different triggering mechanisms, as long as the spatial resolution is sufficiently high, probably because all of the implementations tested here trigger strong AGN feedback under similar conditions. Our simulations confirm that AGN can regulate the thermal state of cool-core galaxy clusters and maintain the core in a state that is marginally susceptible to thermal instability and precipitation.
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    Nonlinear Thermal Instability in Radio Mode AGN Feedback
    (2016-08-24) Nulsen, Paul
    Substantial masses of cold gas found at the centers of cool core clusters are widely believed to be produced by thermally unstable cooling of the hot intracluster medium. A small part of the cold gas can power the radio jets that prevent much greater quantities of gas from cooling, sustaining a radio mode feedback cycle. I will argue that nonlinear hydrodynamic processes play a critical role in the onset of thermally unstable cooling, helping to define the necessary conditions for thermal instability. I will also show that uplift by radio lobes is a likely mechanism for driving this nonlinear thermal instability.