Credit: X-ray: NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScI

This mini-workshop intends to bring together Heidelberg astronomers working on different aspects of galaxy clusters. The workshop is open for everyone interested!

Final Program:

9:30
Welcome address

9:40
Kris Blindert
MPIA
Mass and light in galaxy clusters from the red-sequence cluster survey (RCS)
I will discuss the results of a large spectroscopic survey of optically-selected clusters of galaxies, drawn from the Red-sequence Cluster Survey (RCS). Using galaxies as dynamical tracers of the cluster potential, I will explore the properties of RCS clusters such as optical richness as a function of total cluster mass. I will also present a more detailed dynamical analysis, recovering density profiles for ``ensemble'' clusters of various average masses, thus constraining the concentrations and orbital anisotropies of galaxy clusters.

10:10
Matthias Bartelmann
ZAH / ITA
Magnetic fields and radio emission in clusters
Magnetic fields are ubiquitous in galaxy clusters. Although their origin is unclear, their evolution has been studied in detail in the past years, showing that they can grow substantially beyond adiabatic compression and lose all memory of their initial structure in a cluster collapse. Relativistic charged particles gyrate in the cluster magnetic fields and emit synchrotron radiation in radio wavebands. The discussion of their origin is ongoing, but recent studies hint at a hybrid picture. I shall discuss recent results and their possible implications.

— 10:40 Coffee break —

11:00
Robert Schmidt
ZAH / ARI
X-ray observations of galaxy clusters
In this talk I use X-ray observations of galaxy clusters to study cluster physics and the properties of their dark matter halos. In particular, I will show how the most X-ray luminous, dynamically relaxed clusters can be used to test predictions from cold dark matter simulations, such as the universal mass profile, kinematics, or the mass-concentration relation. I will discuss the power and promise of relaxed clusters to constrain cosmological parameters.

11:30
Wilfried Domainko
MPI-K
H.E.S.S. observations of galaxy clusters at very-high energy gamma rays
Clusters of galaxies are expected to contain a significant population of hadronic and leptonic cosmic rays. Potential sources for these particles are merger and accretion shocks, starburst driven galactic winds and radio galaxies. Furthermore, since galaxy clusters confine cosmic ray protons up to energies of at least 1 PeV for a time longer than the Hubble time they act as storehouses and accumulate all the hadronic particles which are accelerated within them. Consequently clusters of galaxies are potential sources of VHE (> 100 GeV) gamma rays. Motivated by these considerations, promising galaxy clusters are observed with the H.E.S.S. experiment as part of an ongoing campaign. Results from this campaign will be reported.

12:00
Giulia Vannoni
MPI-K
Acceleration of UHE Protons at Cluster Accretion Shocks and Related Non-thermal Emission
Clusters of galaxies are believed to be able to accelerate Cosmic Rays to ultrahigh energies (~10^18 eV and beyond) at accretion shocks. At this energies, the energy losses induced by the interaction with the CMB radiation become effective and determine the maximum energy of protons and the shape of the cutoff in the proton energy spectrum. In particular, the dominant energy loss channel becomes pair production. We present a time dependent numerical calculation of the shock acceleration process where we include self-consistently the presence of energy losses. We accurately calculate the spectra of the produced electron-positron pairs and the X-ray and gamma-ray emission they produce via synchrotron and inverse Compton scattering processes respectively. We find that the radiation spectra show a cutoff harder than exponential and that the downstream and upstream regions contribute almost at the same level to the emission. For the typical characteristics of galaxy clusters, the synchrotron and IC peaks of the electron broadband emission are at comparable levels. The expected emission in gamma-rays is at the limit of the sensitivity of present generation instruments, however it may be detectable with the future generation of detectors.

— 12:30 Lunch break —

14:20
Björn Schäfer
ZAH / ARI & HGSFP
Spherical collapse and cluster abundance in modified gravity
Modified gravity theories, in particular DGP-gravity, aim to be an alternative to dark energy cosmologies. They can be tested by observations of the growth of the cosmological large-scale structure for which they make very different predictions compared to the traditional LCDM model. By solving the spherical collapse equation for DGP-type gravity theories the number density of clusters as a function of redshift and mass can be predicted, and it can be shown that the number of clusters is largely enhanced in DGP gravity, making it a viable test. Constraints on the gravitational theory from PLANCK number counts can be derived, which suggests that DGP could be ruled out with high significance close to 10sigma.

14:50
Jochen Heidt & André Germeroth
ZAH / LSW
Cluster environment of QSOs
The canonical LCDM model predicts that structures grow hierarchically, i.e. one would expect that in the galaxy cluster dark matter haloes the most massive galaxies evolve. If one further takes into account that the mass of a bulge-dominated galaxy and the mass of its SMBH are linked, one would also expect that the most massive SMBHs are found in the most massive galaxy cluster dark matter haloes. This has in fact be found in the Millennium Simulation. The analysis of the cluster environment of QSOs at higher redshifts is challenging even for teleskops of the 8m-class. At a redshift of z=0.5 the apparent magnitude of a L* galaxy is of the order of m=21.1 (V-Band). Therefore, a spectroscopic study of those environments with a larger redshift is a very time-consuming and it is feasible for a few QSOs only. A less precise but more time-efficient method of estimating the redshifts is the combination of photometric measurements and statistical analyses. The talk will give an overview of the recently started project for studying the environment of QSOs with the two LBCs (widefield cameras at the Large Binocular Telescope, Arizona). To find candidates of galaxies physically associated with the QSOs, the publicly available photo-z code Hyperz will be used. A follow-up spectroscopy with LUCIFER (NIR-Imager and -Spectrograph at the LBT) and/or MODS of possible high-redshift clusters will give the cluster member velocity dispersion that leads to an estimation of the mass of the cluster.

— 15:20 Coffee break —

15:40
Thorsten Lisker
ZAH / ARI & HGSFP
What dwarf galaxies can tell us about galaxy cluster evolution
Early-type dwarf galaxies are the most numerous objects in galaxy clusters. I will give an overview of their diverse characteristics, which appear to be linked to environmental density. Do these dwarfs carry the imprint of physical processes typical for a cluster environment, like ram-pressure stripping or galaxy harassment? Did they form within the cluster, or within groups prior to accretion? What properties of the dwarf galaxy population do we expect to find in intermediate-redshift clusters with future telescopes?

16:10
Hans Hippelein
MPIA
HIROCS – The Heidelberg InfraRed/Optical Cluster Survey
HIROCS (Heidelberg InfraRed / Optical Cluster Survey) is a multi-colour survey aiming at establishing a sample of high-redshift galaxy clusters out to z ~ 1.5. The clusters are found as galaxy overdensities using photometric redshifts; the cluster sample will be used to study the evolution of galaxies as well as the evolution of the clusters themselves as a function of redshift. In my talk, I will introduce the survey and present the first results: A sample of cluster candidates at 0.5 < z < 1.6 in the COSMOS field, and cluster candidates at z > 1 in the one square degree 03h field.

16:40
Joint discussion

— 17:30 End —

SLOC:
Thorsten Lisker, Wilfried Domainko, Robert Schmidt