6th ICM Theory and Computation Workshop

Auditorium A (Niels Bohr Institute)

Auditorium A

Niels Bohr Institute

Blegdamsvej 17 Copenhagen

  • Alankar Dutta
  • Alex Lazarian
  • Alexander Schekochihin
  • Amit Singh
  • Annalisa Pillepich
  • August Evrard
  • Brian McNamara
  • Carter Rhea
  • Christoph Pfrommer
  • Christopher Chen
  • Daisuke Nagai
  • Damiano Caprioli
  • Deovrat Prasad
  • Dylan Nelson
  • Elena Bellomi
  • Eugene Churazov
  • Evan Scannapieco
  • Evan Yerger
  • Francisco Ley
  • Gianfranco Brunetti
  • Hao Zhang
  • Hsiang-Yi Karen Yang
  • Iraj Vaezzadeh
  • Irina Zhuravleva
  • Jean-Paul Breuer
  • Jennifer Stafford
  • John ZuHone
  • Julie Hlavacek-Larrondo
  • Lorenzo Maria Perrone
  • Lynn Wilson
  • Léna Jlassi
  • Marcus Bruggen
  • Mark Voit
  • Martin Pessah
  • Mateusz Ruszkowski
  • Max Gronke
  • Maxim Markevitch
  • Megan Donahue
  • Nhut Truong
  • Norbert Werner
  • Paul Nulsen
  • Philipp Grete
  • Prakriti Pal Choudhury
  • Prateek Sharma
  • Rainer Weinberger
  • S. Peng Oh
  • Sebastian Heinz
  • Thomas Berlok
  • Thomas Jones
  • Tomáš Plšek
  • Tsun Hin Navin Tsung
  • Urmila Chadayammuri
  • Wonki Lee
  • Yuan Li
Email inquires to
    • 1:00 PM 1:30 PM
      Registration Auditorium A

      Auditorium A

      Niels Bohr Institute

      Blegdamsvej 17 Copenhagen
    • 1:30 PM 1:45 PM
      Opening Remarks Auditorium A

      Auditorium A

      Niels Bohr Institute

      Blegdamsvej 17 Copenhagen
      Convener: Martin Pessah (Niels Bohr International Academy)
    • 1:45 PM 5:15 PM
      Monday afternoon: Cosmological simulations
      Convener: Christoph Pfrommer (AIP Potsdam)
      • 1:45 PM
        The Physics of Galaxy Cluster Outskirts 25m

        The outskirts of galaxy clusters is one of the new frontiers at the crossroads of cosmology and astrophysics, promising to serve as a powerful laboratory for studying the cosmological growth of structure as well as rich gastrophysics in the infall regions around the most massive dark matter halos. In this talk, I will discuss recent advances in our understanding of the physics of the virialization region in the outskirts of galaxy clusters based on a suite of cosmological simulations, including Omega500 and Illustris-TNG hydrodynamical simulations and the Uchuu-Universe Machine mock galaxy catalogs. Specifically, I will introduce (a) the puzzling offset between the locations of the splachback and the accretion shock radius and (b) how galaxies trace the locations of the splashback radius, with highlights on their possible physical origins and impact for modeling and interpreting large datasets from ongoing and future multi-wavelength (X-ray, SZ, and optical) cluster surveys.

        Speaker: Daisuke Nagai (Yale University)
      • 2:10 PM
        X-ray manifestations of AGN feedback, mergers and satellite accretion with the IllustrisTNG simulations 25m

        With the cosmological magneto-hydrodynamical simulations IllustrisTNG (https://www.tng-project.org/), we are putting together ever more quantitative and plausible evidences as to the role that feedback from SMBHs can have in determining the physical properties of the gas in and around galaxies, all the way up to the mass scale of galaxy clusters. Powerful energy injections originating from the innermost regions of massive galaxies can drive 100s-1000s km/s gas outflows (Nelson+2019a, Pillepich+2021) that in turn shock heat the gas within and around them (Weinberg+2017), decreasing the density (Terrazas+2020, Davies+2020) and increasing the entropy and cooling times of the gaseous haloes (Zinger+2020). In practice, according to our models, SMBH feedback strongly modulates the physical properties of the gas that determine its X-ray emission and the kinematics of the hot gas (Truong+2020, Truong+2021a,b). The same model for SMBH feedback produces the most diverse manifestations in the gas distribution, thermodynamics and kinematics: from large-scale, coherent features of overpressurized and X-ray emitting gas that impinge into the gaseous halo of simulated Milky Way- and Andromeda-like galaxies and that are reminiscent of the eROSITA and Fermi bubbles in the Galaxy (Pillepich+2021); to pressure waves around brightest cluster galaxies that are reminiscent of the ones observed in Perseus. At the same time, the intra-halo gas is also affected by accretion events, such as satellite galaxies plunging into massive clusters at supersonic speeds. With IllustrisTNG, we predict that such galaxies, which undergo ram pressure stripping and are often called jellyfish, frequently produce large-scale bow shocks in their surrounding intra-cluster medium (Yun+2019). I will give an overview of these simulation-based results, our attempts to connect them to future X-ray imaging and IFU observations, and introduce a new simulation suite: TNG-Cluster (co-PIs: Pillepich/Nelson), where we deliver ~350 haloes more massive than 10^14.5 Msun, including ~90 truly massive clusters (~10^15 Msun), with the same all-encompassing underlying full-physics IllustrisTNG model and with a baryonic mass  resolution of ~10^7 Msun and ~kpc-level spatial resolution.

        Speaker: Annalisa Pillepich (MPIA Heidelberg)
      • 2:35 PM
        The interplay of SMBH feedback and the ICM in cosmological simulations 25m

        I will discuss highlights from our recent simulation results, focusing on the interplay of supermassive black hole feedback and the halo gas of massive groups and clusters. I will touch on (i) baryon redistribution and clusters as closed boxes, (ii) turbulence and kinematics in the ICM due to cosmological assembly versus feedback, (iii) the virial shock and the outskirts/IGM interface, (iv) cooling, cool gas, and star-formation in clusters, including its episodic nature. These studies are based on cosmological magneto-hydrodynamical simulations, primarily with the IllustrisTNG model, including the upcoming TNG-Cluster project.

        Speaker: Dylan Nelson (Heidelberg University)
      • 3:00 PM
        Coffee break 45m
      • 3:45 PM
        Magnetic Fields, Jets, Velocities, and Cosmic Rays in Merging Galaxy Clusters (via Zoom) 25m

        As evidenced by cold fronts, surface brightness fluctuations, and the observations of the Perseus Cluster by Hitomi, cool-core clusters have subsonic ("sloshing") gas motions in their cores. The same clusters typically host AGN jets and bubbles in the core region. These two processes can interact with each other in interesting ways. In this talk, I will show recent results from MHD simulations of such motions in the ICM to discuss their effects in the context of their interaction with AGN jets and bubbles, and the transport of AGN-injected cosmic rays. I will close the talk by switching gears slightly and presenting what velocity measurements we may expect from mock observations of a major merging cluster using microcalorimeters such as XRISM and Athen

        Speaker: John ZuHone
      • 4:10 PM
        Effects of supermassive black hole feedback on galactic hot atmospheres 25m

        Current observations reveal that supermassive black holes (SMBHs) are nearly ubiquitous in massive galaxies in the local Universe. They are thought to be the central engines powering the feedback from active galactic nuclei (AGN), which is a standard ingredient in current cosmological simulations of galaxies and clusters of galaxies. In this talk I will present our recent results on the effects of SMBH-driven energetic feedback on galactic hot atmospheres, i.e. on the hot gas in and around galaxies. These are based on analyzing state-of-the-art cosmological simulations, such as IllustrisTNG (https://www.tng-project.org/) and others, in comparison to current X-ray observations. In particular, I will focus the discussion on two main concepts: i) X-ray signatures of SMBH feedback as quenching mechanism, and ii) effects of SMBH feedback on the spatial distribution of the circumgalactic gas.

        Speaker: Nhut Truong (Max Planck Institute for Astronomy)
      • 4:35 PM
        Reconstructing Cluster Merger Histories using Triple Merger Simulations 5m

        We present a suite of 13 triple merger simulations using the FLASH hydrodynamic + N-body code which we use to derive a means of distinguishing a multi-merger system from a simple binary merger system using sloshing cold fronts. Our simulations explore different trajectories of a 1:10:10 triple merger. In particular we focus on the growth rate of sloshing cold fronts and the potential impact multi-mergers have on this. We see that the growth of sloshing cold fronts is more complicated than previously thought.

        Speaker: Iraj Vaezzadeh (University of Hull)
      • 4:40 PM
        Simulations of Large-Scale Cold Fronts and Subcluster Interactions in the Perseus Cluster 5m

        The Perseus Cluster is a well-studied system in the X-rays, presenting spiral-shaped cold fronts which extend to large radii. These features are believed to be caused by the sloshing motion of the gas after the passage of a subcluster. We present simulations of the formation of sloshing cold fronts in Perseus using the AREPO magnetohydrodynamics code, with the aim of reproducing the positions of the observed fronts. Our simulations explore a range of initial conditions, including different subcluster masses, impact parameters, and initial magnetic fields. We will show how these parameters affect the formation and propagation of the cold front.
        How old is the large-scale cold front? Does its existence constrain the space of initial parameters? What can we learn about the perturbing subcluster from the observations of Perseus cold fronts?

        Speaker: Elena Bellomi (Harvard-Smithsonian Center for Astrophysics)
      • 4:45 PM
        Discussion: Cosmological simulations 30m
    • 5:15 PM 6:30 PM
      Welcome reception 1h 15m
    • 8:30 AM 9:00 AM
      Morning coffee 30m
    • 9:00 AM 12:30 PM
      Tuesday morning: Turbulence
      Convener: Prateek Sharma (IISc Bangalore)
      • 9:00 AM
        Dynamical impact of magnetic fields and stratification on turbulence in massive galactic halos 25m

        I will discuss recent theoretical work related to the impact of magnetic fields and gravitational stratification in the hot halos of massive galaxies and clusters. Specifically, I will demonstrate that even extremely weak magnetic fields can dramatically alter the feeding the supermassive black holes and the dynamics of cold gas precipitating out of the circumgalactic (CGM) and intracluster (ICM) medium. Furthermore, I will show that, contrary to textbook expectation, turbulence in the CGM/ICM should be non-Kolmogorov in nature. These findings have implications for heating of the CGM/ICM, the interpretation of observations, and for our understanding of AGN feedback in general.

        Speaker: Mateusz Ruszkowski (University of Michigan)
      • 9:25 AM
        Turbulence in the Intra-cluster Medium 25m

        The intra-cluster medium (ICM) in the centers of galaxy clusters is heavily influenced by the ``feedback’’ from supermassive black holes (SMBHs) jets, which prevents a catastrophic cooling and suppresses star formation. However, exactly how jets influence the surrounding ICM is unclear. Due to the limited spatial and spectral resolutions of X-ray telescopes, it has been rather challenging to observe turbulence in the hot ICM driven by SMBHs. Recently, my group developed a new method to measure turbulence in the hot ICM using high-resolution optical data of the multiphase filaments that are ubiquitous in cluster centers. We study the kinematics of the filaments by measuring their velocity structure function (VSF) over a wide range of scales in the centers of nearby galaxy clusters. The motions of the filaments are turbulent in all clusters. There is a clear correlation between features of the VSFs and the activities of the SMBHs, suggesting that SMBHs are the main driver of turbulent gas motions in the centers of galaxy clusters. The slope of the VSF differs from the classical Kolmogorov expectation and varies from system to system. Several theoretical explanations have been proposed but further studies are needed from both the simulation side and the observational side.

        Speaker: Yuan Li (University of North Texas)
      • 9:50 AM
        Cavity Detection Tool (CADET) 5m

        The study of jet-inflated X-ray cavities in early-type galaxies provides a powerful insight into the energetics of galactic atmospheres and the AGN feedback phenomenon. Properly estimating their total extent is, however, non-trivial, prone to biases and nearly impossible for poor-quality data. We present a novel and automized method developed for producing unbiased size estimates of X-ray cavities from raw Chandra images.

        Speaker: Tomáš Plšek (Masaryk University)
      • 9:55 AM
        Idealized cloud crushing simulations in expanding background 5m

        Cold gas is ubiquitously found in the circumgalactic and intracluster medium (CGM/ICM). Clouds can form and be accelerated in the expanding winds generated by galactic outflows. I present our recent work where we study the growth and survival of cold, dense structures/clumps in these environments. We have designed a cloud crushing setup that accounts for the movement of the dense cloud in a dynamically expanding background wind. This is a novel simulation setup and more realistically models the background outflow compared to the "vanilla" cloud crushing setup studied extensively in the literature. We also quantify the effect of cloud geometry on the survival and growth of these clouds. The motivation is to generalize cloud crushing in order to encompass CGM environments with cold clumps as well as the filamentary structures in the ICM or ram-pressure stripped tails of jellyfish galaxies.

        Speaker: Alankar Dutta (Indian Institute of Science)
      • 10:00 AM
        Science communication with the Galactic Atmospheres Forum 5m


        Speaker: Mark Voit (Michigan State University)
      • 10:05 AM
        Numerical simulation on the redistribution of AGN jet tails in merging cluster: Example of Abell 514 5m

        The bulk motion of ICM determines the morphology of AGN jet tails in galaxy clusters. In this study, we use idealized simulations to understand the formation process of the 800 kpc-size bent AGN radio jet tail that we recently discovered in the merging cluster Abell 514. By comparing the observed X-ray morphology with simulations, we constrain the merger history of Abell 514 as a ~2:1 mass ratio collision. We will present how injected jet clouds are redistributed during cluster merger and discuss the remaining challenges in explaining Abell 514.

        Speaker: Wonki Lee (Yonsei University)
      • 10:15 AM
        Coffee break 45m
      • 11:00 AM
        Magneto-Thermal Instability in Galaxy Clusters 25m

        In the intracluster medium (ICM) of galaxies, exchanges of heat across magnetic field lines are strongly suppressed. This anisotropic heat conduction, in the presence of a large-scale temperature gradient, destabilizes the outskirts of galaxy clusters via the magneto-thermal instability (MTI), and thus supplies a source of observed ICM turbulence. Our aim is to take a fresh look at the problem and construct a general theory that (a) explains the MTI saturation mechanism and (b) provides scalings and estimates for the turbulent levels. We simulate MTI turbulence with a Boussinesq code, SNOOPY, which allows us to carry out an extensive sampling of the parameter space. In two dimensions the saturation mechanism involves an inverse cascade carrying kinetic energy from the short MTI injection scales to larger scales, where it is arrested by the stable entropy stratification; at a characteristic ‘buoyancy scale’, the energy is dumped into large-scale g-modes, which subsequently dissipate. In three dimensions most of the energy is dissipated at the same scale as its injection, and turbulent eddies are vertically elongated at or below the thermal conduction length, but relatively isotropic on larger scales. Similar to 2D, however, the saturated turbulent energy levels and the integral scale follow clear power-laws that depend on the thermal diffusivity, temperature gradient, and buoyancy frequency. We also show that the MTI amplifies magnetic fields, through a fluctuation dynamo, to equipartition strengths provided that the integral scale of MTI turbulence is larger than the viscous dissipation scale. Finally, we show that our scaling laws are consistent with extant observations of ICM turbulence if the thermal conductivity is reduced by a factor of $\sim 10$ from its Spitzer value, and that on global cluster scales the stable stratification significantly reduces the vertical elongation of MTI motions.

        Speaker: Lorenzo Maria Perrone (University of Cambridge)
      • 11:25 AM
        As a matter of dynamical range – scale-dependent energy dynamics in magnetized turbulence 25m

        Magnetized turbulence is ubiquitous in many astrophysical and terrestrial plasmas but no universal
        theory exists. Even the detailed energy dynamics in magnetohydrodynamic turbulence are still not
        well understood. We present a suite of ICM-like, subsonic, super-Alfvénic, high plasma-beta MHD turbulence
        simulation that only vary in their dynamical range, i.e., in their separation between the large-scale
        forcing and dissipation scales. From a practical point of view, we show how numerical dissipation can
        be estimated using an energy transfer analysis framework and that implicit large eddy simulations
        match direct numerical simulations. From a theoretical point of view, we use the same framework
        to demonstrate that – contrary to hydrodynamic turbulence – the cross-scale energy fluxes are not
        constant in MHD turbulence. This applies both to different mediators (such as cascade processes or
        magnetic tension) for a given dynamical range as well as to a dependence on the dynamical range
        itself. We do not observe any indication of convergence even at the highest resolution simulation at
        2048^3 cells. This raises the question on whether an asymptotic regime in MHD turbulence exists, and,
        if yes, what it looks like.

        Speaker: Philipp Grete (University of Hamburg)
      • 11:50 AM
        A New Paradigm in X-ray Spectral Analysis 5m

        X-ray spectral analysis is a powerful tool available to astronomers to study differing astrophysical phenomena from X-ray binaries, galactic black holes, and the intracluster medium.
        A new Bayesian paradigm is emerging in the field of X-ray spectral analysis. However, continued concerns over the choice of priors dominate the conversation. With our new machine learning methodology employing Mixture Density Networks (MDN), we use posterior target distributions calculated by an MDN as the priors for a full Bayesian inference approach to X-ray spectroscopy. Additionally, we discuss the potential of deconvolving observed X-ray spectra from the instrumental response using a Recurrent Inference Machine (RIM). Our findings indicate that using a RIM to deconvolve the spectrum and then passing the deconvolved spectrum to well-tuned MDN results in inaccurate estimates of the temperature and metallicity values which are critical in the study of galaxy clusters, plasma physics, and feedback astrophysics. In this talk, we will also discuss the implications for use cases and demonstrate the power of this exciting new methodology in our exploration of galaxy clusters.

        Speaker: Carter Rhea (L'Université de Montréal)
      • 11:55 AM
        Exploring the X-ray filamentary structure in Frontier Fields Cluster MACSJ0717.5+3745 5m

        We present the results of Chandra and XMM-Newton X-ray imaging and spatially-resolved spectroscopy of the filament in MACSJ0717.5+3745, an intermediate redshift (z=0.5458) and exceptionally massive (3.5 +/- 0.6 * 10^15 solar masses) Frontier Fields cluster experiencing multiple mergers. Tight constraints placed on the thermodynamical properties of the filament are acquired using a joint fitting of spectra using nested parameter sampling within a Bayesian framework.

        Speaker: Jean-Paul Breuer (Masaryk University)
      • 12:00 PM
        Discussion: Turbulence 30m
    • 12:30 PM 1:45 PM
      Lunch 1h 15m
    • 1:45 PM 5:05 PM
      Tuesday afternoon: Thermal instability & multiphase ICM
      Convener: Mark Voit (Michigan State University)
      • 1:45 PM
        Cloudy with a Chance of Rain: Accretion Braking of Cold Clouds 25m

        I discuss how turbulent mixing layers in infalling cold clouds serves as a drag force which can be more efficient than standard hydrodynamic drag. In a gravitational field, this leads to a terminal velocity which is smaller than standard terminal velocity, and can explain the sub-virial infall of filaments seen in galaxy clusters.

        Speaker: S. Peng Oh (UCSB)
      • 2:10 PM
        Stratified and multiphase turbulence in the intracluster medium 25m

        The intracluster medium (ICM) is stratified so Kolmogorov’s picture of isotropic/homogeneous turbulence needs to be modified. Similarly, cool cores show multiphase gas with cold/dense clouds embedded in the diffuse hot ICM. I shall present results from idealized stratified turbulence simulations and idealized multiphase periodic box simulations with heating and cooling. I shall present common diagnostics such as structure functions, power spectra, and the scaling between rms density, pressure and turbulent Mach number. The pressure fluctuations, in contrast to density fluctuations, are a much better indicator of the turbulence Mach number. I shall make comparisons with cosmological simulations and discuss observational implications. (This work is mostly based on the PhD thesis of Rajsekhar Mohapatra)

        Speaker: Prateek Sharma (IISc Bangalore)
      • 2:35 PM
        The problems of energy transport and its multi-scale impact in the intracluster medium 25m

        Energy transport across a wide range of dynamical scales in the intracluster medium is one of the most interesting topics in current research and future interest. Hot baryons, visible in the X-rays, need to be stably sustained against radiative cooling over a large inner fraction of the cluster virial radius. A historical motivation has been the lack of sufficient observed cold gas in the cluster cores that is expected in the absence of efficient heating. Quantitatively, there is enough energy from active galactic nuclei to solve the problem at the simplest level, but the complexity of how that energy flows around is not well understood. Multiple transport mechanisms are being actively discussed including long wavelength, nearly isotropic sound waves, anisotropic heat conduction along local magnetic fields (depending on the local temperature gradient), generation and dissipation of volume-filling turbulence, etc. While sound waves and turbulence have been strong contenders, thermal conduction has been claimed to be further suppressed by gyro-scale whistlers that scatter thermal electrons efficiently in the weakly magnetised ICM. In the latter scenario, thermal instability domain may be enhanced leading to excess and/or smaller scale cold gas. This further implies that observations may need to account for excess cold/mixed phase gas. In my talk, I will discuss these topics of energy transport in the ICM and the consequences.

        Speaker: Prakriti Pal Choudhury (Institute of Astronomy, University of Cambridge)
      • 3:00 PM
        Coffee break 45m
      • 3:45 PM
        Cooling & the Origins of Cluster Atmospheres (via Zoom) 25m

        The conditions leading to large reservoirs of molecular clouds and star formation in central cluster galaxies are determined by atmospheric conditions on large scales. The thermodynamic properties of cooling core and non-cooling core cluster atmospheres diverge at radii approaching R_2500, or roughly 400 kpc radius in a 10E14 solar mass cluster. These conditions are driven
        by an excess of 10E12 solar masses of atmospheric gas in a 10E14 solar mass cool core cluster with respect to a non-cool core cluster of similar mass. The high pressure environment in the inner one hundred kpc or so promotes thermally unstable cooling that leads to cloud condensation and star formation. We will discuss how these conditions may have arisen.

        Speaker: Brian McNamara (Physics & Astronomy, University of Waterloo)
      • 4:10 PM
        Dynamics of a turbulent, multiphase medium (via Zoom) 25m

        In this talk, I want to describe what sets the mass transfer between the phases in a turbulent, multiphase medium. I will show analytic and simulation results which suggest (i) a "survival criterion", (ii) mass growth rates, and (iii) a power-law mass function of the cold gas.

        Speaker: Max Gronke (MPA)
      • 4:35 PM
        Discussion: Thermal instability & multiphase ICM 30m
    • 6:00 PM 10:00 PM
      Conference Dinner: Restaurant Allegade 10 (https://allegade10.dk/)

      The conference dinner will take place at restaurant Allegade 10, https://allegade10.dk/.

    • 8:30 AM 9:00 AM
      Morning coffee 30m
    • 9:00 AM 12:30 PM
      Wednesday morning: AGN feedback
      Convener: Thomas Berlok (Leibniz Institute for Astrophysics (AIP))
      • 9:00 AM
        Expanding Gaseous Halos with AGN Feedback 25m


        Speaker: Mark Voit (Michigan State University)
      • 9:25 AM
        A relic of the past cluster-size AGN explosion in the Ophiuchus cluster 25m

        I will show new X-ray results confirming the existence of the most
        energetic AGN explosion ever found in a galaxy cluster. New radio data for the relic lobe will also be shown.

        I will also show a mission concept for a new X-ray Probe, Line Emission Mapper, a microcalorimeter array with a large field of view sensitive to soft X-rays (exceeding Athena in grasp by factor 15), and solicit ideas for investigations of ICM physics.

        Speaker: Maxim Markevitch (NASA GSFC)
      • 9:50 AM
        Steep Circumgalactic Entropy Profiles Around Powerful Radio Sources 25m

        I will present (relatively) high resolution Chandra X-ray profiles around (again, relatively) nearby but powerful radio sources. These profiles show steeply increasing gas entropy profiles from the central black holes to the outskirts. I will explain why these profiles are not "runaway" events, but evidence of how AGN regulation works when the AGN feedback energy is dumped far from the central source.

        Speaker: Megan Donahue (Michigan State University)
      • 10:15 AM
        Coffee break 45m
      • 11:00 AM
        Simulating AGN jet feedback effects in galaxy clusters 25m

        The intra-cluster medium and its x-ray dark cavities represent some of the most direct evidence of feedback effects from AGN jets on galactic halo scales. Yet, modelling jets in order to understand the detailed process that leads to a reduction of the cooling-flow and a regulation of star formation and black hole accretion poses substantial challenges even with state-of-the-art computational resources. I will present developments towards predictive simulations of jet-ICM interactions, present some recent results on short term ($< t_{c}$) feedback effects and discuss their implications in the self-regulated heating-cooling cycle of the ICM. Finally, I will propose a new method to model cold gas in global galaxy cluster and even cosmological simulations based on a 2-fluid formulation of hydrodynamics. This new method will open up the possibility to study cold, spatially unresolved clouds in global simulations and their evolution over Gyr timescales, thereby providing an opportunity to include small-scale jet feedback on the interstellar medium, global ICM simulations and cold filaments in the ICM in the same simulation.

        Speaker: Rainer Weinberger (Canadian Institute for Theoretical Astrophysics)
      • 11:25 AM
        Self-regulated AGN feedback of light jets in cool-core galaxy clusters 25m

        Heating from active galactic nuclei (AGN) is thought to stabilize cool-core clusters, limiting star formation and cooling flows. We employ radiative magneto-hydrodynamic (MHD) simulations to model light AGN jet feedback with different accretion modes (Bondi-Hoyle-Lyttleton and cold accretion) in an idealised Perseus-like cluster. Independent of the probed accretion model, accretion efficiency, jet density and resolution, the cluster self-regulates with central entropies and cooling times consistent with observed cool-core clusters in this non-cosmological setting. We find that increased jet efficiencies lead to more intermittent jet powers and enhanced star formation rates. Our fiducial low-density jets can easily be deflected by orbiting cold gaseous filaments, which redistributes angular momentum and leads to more extended cold gas distributions and isotropic bubble distributions. In comparison to our fiducial low momentum-density jets, high momentum-density jets heat less efficiently and enable the formation of a persistent cold-gas disc perpendicular to the jet that is centrally confined. Cavity luminosities measured from our simulations generally reflect the cooling luminosities of the intracluster medium (ICM) and correspond to averaged jet powers that are relatively insensitive to short periods of low-luminosity jet injection. Cold gas structures in our MHD simulations with low momentum-density jets generally show a variety of morphologies ranging from discy to very extended filamentary structures. In particular, magnetic fields are crucial to inhibit the formation of unrealistically massive cold gas discs by redistributing angular momentum between the hot and cold phases and by fostering the formation of elongated cold filaments that are supported by magnetic pressure.

        Speaker: Christoph Pfrommer (AIP Potsdam)
      • 11:50 AM
        AGN Feedback from Jets of Different Composition 25m

        Feedback from AGN jets is critical for heating the intracluster medium (ICM) and prevent cooling catastrophe in cool-core (CC) clusters. However, the composition of AGN jets and the resulting feedback processes remain uncertain. In this talk, I will present results from our recent simulations comparing leptonic vs. hadronic jets and their implications. I will also discuss their observational signatures and whether they could allow us to distinguish the different scenarios.

        Speaker: Hsiang-Yi Karen Yang (National Tsing Hua University)
      • 12:15 PM
        Environmental Dependence of Self-Regulating Black-hole Feedback in Massive Galaxies 15m

        The ability of AGN feedback to self-regulate in massive galaxies depends critically on environmental factors like the depth of the potential well and the pressure of the surrounding circumgalactic medium (CGM). I have carried out high resolution 3D hydrodynamic simulations exploring the dependence of AGN feedback in galaxies on those environmental factors with a range of halo masses. These simulations also include in situ star formation and stellar feedback along with feedback from massive galaxy’s old stellar population. Our simulations show that this feedback mechanism is tightly self-regulating in a massive galaxy with a deep central potential and low CGM pressure, permitting only small amounts of multiphase gas to accumulate and allowing no star formation. In a similar mass galaxy with shallower central potential and greater CGM pressure, the feedback mechanism is more episodic, producing extended multiphase gas and allowing small rates of star formation. Another important question I will touch upon is “how does kinetic AGN feedback with a strong momentum flux interacts with the CGM?” Our analysis shows that large scale CGM circulation plays an important role in reconfiguring the galactic atmosphere and regulating the atmosphere’s central entropy level. We find that most of the AGN’s energy output goes into lifting of circumgalactic gas rather than heating of atmospheric gas within the galaxy, consequently reconfiguring the circumgalactic medium (CGM) during our simulations.

        Speaker: Deovrat Prasad (Michigan State University)
    • 12:30 PM 1:45 PM
      Lunch 1h 15m
    • 1:45 PM 5:30 PM
      Wednesday afternoon: AGN feedback
      Convener: Hsiang-Yi Karen Yang (National Tsing Hua University)
      • 1:45 PM
        Interaction between a central radio galaxy and the ICM 25m

        Chandra observations of radio galaxies hosted by the central galaxy of a group or cluster continue to provide insights into the makeup of jets and lobes and their interactions with the environment. My main focus will be implications of the rich X-ray and radio structure observed in Cygnus A. Among other things, I will argue that the feature known as the X-ray jet in Cygnus A is formed by encounters between gas clouds and the jet, which leave trails of remnant plasma scattered along the path of the jet. I will also discuss early results of numerical simulations for the formation of the hole found in the X-ray emission around the primary hotspot in the eastern lobe of Cygnus A.

        Speaker: Paul Nulsen (Harvard-Smithsonian Center for Astrophysics)
      • 2:10 PM
        AGN feedback in groups 25m

        Our understanding of how active galactic nucleus feedback operates in galaxy clusters has improved in recent years owing to large efforts in multiwavelength observations and hydrodynamical simulations. However, it is much less clear how feedback operates in galaxy groups, which have shallower gravitational potentials.
        We present recent observational work using a combination of eROSITA and MeerKAT (+ other radio) observations to study feedback processes in groups.
        Among other things, we find that (i) galaxy groups are more likely than clusters to host large radio galaxies and (ii) that clusters and groups follow the same correlation between X-ray and radio emission.
        In one particularly interesting case, we image the evolution of multiple generations of cosmic-ray active-galactic-nuclei bubbles in a galaxy group with LOFAR observations below 200 MHz. After hundreds of million years, the bubbles still have not fully mixed with the thermal gas, probably under the action of magnetic fields. This has implications for simulations of AGN feedback and we present new MHD simulations that attempt to reproduce our observations.

        Speaker: Marcus Bruggen (University of Hamburg)
      • 2:35 PM
        Cooling and AGN feedback in giant elliptical galaxies 25m


        Speaker: Norbert Werner (Masaryk University)
      • 3:00 PM
        Coffee break 45m
      • 3:45 PM
        Constraining The Thermal History of Groups and Clusters with the Sunyaev-Zel'dovich Effect 25m

        Energetic feedback from active galactic nuclei (AGNs) is often invoked to explain the thermal properties of galaxy groups and clusters, and to resolve several outstanding issues in galaxy formation, but its impact is still not fully understood. AGN feedback may have been gradual, occurring mostly during periods of lower accretion rates, as observed in cool core clusters today, or it may have been dominated by powerful outbursts that occurred when supermassive black holes were more rapidly accreting. The most promising methods for distinguishing between these models is by looking for cosmic microwave background (CMB) anisotropies due to the thermal Sunyaev–Zel’dovich (tSZ) effect. I will describe our analysis of this effect in South Pole Telescope and Atacama Cosmology Telescope data to constrain the gas around massive galaxies at z~1 at a resolution of ~ 500 comoving kpc, and I will also present our analysis of data from the TolTEC instrument, which constrains feedback on scales of ~ 50 comoving kpc. Future such measurements will continue to provide essential information on the thermal history of groups and clusters.

        Speaker: Evan Scannapieco (Arizona State University)
      • 4:10 PM
        Decoding AGN/ICM Structures and Emissions: Probes of Cluster Dynamics and History 25m

        As they penetrate tens to hundreds of kpc through ICMs, AGN jets and their
        back-flows often can highlight distinct ICM structures encountered along the
        way. Those ICM structures frequently represent important, "tell-tale"
        signatures of that cluster's environment and dynamical history, so of its
        formation and evolution. Consequently, characterizing such AGN distortions
        and, especially the physical properties of the associated ICM structures
        encountered and their relationships to a cluster's dynamical history can
        provide unique and valuable probes of cluster formation and ICM physics. In
        this talk I will summarize some of the MHD simulations by our group that are
        designed to refine understanding of these relationships. The simulations
        include energy-dependent transport of cosmic ray electrons (CRe) and their
        emissions needed to clearly establish unique and diagnostic signatures of these interactions.

        Acknowledgements: This work was supported at the University of Minnesota by
        NSF grant AST-1714205 and by the University of Minnesota Supercomputing
        Institute. We also thank multiple contributors over the years to development
        of the codes used in the simulations reported.

        Speaker: T. W. Jones (University of Minnesota)
      • 4:35 PM
        Magnetic field in galaxy clusters and their effect on intracluster physics (via Zoom) 25m

        I shall discuss the gradient technique that has recently been applied to the galaxy clusters in order to obtain the properties of the magnetic field there. I shall also discuss how magnetic field affect the thermal conductivity and CR transport/acceleration in galaxy clusters.

        Speaker: Alex Lazarian
      • 5:00 PM
        Discussion: AGN feedback 30m
    • 8:30 AM 9:00 AM
      Morning coffee 30m
    • 9:00 AM 12:30 PM
      Thursday morning: New frontiers of theory and observations
      Convener: Gianfranco Brunetti (IRA INAF)
      • 9:00 AM
        Micro- vs Macroscopic Transport in Clusters 25m

        I will discuss large scale transport in the ICM, mitigated by AGN jet feedback, and how it affects the evolution of thermal and non-thermal plasma in ICM and, in turn, the X-ray and radio properties of clusters. In particular, I will address the ability of AGN feedback to enhance conductive heat transport through a mechanism akin to the way geothermal heat pumps operate, based on a set of MHD feedback simulations of the Perseus Cluster.

        Speaker: Sebastian Heinz (University of Madison-Wisconsin)
      • 9:25 AM
        Signatures of turbulent magnetic fields in merging galaxy clusters 25m

        Kelvin-Helmholtz Instabilities (KHI) along contact discontinuities in galaxy clusters have been used to constrain the strength of magnetic fields in galaxy clusters, following the assumption that, as magnetic field lines drape around the interface between the cold and hot phases, their magnetic tension resists the growth of perturbations. This has been observed in simulations of rigid objects moving through magnetised media and sloshing galaxy clusters, and then applied in interpreting observations of merger cold fronts. Using a suite of MHD simulations of binary cluster mergers, we show that even magnetic field strengths stronger than yet observed ($\beta = P_{\rm th}/P_B = 50$) show visible KHI features. This is because our initial magnetic field is tangled, producing Alfven waves and associated velocity fluctuations in the ICM; stronger initial fields seed larger fluctuations, so that even a reduced growth rate due to magnetic tension produces a significant KHI eddies. The net result is that a stronger initial magnetic field produces more dramatic fluctuations in surface brightness and temperature, not the other way around. We show that this is difficult to distinguish from the evolution of turbulent perturbations of the same initial magnitude. In order to use observations of KHI in the ICM to infer magnetic field strengths by comparing to idealized simulations, the perturbations which seed the KHI must be well-understood and (if possible) carefully controlled. Since the turbulent simulations are resolution-dependent, I will also present preliminary work on modeling sub-grid turbulence in the ICM.

        Speaker: Urmila Chadayammuri (Center for Astrophysics - Harvard & Smithsonian)
      • 9:50 AM
        Pushing the frontier of ICM physics with novel observations : new SITELLE observations of the filaments in the Perseus cluster 25m

        Clusters of galaxies exhibit some of the most spectacular examples of optically bright, line emitting nebulae. These nebulae surround the central galaxies, are filamentary in nature and can extend over 100 kpc in size. Recently, Gendron-Marsolais et al. produced for the first time a detailed velocity map at optical wavelengths of the giant filamentary nebula in the Perseus cluster and revealed a previously unknown rich velocity structure across the entire nebula. These observations were obtained with the optical imaging Fourier transform spectrometer SITELLE at CFHT, which has an outstanding field of view of 11 arcmin by 11 arcmin. Here, we present new SITELLE observations of NGC 1275, taken at 4 times higher spectral resolution compared to the initial SITELLE observations. These data reveal is remarkable detail the kinematic and quiescent nature of the filaments. They also suggest that there are 2 distinct formation mechanisms for the creation of filaments: one responsible for the large-scale filamentary nature of the filaments, as well as a distinct and more turbulent mechanism that gives rise to filaments in the wake of X-ray cavities.

        Speaker: Julie Hlavacek-Larrondo (Université de Montréal)
      • 10:15 AM
        Coffee break 45m
      • 11:00 AM
        In search of universality: towards a statistical mechanics of collisionless plasma 25m

        This talk is a piece of light but relevant theoretical-physicsy entertainment in the midst of serious astro content. Much of existing plasma (astro)physics (and indeed much of the rest of kinetic physics) is done hovering in the vicinity of a Maxwellian equilibrium, which is the maximum point of the standard Gibbs entropy and is achieved dynamically by means of two-particle collisions. In this talk, I would like to discuss what I believe to be the next frontier for plasma theoreticians---and, hopefully, also observers---and attempt to grapple with the fact that many astrophysical plasmas (solar wind being the most accessible of them, and ICM the grandest) are too collisionless to be Maxwellian (in the sense that they have dynamics that occur on shorter timescales than interparticle collisions). The central question is then whether there exist universal collisionless equilibria, or classes thereof, and what they are. What is the meaning of entropy in a collisionless plasma? (Similar questions are asked in galactic dynamics, where the collisionless particles are stars.) I will discuss some simple ideas, going back to the work of Lynden-Bell in the 1960s, about the statistical mechanics of a collisionless plasma, leading to a class of universal collisionless equilibria---these are reminiscent of the equilibria of Fermi gases, with phase-volume conservation in a collisionless plasma imposing (an infinite set of) constraints that are analogous to the Pauli exclusion principle. I will then outline a programme for how one might do to this statistical mechanics what Boltzmann did to Gibbs: derive a “collisionless collision integral” that describes the dynamical relaxation of a plasma towards the Lynden-Bell equilibria. It turns out that in order to make progress in this task, one must understand the structure of chaotic fluctuations in phase space. Lynden-Bell-like equilibria are recoverable under some very restrictive assumptions---roughly speaking, when these fluctuations are treated as structurally similar to a thermal noise. In fact, they are more likely to behave like fully-fledged turbulence---with phase mixing (“Landau damping”) and stochastic echoes conspiring to process a constant flux of energy. What universal equilibria (if any) exist against such a background is a topic of ongoing research.

        Speaker: Alexander Schekochihin (University of Oxford)
      • 11:25 AM
        On the acceleration efficiency of low-Mach number shocks 25m

        The recent discoveries in the theory of diffusive shock acceleration (DSA) that originate from first-principle kinetic plasma simulations are discussed. We show that, when ion acceleration is efficient, the back-reaction of non-thermal particles and self-generated magnetic fields becomes prominent and leads to both enhanced shock compression and particle spectra appreciably different from the standard test-particle DSA theory. In particular, we present hybrid simulations of ion acceleration at low-Mach number shocks for different values of plasma beta and we discuss how these results may solve the discrepancy reported between the Mach numbers inferred from radio and X-ray observations of shocks in galaxy clusters.

        Speaker: Damiano Caprioli
      • 11:50 AM
        An Effective Collision Operator for Heat-Flux-Generated Whistler Turbulence 5m

        A large proportion of galaxy clusters contain an ICM which supports temperature gradients that are inconsistent with the classical coulomb scattering rate. Turbulence has long been cited as a possible mechanism for enhanced scattering. In particular, the role of the whistler instability in limiting electron heat flux has been a recent area of interest. Numerical results have demonstrated the saturated heat flux scales as 1/beta_e (Roberg-Clark 2016 and Komarov 2018) and a quasi-linear form for the whistler scattering operator has been proposed (Drake 2021). In this work, we run numerical simulations of the whistler instability and confirm the heat flux scaling in a setup similar to previous work as well as a stratified setup with gravity. We then recover the form of the turbulence collision operator using a Fokker-Planck method as well as a Chapman-Enskog method and discuss in detail the comparative strengths of each method. We then use this information to construct a collision operator for whistler turbulence and compare with existing models. Finally, we discuss preliminary results from our simulations of an analogous ion heat flux instability.

        Speaker: Evan Yerger (Princeton University)
      • 11:55 AM
        A Turbulent Heating Mechanism via Magnetic Pumping in the Intracluster Medium of Galaxy Clusters (via Zoom) 5m

        Turbulence driven by AGN activity constitutes an attractive energy source for heating the intracluster medium (ICM) in galaxy clusters. How this energy dissipates into the ICM plasma remains unclear, given its low collisionality and high magnetization, which precludes viscous heating in the ordinary gas dynamics sense. However, Kunz et al. 2011 proposed that gyroviscous heating, a form of heating based on the anisotropy of the plasma pressure with respect to the local magnetic field, could be a viable heating mechanism. In this work, we build upon this idea by studying how the anisotropy evolves under a range of forcing frequencies, what waves and instabilities are generated, and demonstrating that the particle distribution function acquires a high energy tail. We perform particle-in-cell (PIC) simulations of a plasma subject to periodic variations of the mean magnetic field $\textbf{B}(t)$ to show that particles can be gyroviscously heated by large-scale turbulent fluctuations via magnetic pumping. When $\textbf{B}(t)$ grows (dwindles), a pressure anisotropy $P_{\perp,j}>P_{\parallel,j}$ ($P_{\perp,j}< P_{\parallel,j}$) builds up ($P_{\perp,j}$ and $P_{\parallel,j}$ are, respectively, the pressures of species $j$ perpendicular and parallel to $\textbf{B}(t)$). These pressure anisotropies excite mirror and oblique firehose instabilities, which pitch-angle scatter the particles and limit the anisotropy level, thus providing a channel to heat the plasma. The efficiency of this heating mechanism depends on the frequency of the large-scale turbulent fluctuations and the efficiency of the scattering the instabilities provide in their nonlinear evolution. Our results show that this process can be relevant in dissipating and distributing turbulent energy at kinetic scales in weakly collisional plasmas such as the ICM.

        Speaker: Francisco Ley (University of Wisconsin Madison)
      • 12:00 PM
        Discussion: New frontiers of theory and observations 30m
    • 10:20 AM 10:50 AM
      Tour of NBI - Group 1 30m Auditorium A

      Auditorium A

      Niels Bohr Institute

      Blegdamsvej 17 Copenhagen

      Elena Bellomi
      Thomas Berlok
      Jean-Paul Breuer
      Marcus Bruggen
      Gianfranco Brunetti
      Damiano Caprioli
      Urmila Chadayammuri
      Christopher Chen
      Megan Donahue
      Alankar Dutta
      August Evrard
      Philipp Grete
      Sebastian Heinz
      Julie Hlavacek-Larrondo
      Léna Jlassi

      Speaker: Christian Joas
    • 12:30 PM 1:45 PM
      Lunch 1h 15m
    • 12:35 PM 1:05 PM
      Tour of NBI - Group 2 30m Auditorium A

      Auditorium A

      Niels Bohr Institute

      Blegdamsvej 17 Copenhagen

      Wonki Lee
      Yuan Li
      Maxim Markevitch
      Daisuke Nagai
      Dylan Nelson
      Paul Nulsen
      S. Peng Oh
      Prakriti Pal Choudhury
      Lorenzo Maria Perrone
      Martin Pessah
      Christoph Pfrommer
      Annalisa Pillepich
      Tomáš Plšek
      Deovrat Prasad

      Speaker: Christian Joas
    • 1:15 PM 1:45 PM
      Tour of NBI - Group 3 30m Auditorium A

      Auditorium A

      Niels Bohr Institute

      Blegdamsvej 17 Copenhagen

      Carter Rhea
      Mateusz Ruszkowski
      Evan Scannapieco
      Alexander Schekochihin
      Prateek Sharma
      Amit Singh
      Jennifer Stafford
      Nhut Truong
      Tsun Hin Navin Tsung
      Iraj Vaezzadeh
      Mark Voit
      Rainer Weinberger
      Norbert Werner
      Lynn Wilson
      Hsiang-Yi Karen Yang
      Evan Yerger

    • 1:45 PM 3:55 PM
      Thursday afternoon: Connecting solar wind and ICM physics
      Convener: Alexander Schekochihin (University of Oxford)
      • 1:45 PM
        Recent progress in plasma turbulence, heating, and related processes, from solar wind observations 35m

        Plasmas in the solar system represent an unprecedented opportunity to learn about how basic processes such as turbulence and dissipation work in astrophysical environments, as well as their impacts on the large-scale dynamics. This is because they are the only astrophysical plasmas that can be directly probed in situ by spacecraft, which provide a wealth of data to characterise these processes in great detail. The solar wind, in particular, is well-suited to this, since the system-size and plasma microscales are well-separated, allowing potentially universal physics to be probed, and the plasma is fast flowing, allowing spatial structure to be measured with a single spacecraft. In this talk, I will describe some of the recent progress we have made in understanding solar wind processes such as turbulence (at both MHD scales and in the small-scale kinetic range), pressure-anisotropy instabilities (such as firehose and mirror), dissipation mechanisms (such as Landau damping), and some recent measurements of the effective collisionality. I will also talk about the impacts of these processes, e.g., for how the solar wind is generated and how its large-scale structure originates. I hope this will be of interest to those studying similar processes in the ICM, and that interesting discussions on the comparisons will be stimulated.

        Speaker: Christopher Chen
      • 2:20 PM
        Particle Energization at Collisionless Shock Waves and Particle Transport in the Interplanetary Medium 25m

        Collisionless shocks are an ubiquitous phenomena in astrophysical plasmas in the form of bow shocks upstream of magnetized planetary bodies (and unmagnetized objects like comets and Venus), interplanetary shocks, supernova blast waves, and stellar astrospheres. Despite their ubiquity, there are still numerous processes associated with collisionless shocks that are not well understood. One of the more critical unknowns involves the processes by which particles are energized at collisionless shocks and how they are transported throughout the surrounding medium. This is a critical issue as collisionless shocks are thought to generate some of the most energetic particles in the universe in addition to causing space weather hazards for robotic assets and humans in space. We will discuss some of the known and observed energization mechanisms and discuss unknowns that motivate future work.

        Speaker: Lynn Wilson (NASA Goddard Space Flight Center)
      • 2:45 PM
        Coffee break 30m
      • 3:15 PM
        Discussion: Connecting solar wind and ICM physics 40m
    • 8:30 AM 9:00 AM
      Morning coffee 30m
    • 9:00 AM 12:30 PM
      Friday morning: Cosmic rays
      Convener: Mateusz Ruszkowski (University of Michigan)
      • 9:00 AM
        Particle acceleration and non-thermal emission from galaxy clusters and large scale structure 25m

        Cluster scale radio emission traces relativistic particles and magnetic fields in the ICM, the origin of these non-thermal components is a long-standing problem. More recent observations with LOFAR have revealed synchrotron emission extending beyond the central, Mpc, regions of galaxy clusters, in cluster outskirts or in cosmic filaments that connect pairs of massive clusters.
        Addressing the mechanisms at the origin of these phenomena and their interplay with the ICM is important to constrain magneto-genesis in LSS, and to explore the physics of particle acceleration in novel regimes and the collisionless processes that govern ICM (micro-)physics.
        In this talk I will briefly review the most important observational findings obtained with modern radio telescopes and their impact on our theoretical understanding of these phenomena. In particular I will focus on the role of turbulence in the (re)acceleration of particles and amplification of magnetic fields in clusters and LSS.

        Speaker: Gianfranco Brunetti
      • 9:25 AM
        Radio relics and their substructure (via Zoom) 25m
        Speaker: Paola Dominguez Fernandez
      • 9:50 AM
        Thermal Instability with Cosmic Ray Heating 25m

        Observations have revealed clusters having high core densities; the cooling times of these clusters are much less than the Hubble time and should result in cooling flows, which were not seen. This means there is global heating supporting clusters from the cooling catastrophe. On the other hand, H-alpha observations have shown that despite being in global thermal equilibrium, cold gas do exist within clusters. Thus any heating mechanism proposed must not preclude the possibility of a local thermal instability. Cosmic ray heating has been proposed as a crucial candidate, and simulations including diffusive transport have demonstrated it can indeed support a globally stable atmosphere while allowing for local thermal instability. However, if cosmic ray transport were streaming dominated, we demonstrate that the bottleneck effect will redistribute the heating such that only the cold gas is heated. The hot gas will collapse from cooling, causing a cooling flow. This will have important implications to modeling cosmic ray transport in the ICM.

        Speaker: Tsun Hin Navin Tsung (University of California, Santa Barbara)
      • 10:15 AM
        Coffee break 45m
      • 11:00 AM
        Cosmological zoom simulations of merging galaxy clusters 25m


        Speaker: Thomas Berlok (Leibniz Institute for Astrophysics (AIP))
      • 11:25 AM
        Simulating the impact of kpc scale jets on multiphase ISM in galaxies (via Zoom) 25m
        Speaker: Dipanjan Mukherjee
      • 11:50 AM
        Discussion of cosmic rays 35m

        Lead by Mateusz Ruszkowski and Damiano Caprioli

      • 12:25 PM
        Farewell 5m
    • 12:30 PM 1:45 PM
      Lunch 1h 15m Canteen


      Niels Bohr Institute