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    The Physics of Galaxy Clusters

    Clusters of galaxies are the largest and most recently gravitationally-collapsed objects in the Universe. Hence they provide us the opportunity to study an "ecosystem" - a volume that is a high-density microcosm of the rest of the Universe. Clusters are excellent laboratories for studying the rich astrophysics of baryons and dark matter. At the same time, they are extremely rare events, forming at sites of constructive interference of long waves in the primordial density fluctuations. Hence, they are very sensitive tracers of the growth of structure in the universe and the cosmological parameters governing it, which puts them into focus of constraining the properties of Dark Energy or to test whether our understanding of gravity is complete.

    This lecture will explain how clusters form and grow. We will encounter the rich and interesting astrophysics that governs the physics of dark matter and baryons in clusters. We will see how we can take advantage of these physical processes to observe clusters and deepen our understanding of the underlying fundamental physics. To this end we will frequently use the powerful technique of order of magnitude estimates, a very useful tool for contemporary research in astrophysics. The lecture aims at students who

    • wish to extend and deepen their understanding of theoretical physics;
    • are interested in astronomy and astrophysics; or
    • (intend to) carry out a masters thesis or Ph.D. dissertation on an astronomical or astrophysical subject.

    The lectures will be held in English because they are part of the Masters programme. Advanced Bachelor students are welcome. The lectures take place every Tuesday, 9:15am to 11am at the seminar room at ARI (Mönchhofstrasse 12-14), starting on April 17, 2012.

    The (hand-written) lecture notes will be continuously updated and is available as a PDF file with the following outline.


    • Overview and background:
      • What is a galaxy cluster? Insights from observations at various wavelengths
      • The homogeneous Universe: geometry, dynamics, and content

    • Evolution of the dark component:
      • Growth of perturbations
      • Statistics and non-linear evolution, power spectra
      • Spherical collapse model and connection to perturbation theory
      • Press-Schechter mass function, halo density profiles, virial masses

    • Evolution of the baryonic component:
      • Non-radiative physics
        • Entropy and convective instability
        • Scaling relations (ideal and real)
        • Shocks and jump conditions
        • Entropy generation by smooth accretion and hierarchical merging

      • Radiative physics
        • Radiative cooling and star formation
        • Energy feedback (supernovae, active galactic nuclei)
        • Transport processes of gas:
          turbulence, conduction, thermal stability (without and with magnetic fields)

      • Non-thermal processes
        • Origin and transport of magnetic fields, magneto-hydrodynamic turbulence
        • Acceleration of cosmic rays (to first and second order), transport equation

    • Cluster physics informed by different observables:
      • Optical: galaxy properties and virial theorem
        • Transforming galaxy populations: ram pressure, tidal effects, dynamical friction
        • Weighting clusters (1): virial theorem

      • Gravitational lensing
        • Deflection angle, lens equation, Einstein radius, lensing potential
        • Weighting clusters (2): strong and weak cluster lensing

      • X-rays: gastrophysics at high-resolution
        • Weighting clusters (3): hydrostatic equilibrium masses (and biases)
        • Kinematics of shocks and cold fronts
        • Probing kinetic equilibrium with collisionless shocks
        • Width of cold fronts - magnetic draping

      • Sunyaev-Zel'dovich (SZ) effect: the thermal energy content
        • Thermal and kinetic SZ effect
        • Properties and SZ scaling relation, SZ power spectrum

      • Radio halos and relics: watching powerful shocks and plasma physics at work

    Credit Points:

    There won't be exercise classes. Students who wish to obtain credit points can choose one research paper from a selection of "classical papers" (provided by the lecturer) that they have to read and present at a mini-symposium (which will be held in the end of the lecture series). Other students are expected to actively participate with questions. Diploma students can obtain a certificate for this lecture which can be used for acceptance in astronomy as a minor subject for the diploma. Bachelor- and Masters students can use the credit points for this lecture as usual.
    The collection of papers and the assignment can be obtained as a PDF file.