How to weigh clusters of galaxies

Description and Aim

How to weigh clusters of galaxies?

At the intersections of the cosmic web, clusters of galaxies correspond to the highest density regions in the universe. In such an extreme environment, the evolution of galaxies is altered dramatically, due to the frequent interactions between galaxies. Hence galaxy clusters provide an excellent laboratory to study key aspects of galaxy formation, including the various feedback processes that are thought to play a central role. The study of the latter is facilitated in clusters because multi-wavelength observations can provide a fairly complete census of the various constituents that make up a cluster. Most notably, almost all the baryons can be accounted for, thanks to the X-ray emission of the hot intracluster medium.

It is clear that the mass of a cluster is a key quantity in such studies. They are, however, dark matter dominated systems, which complicates a direct measurement of their masses. Improving our understanding of the observable properties of clusters and their underlying dark matter distribution is important for another reason: the number density of clusters as a function of mass and redshift is a sensitive probe of our cosmology. Careful measurements can provide unique constraints on the properties of dark energy by measuring  the growth of structure. Furthermore, studies of the mass distribution in the central regions can place constraints on the interaction cross section of the still elusive dark matter particle.

There are many open questions regarding the measurements of cluster masses. This is because many different methods can be employed, each providing a somewhat different probe of the gravitational potential, and with different systematic biases. Numerical simulations are playing an increasingly important role in understanding biases. Despite the promise of cosmology using galaxy clusters, the wider community has been skeptical, because of the issues surrounding cluster masses. However, weak gravitational lensing studies are providing a unique way to study the mass distribution and linking these observations to other probes as well as simulations will form a key aspect of this workshop.

The aim of this workshop is to bring together experts on the various probes of cluster masses to frankly discuss the issues at hand, with a particular focus to separate perceived strengths and weaknesses from actual ones. The range in expertise in terms of wavelength (but also including theorists) ensures we will be able to explore the full benefits of multi-wavelength studies.

Key science questions that we wish to address/answer are:

How well can we measure cluster masses?

How important are multi-wavelength observations? Which combinations are best?

What needs to be done to make cluster studies an accepted key probe of cosmology?

Which mass range can be addressed? Which mass range needs to be addressed?

What can we learn about galaxy formation using large, multi-wavelength samples of clusters?