3rd ICM Theory and Computation Workshop

11 Aug 2014, 09:00 → 14 Aug 2014, 14:00 Europe/Copenhagen

Auditorium A (Niels Bohr Institute)

Monday 11 August

11:30 → 12:30 Registration

12:30 → 13:45 Welcoming Lunch

13:45 → 14:00 Opening Remarks

Convener: Dr Martin Pessah (Niels Bohr International Academy)

slides nbia-icm-2014-welcome.pdf

14:00 → 17:30 Monday afternoon

Convener: Prof. August Evrard (University of Michigan)

14:00 Gas Flows in Galaxy Clusters

Galaxies and clusters are embedded in gaseous hot atmospheres that serve as repositories of unused fuel for galaxy formation, ejecta from evolved stars, and mechanical energy released by supermassive black holes. The hot gas located within the central galaxy is expected to cool and fuel star formation. Yet most giant elliptical galaxies are "red and dead."  Instead, cooling is suppressed by powerful radio jets that periodically heat the gas in a self-regulating feedback loop.  Radio-mechanical feedback may explain the the dearth of  luminous, blue galaxies predicted by standard Lambda-CDM models and the excess of hot baryons in the Universe.  I will discuss recent studies showing that radio-mechanical feedback  also drives hot outflows at rates of tens of solar masses per year from central cluster galaxies.  I will highlight new results from the Atacama Large Millimeter Array (ALMA) showing that the hot gas that has cooled resides in nascent molecular gas disks and plumes of molecular gas clouds flying in and out of the galaxy.  The ALMA data for the Abell 1835 BCG indicate a molecular outflow at a rate of ~200 solar masses per year behind a pair of buoyantly rising radio/X-ray bubbles.  Apparently, X-ray bubbles couple efficiently to molecular clouds, and this may have broader implication for the evolution of galaxies and supermassive black holes.

Speaker: Dr Brian McNamara (University of Waterloo)

Slides copenhagen_BRM_post.pdf

14:25 Witnessing the ongoing growth of nearby clusters of galaxies

Over the past several years, the comparatively low background of the Suzaku satellite has enabled measurements of the thermodynamic properties of the faint cluster outskirts, opening a new window for the exploration of the ongoing virialization and equilibration processes that occur as part of large-scale structure formation. This has been complemented by several subsequent studies with Chandra, XMM-Newton, and Sunyaev-Zel’dovich experiments such as Planck. I will briefly summarize the highlights from recently finalized Key/Large programs with the Suzaku satellite on the Perseus and Coma clusters, before discussing new results from the Key Project targeting the Virgo Cluster. These nearest, brightest clusters of galaxies are ideal targets for studying in detail the physics of the hot X-ray emitting plasma near their virial radii. I will compare and contrast the results from these different projects in order to shed light on how the thermodynamic properties in the cluster outskirts depend upon the cluster mass, dynamical state, and large-scale structure environment.

Speaker: Dr Aurora Simionescu (ISAS/JAXA)

Slides Copenhagen.pdf

14:50 New observational constraints on microphysical properties of the intracluster plasma

I will present new observational constraints on large-scale thermal conductivity of the ICM, based on radial profiles for relaxed clusters. Constraints on electron-proton equilibration timescale from new analysis of the shock front in A520 and reanalysis of the shock in Bullet will also be presented.

Speaker: Dr Maxim Markevitch (NASA/GSFC)

Slides markevitch.pdf

15:15 Coffee break

16:00 Stratified Turbulence, Heating, Cooling and Diffusion in Galaxy Clusters

I will discuss what can be inferred about the structure of stratified turbulence in galaxy clusters from basic assumptions such as a generalised principle of critical balance [3,4]. I will then discuss how the conclusions from this exercise lead to a useful prescription for deducing velocity spectra in the ICM from the density fluctuation spectra [2] (measurable by existing X-ray observatories). One of the most striking observational consequences one can draw from such observations is that turbulent heating and radiative cooling in the cores of the brightest galaxy clusters appear to match each other locally in radial location (upcoming paper by I. Zhuravleva et al. [1]). Finally, I will discuss the role of turbulent diffusion vs. turbulent heating in a stratified ICM. References: [1] I. Zhuravleva, E. M. Churazov, A. A. Schekochihin et al. 2014, submitted [2] I. Zhuravleva et al. 2014, ApJ 788, L13 [arXiv:1404.5306] [3] S. V. Nazarenko & A. A. Schekochihin 2011, JFM 677, 134 [arXiv:0904.3488] [4] A. A. Schekochihin, I. Zhuravleva & E. M. Churazov 2014, in preparation

Speaker: Prof. Alexander Schekochihin (University of Oxford)

16:25 High-order Numerical Methods for Plasma Simulations on Large-scale High-Performance Computing Architectures

Modeling diverse physical processes using mathematical algorithms has become a successful tool in modern science and engineering. The underlying mathematical models are carefully designed to perform large-scale computer simulations that involve disparate scales of space and time. Such complexities often arise when incorporating various multi-physics components that can be represented by classes of partial differential equations. In the first part, I will discuss key issues in seeking computational solutions on large-scale high-performance Computing (HPC) architectures, and the need for using high-order numerical algorithms on HPC. I describe mathematical algorithms with special attention to two numerical approaches: first, the traditional formulations based on high-order polynomials; second, a new innovative exponentially converging formulation based on Gaussian Process Modeling. Moreover, I will show the importance of fast convergent, high-order accurate numerical methods and how they are crucial for future high-performance exascale computing architectures. In the second part, I will present laboratory astrophysics scientific simulations using the numerical algorithms introduced in the first part. They include large-scale computer simulations of astrophysics and high-energy-density plasma physics, with special emphasis on laser-driven shock experiments to shed lights on the processes behind magnetic field generation and amplification.

Speaker: Dr Dongwook Lee (The University of Chicago)

Slides DongwookLee_ICM.key.pdf

16:50 Discussion

Tuesday 12 August

09:00 → 12:30 Tuesday morning

Convener: Prof. Mateusz Ruszkowski (University of Michigan)

09:00 The origin of cold gas in giant ellipticals and its role in fueling AGN feedback

I will present a multi-wavelength study of the nature and origin of the multi-phase medium in giant ellipticals at the centers of low mass groups of galaxies. All systems with extended Halpha emission in our sample contain significant amounts of cold gas, which is co-spatial with the line emitting nebulae and the lowest entropy X-ray emitting plasma. I will show that while the hot atmospheres of the cold-gas-poor galaxies are thermally stable outside of their innermost cores, the atmospheres of the cold-gas-rich systems are prone to cooling instabilities. This indicates that cold gas in giant ellipticals is produced chiefly by cooling from the hot phase. I will also show that cooling instabilities may develop more easily in rotating systems and discuss the role of cold gas in AGN feedback.

Speaker: Dr Norbert Werner (Stanford University)

Slides Werner_Norbert_CPH.pdf

09:25 Raining onto black holes: chaotic cold accretion driving AGN feedback

It is commonly thought that supermassive black holes mainly accrete the hot gas from the surrounding intracluster medium, following the classic Bondi theory. However, in the presence of heating and turbulence due to the AGN feedback, cold clouds and filaments condense out of the hot phase via nonlinear thermal instability, up to 10s kpc radii. Through unprecedented 3D hydrodynamic simulations reaching a dynamical range up to 10 million, we show that the chaotic inelastic collisions between the cold clouds, filaments, and central torus, promote angular momentum cancellation down to the inner sub-pc region, thus boosting the accretion rate up to 100 times the hot Bondi rate, or comparable to the gas cooling rate. We also present new AMR simulations studying the impact of rotation on the accretion flow with different ICM physics. Chaotic cold accretion is crucial to trigger powerful AGN outflows, which can quench the cooling flow and star formation. We discuss how cold accretion creates a symbiotic link between the black hole and the host galaxy, leading to a tight self-regulated feedback loop, which preserves the cores of groups and clusters in quasi thermal equilibrium throughout cosmic history.

Speaker: Dr Massimo Gaspari (Max Planck Institute for Astrophysics)

Slides Gaspari 2014 - Copenhnagen

09:50 Modeling AGN Feedback in Cool-Core Galaxy Clusters

We study the influence of momentum-driven AGN feedback on cool-core clusters using high-resolution adaptive mesh refinement (AMR) simulations. Run-away cooling first happens only in the central 50 pc region while no local instability develops outside the very center of the cluster. The gas is accreted onto the super-massive black hole which powers AGN jets at an increasing rate as the entropy continues to decrease in the core. The ICM first cools into clumps along the propagation direction of the AGN jets. As the jet power increases, gas condensation occurs isotropically, forming spatially extended (up to a few tens kpc) structures that resemble the observed Hα filaments in Perseus and many other cool-core cluster. Jet heating elevates the gas entropy and cooling time, halting clump formation. The cold gas that is not accreted onto the SMBH settles into a rotating disk. The mass cooling rate averaged over 7 Gyr is ~ 30 M_sun/yr, an order of magnitude lower than the classic cooling flow value (which we obtain in runs without the AGN). Owing to its self-regulating mechanism, AGN feedback can successfully balance cooling with a wide range of model parameters. Besides suppressing cooling, our model produces cold structures in early stages (up to ∼ 2 Gyr) that are in good agreement with the observations. However, the long-lived massive cold disk is unrealistic, suggesting that additional physical processes are still needed.

Speaker: Ms Yuan Li (Columbia University)

Slides Copenhagen2014_Yuan.pdf

10:15 Coffee break

11:00 Thermal instability paradigm for cold gas in cluster cores: effects of thermal conduction, turbulence and AGN jets

I will summarize the thermal instability model for cold gas in cool core clusters. I will describe some recent results from our simulations with thermal conduction, turbulence, and AGN jets.

Speaker: Prateek Sharma (IISc Bangalore)

Slides copenhagen_prateek.pdf

11:25 Spherical accretion and AGN feedback

For a supermassive black hole accreting from a hot, quasi-spherical atmosphere, it is almost inevitable that the fluid approximation fails because particle mean free paths exceed the radius well outside the black hole event horizon. Within this region, the flow needs to be modeled using the Fokker-Planck equation. In the absence of magnetic fields, the flow is analogous to the "loss cone" problem for consumption of stars by a black hole. The accretion rate is suppressed well below the Bondi accretion rate and a significant power must be conveyed outward for the flow to proceed. This situation is complicated significantly by the presence of a magnetic field, but I will argue that the main outcomes are similar. I will also argue that the power emerging from such a flow, although generally far too small to suppress cooling on large scales, is an important ingredient of the AGN feedback cycle on scales comparable to the Bondi radius.

Speaker: Dr Paul Nulsen (Harvard-Smithsonian Center for Astrophysics)

Slides Pnulsen.pdf

11:50 Discussion

12:30 → 13:45 Lunch

13:45 → 16:30 Tuesday afternoon

Convener: Prof. Alexander Schekochihin (University of Oxford)

13:45 Magnetohydrodynamics in a Weakly Collisional Intracluster Medium

The intracluster medium (ICM) of galaxy clusters is a weakly collisional plasma in which the transport of heat and momentum occurs primarily along magnetic-field lines. Anisotropic heat conduction allows convective instabilities to be driven by temperature gradients of either sign: the magnetothermal instability (MTI) in the outskirts of clusters and the heat-flux buoyancy-driven instability (HBI) in their cooling cores. We report on several recent studies we have performed to investigate the MHD of cluster gas in this regime. These include (1) a study of the effect of both anisotropic conduction and viscosity on the nonlinear regime of the MTI and HBI, (2) a study of the role of anisotropic viscosity in inhibiting fluid instabilities at sloshing cold fronts observed in merging clusters, and (3) a study of the saturation of firehose and mirror instabilities in shearing plasmas, which is relevant to the turbulent tangling of magnetic field at microscopic scales (which, in turn, may affect anisotropic transport at macroscopic scales).

Speaker: Prof. James Stone (Princeton University)

Slides ICM.pdf

14:10 Magnetic fields and the Helium content in the intracluster medium

Understanding whether Helium can sediment to the core of galaxy clusters is important for a number of problems in cosmology and astrophysics. For example, our ignorance in the distribution of Helium leads to systematic uncertainties in estimating the density and masses of galaxy clusters. All current models addressing this question are one-dimensional, and ignore the fact that the intracluster medium is a dilute, magnetized plasma, which can effectively channel ions and electrons, leading to anisotropic transport of momentum, heat, and particle diffusion. This anisotropy can lead to a wide variety of instabilities, which could be relevant for understanding the dynamics of the heterogeneous medium. As part of my thesis work, I am shedding light on this problem, by investigating the dynamical role played by gradients in the temperature and mean molecular weight in a magnetized tenuous plasma, such as the ICM. I will present the results of analytical and numerical studies analyzing the wide spectrum of instabilities, and discuss the future prospects of studying the long term evolution of Helium sedimentation in more realistic settings.

Speaker: Mr Thomas Berlok (NBIA)

Slides 3rd_ICM_workshop_2014_-_Berlok.pdf

14:35 Coffee break

15:05 Fluid flow experiments in galaxy clusters: Using cluster galaxies and minor mergers as probes for transport coefficients of the ICM

The ICM transport properties (viscosity, thermal conductivity) and magnetic field structure are still ill-constrained. We use the ICM flows around gas-stripped elliptical cluster galaxies and in merging clusters as direct probes of these ICM properties. Galaxies moving through the ICM experience a head wind that strips off their gaseous atmospheres. The structure of the galaxy-ICM interface and of the stripped gas tails depends on ICM transport coefficients and magnetic field structure, but also on the galaxy infall dynamics. Minor mergers set the ICM in clusters sloshing, which in turn leads to sloshing cold fronts, whose fine-structure depends on the ICM properties. Our team has in hand deep high resolution X-ray data, e.g. of the stripped cluster ellipticals M89, M49, M60, M86, and NGC 1404. These data set have sufficient quality to distinguish different ICM properties -- if we can disentangle the effects of ICM transport properties and ICM dynamics. We do so by one-to-one comparisons between observations and specifically tailored numerical simulations of these galaxies and merging clusters. We report on first evidence for a highly suppressed ICM viscosity in the Virgo cluster.

Speaker: Dr Elke Roediger (Hamburger Sternwarte)

Slides ERoediger_Copenhagen.pdf

15:30 Evolution (or not) of Cool Cores

Observational surveys seeking to measure evolution in the incidence and properties of cool cores in galaxy clusters have obtained seemingly contradictory results. Some claim to detect evolution. Others find no evolution. Reconciling these observations requires a closer look at how cool cores are defined and the physics that governs their characteristics.

Speaker: Mark Voit (Michigan State University)

Slides Copenhagen_12Aug14.pdf

15:55 Discussion

16:30 → 21:00 Conference Dinner: Louisiana Museum of Modern Art

http://www.louisiana.dk/ We have organised a coach that will take us from NBI to the Museum Restaurant. We expect to be back in Copenhagen around 9pm.

Wednesday 13 August

09:00 → 12:30 Wednesday morning

Convener: Prof. Daisuke Nagai (Yale University)

09:00 Cosmic ray heating in cool core clusters

Feedback by active galactic nuclei appears to be critical in balancing radiative cooling of the low-entropy gas at the centers of galaxy clusters and in mitigating the star formation of the brightest cluster galaxies. I consider a model where the heating is provided by the damping of Alfvén waves that are excited by streaming cosmic rays (CRs). Recent radio observations of M87 by LOFAR suggest effective mixing of CRs with the cluster gas and the amount of CRs necessary to explain the observed gamma rays by Fermi and HESS is just right to balance the radiative cooling observed in the X-rays, providing a natural explanation for the observed temperature floor as shown by a thermal instability analysis. I will show first AREPO simulations of the non-linear interplay of CRs that are coupled to magneto-hydrodynamics with the goal to support this picture.

Speaker: Dr Christoph Pfrommer (HITS)

Slides Pfrommer_Copenhagen14.pdf

09:25 Gas density and velocity power spectra in galaxy clusters

Measurements of gas density fluctuations in galaxy clusters using X-ray images are discussed. We argue that for relaxed clusters there is a linear relation between the rms density and velocity fluctuations across a broad range of scales. The normalization of this relation can be predicted, provided that the gas motions are slow enough. The normalization is set at large scales by buoyancy physics, while at small scales the density and velocity power spectra remain proportional to each other because the gas entropy serves as a passive scalar advected by the gas motions. This opens an interesting possibility to measure the velocity power spectra in relaxed clusters using existing Chandra and XMM-Newton images.

Speaker: Dr Eugene Churazov (MPA)

09:50 The Generation, Evolution and Transport of Turbulence During Structure Formation

Turbulence in the ICM is a natural consequence of shocks and large-scale sheared flows produced in structure formation during mergers and from accretion, more generally. This is demonstrated in many simulations. However, the physical conditions involved in turbulence generation and evolution in this context are complex and intermittent in both time and space. Consequently they and their relationships are not yet well characterized. To improve this understanding we have undertaken a high resolution simulation study designed to isolate the dynamical processes responsible for production, evolution and transport of turbulence in the ICM. The study is based on nested grid, zoomed simulations of multiple cluster formation events with uniform spatial resolution (currently 20 kpc) inside the virial radius and high time resolution (currently ~ 50 Myr) outputs throughout the formation history of each cluster. This talk will outline this effort and present initial results.

Speaker: Prof. Tom Jones (U Minnesota)

Slides icm3_twj.pdf

10:15 Coffee break

11:00 Hydrodynamical Simulations of Non-thermal Pressure in Galaxy Clusters

Cosmological constraints from X-ray and microwave observations of galaxy clusters are subjected to systematic uncertainties. Non-thermal pressure support due to internal gas motions in galaxy clusters is one of the major sources of astrophysical uncertainties. Using mass-limited samples of galaxy clusters from high-resolution hydrodynamical cosmological simulations, we characterize the non-thermal pressure fraction profile and study its dependence on redshift, mass, and mass accretion rate. We find that the non-thermal pressure fraction profile exhibit universality across redshift when galaxy cluster radii are defined with respect to the mean matter density of the universe instead of the commonly used critical density. We also find that the non-thermal pressure is predominantly radial, and the gas velocity anisotropy profile exhibits strong universality when galaxy cluster radii are defined with respect to the mean matter density of the universe. However, we find that the non-thermal pressure fraction is strongly dependent on the mass accretion rate of the galaxy cluster. We provide fitting formulae for the universal non-thermal pressure fraction and velocity anisotropy profiles of gas in galaxy clusters. We will discuss implications of our results for cosmological studies based on galaxy cluster counts and SZ power spectrum.

Speaker: Ms Kaylea Nelson (Yale University)

Slides Nelson.pdf

11:25 Temperature Structure of the Intra-Cluster Medium from SPH and AMR Simulations

Analyses of cosmological hydrodynamic simulations of galaxy clusters suggest that X-ray masses can be un- derestimated by 10% to 30%. The largest bias originates by both violation of hydrostatic equilibrium and an additional temperature bias caused by inhomogeneities in the X-ray emitting intra-cluster medium (ICM). To elu- cidate on this large dispersion among theoretical predictions, we evaluate the degree of temperature structures in cluster sets simulated either with smoothed-particle-hydrodynamics (SPH) and adaptive-mesh-refinement (AMR) codes. We find that the SPH simulations produce larger temperature variations connected to the persistence of both substructures and their stripped cold gas. This difference is more evident in no-radiative simulations, while it is reduced in the presence of radiative cooling. We also find that the temperature variation in radiative cluster simulations is generally in agreement with the observed one in the central regions of clusters. Around R500 the temperature inhomogeneities of the SPH simulations can generate twice the typical hydrostatic-equilibrium mass bias of the AMR sample. We emphasize that a detailed understanding of the physical processes responsible for the complex thermal structure in ICM requires improved resolution and high sensitivity observations in order to extend the analysis to higher temperature systems and larger cluster-centric radii.

Speaker: Elena Rasia (University of Michigan)

Slides erasia.pdf

11:50 Discussion

12:30 → 14:00 Lunch

14:00 → 17:05 Wednesday afternoon

Convener: Dr Paul Nulsen (Harvard-Smithsonian Center for Astrophysics)

14:00 Damped and Driven Waves in Galaxy Clusters

Acoustic waves excited by AGN have long been considered an important channel for heating the intracluster medium. Although heating by nearly adiabatic plane waves is quite simple to calculate, heating by spherical waves in a highly dissipative medium is less simple. I'll describe joint work with Vladimir Mirnov (UW-Madison) in which we calculate the driving and damping of acoustic and thermal modes by an oscillating source, and the implications of our results for plasma heating as a function of position within the cluster.

Speaker: Prof. Ellen Zweibel (U Wisconsin-Madison)

Slides icm_3_copenhagen_cp.pdf

14:25 Shock Waves in Galaxy Clusters

The intracluster medium is an extreme plasma that cannot be studied in terrestrial laboratories. Its thermal state is largely set by shocks that occur during virialisation and cluster mergers. Shocks are also a site for particle acceleration. Observations in the X-ray and radio band are beginning to tell us more about the physics of these shocks - however, the results from recent observations have raised a number of fundamental questions. I will present recent observations that challenge our current understanding of shocks in clusters. New ways of studying them are discussed.

Speaker: Prof. Marcus Bruggen (University of Hamburg)

Slides Copenhagen.pdf

14:50 Spectral Composition of Cosmic Rays in Galaxy Clusters

We present cosmological simulations of galaxy clusters following the distribution of cosmic rays as they are injected, accelerated, and transported. Implications for X-ray, radio, and Sunyaev-Zel'dovich effect observations are discussed, in particular in the context of constraining ICM plasma properties near shock fronts.

Speaker: Samuel Skillman (Stanford, SLAC)

15:15 Coffee break

16:00 Kinetic simulations of cosmic ray acceleration at shocks

Hybrid particle in cell simulations (kinetic protons and fluid electrons) are providing us with unprecedented insights into the microphysics of collisionless shocks, also attesting to their ability to accelerate particles and to generate magnetic fields. I present state-of-the-art 2D and 3D simulations of non-relativistic shocks, discussing under which conditions (shock strength and inclination) ions are injected and energized via diffusive shock acceleration. I also show how resonant and non-resonant instability generate magnetic turbulence, and illustrate the energy spectrum of the self-generated turbulence. Finally, also exploiting the results of full PIC simulation, I present the first self-consistent description of how ions and electrons are injected into the acceleration process.

Speaker: Dr Damiano Caprioli (Princeton University)

Slides Caprioli.pdf

16:25 Discussion

17:05 → 18:05 Beer in the Park

Thursday 14 August

09:00 → 12:30 Thursday morning

Convener: Mark Voit (Michigan State University)

09:00 Gas Accretion and Non-Equilibrium Phenomena in the Outskirts of Galaxy Clusters

In recent years, the outskirts of galaxy clusters have emerged as one of the new frontiers for studying the physics of clusters and intergalactic medium. In this talk, I will discuss the physics of galaxy clusters outskirts, focusing on roles of cosmic gas accretion and non-equilibrium phenoma (such as gas inhomogeneities, turbulence, and electron-proton equilibration process operating in the cluster outskirts). I will discuss implications for interpreting deep Chandra X-ray observations of A133.

Speaker: Prof. Daisuke Nagai (Yale University)

Slides Nagai_Copenhagen14.pdf

09:25 Simulating galaxy populations within clusters

Simulations of galaxy clusters have become significantly more realistic over time, and can now reliably reproduce many observable quantities outside of the cluster core. Unfortunately, however, many observables cannot be reproduced by the current generation due to challenges relating to numerical resolution or lack of physics. One set of such observables pertains around cluster galaxies - given the dynamic range required in space and in mass, it is very hard to accurately simulate both the properties of galaxies within clusters and the cluster itself. In this talk, I discuss our efforts to bridge this gap using "galaxy particles" within the Enzo code. These galaxy particles combine cosmological hydrodynamical simulations of the intracluster medium with semi-analytic models for galaxy formation and evolution, and can be used to study the cluster galaxies as well as their interactions with, and effects upon, the intracluster medium. I will present our early results using this technique, and discuss some of the possibilities for the future.

Speaker: Prof. Brian O'Shea (Michigan State University)

Slides OShea_3rd_ICM_final.pdf

09:50 Analytical model for non-thermal pressure in galaxy clusters

I will present an analytical model we recently developed for intracluster non-thermal pressure in the virial region of relaxed clusters. The core of this model is a first-order differential equation describing the evolution of non-thermal velocity dispersion. This equation is based on insights gained from observations, numerical simulations, and theory of turbulence. It describes that the non-thermal energy is sourced, in a self-similar fashion, by the mass growth of clusters via mergers and accretion, and dissipates with a time scale determined by the turnover time of the largest turbulence eddies. Our model predicts a radial profile of non-thermal pressure for relaxed clusters. The non-thermal fraction increases with radius, redshift, and cluster mass, in agreement with numerical simulations. The radial dependence is due to a rapid increase of the dissipation time scale with radii, and the mass and redshift dependence comes from the mass growth history. Combing our model for the non-thermal fraction with the Komatsu-Seljak model for the total pressure, we obtain thermal pressure profiles, and compute the hydrostatic mass bias. We find typically 10% bias for the hydrostatic mass enclosed within r500.

Speaker: Dr XUN SHI (Max Planck Institute for Astrophysics)

Slides nonth_model_copenhagen.pdf

10:15 Coffee break

11:00 The Effects of Accretion History on the Cluster Temperature Profile and Cosmological Observables

We have recently shown using a simple hydrostatic prescription and accretion histories from cosmological simulations that for a group or cluster of a given mass that there is a scatter of temperature and pressure profiles. In particular, we find that for almost every system, even in the presence of conduction and convection, that the temperature decreases outwards with a range of scatter about the mean profile. I will quantify this range of temperature profiles and explain the phenomenology as well as potential for non-thermal pressure support. I will also discuss the effects of this temperature and pressure variation on X-Ray and SZ measurements on both the individual cluster level how it translates into an uncertainty in mass for a given SZ signal. Finally, I will quantify what this implies for cosmological parameter estimation and uncertainty in parameters like sigma-8.

Speaker: Dr Ian Parrish (Canadian Institute for Theoretical Astrophysics)

11:25 Cosmic rays in galaxy clusters and their non-thermal emission

Observations of giant radio halos and radio relics in galaxy clusters demonstrate the presence of synchrotron emitting electrons with GeV energies in more than 50 clusters. The precise origin of these radio emitting electrons is, however, still unclear. In this talk I discuss two classes of cosmic ray models, hadronic and reacceleration, that can explan the origin of electrons in giant radio halos. In addition I use gamma-ray upper limits to constrain these models, and discuss the prospects for detecting the cosmic ray induced gamma-ray emission in the future. Finally, I show how structure formation shocks can leave behind an aged MeV electron population in the cluster outskirts. This population can be revived through diffusive shock acceleration in radio relics and dominate over fresh injection at low Mach numbers, allowing weak shocks which would be otherwise invisible to glow in radio emission.

Speaker: Dr Anders Pinzke (Dark Cosmology Centre, University of Copenhagen)

Slides NBI.pinzke.pdf

11:50 Discussion

12:30 → 14:00 Lunch