Galaxy clusters out of equilibrium
In galaxy clusters, gravity binds hundreds of thousands of galaxies together in such large collections that the fabric of space-time is distorted. According to present understanding, these massive astrophysical objects take billions of years to form and grow by accreting mass from their surrounding intracluster medium (ICM). Gas accreted is thought to heat up and slow down in large-scale shock waves surrounding the clusters. The EU-funded astrophysicists identified the signature of such a wave around one of our richest nearby clusters, the Coma cluster, which lies about 100 megaparsecs away from Earth. In the context of the NEPAL (Non-equilibrium processes in galaxy clusters) project, the astrophysicists discovered its gamma-ray signature through observations from the Very Energetic Radiation Imaging Telescope Array System (VERITAS). This long-anticipated detection tool provides a new cosmological probe of the ICM. Unlike gas accreted from the ICM, gas at the core of galaxy clusters is expected to cool over time, forming a gas flow cold enough to condense and form new stars. The NEPAL team was able to identify, in high-resolution X-ray, images of the cold front inside the core and sometimes beyond it. Specifically, shear flow beneath the cold fronts can produce the magnetic fields needed to stabilise it against Kelvin-Helmholtz instabilities. Such shear flow-induced magnetic fields have in the past been reproduced in computer simulations. Their existence had, however, remained unconfirmed. NEPAL's findings shed new light on the colder core of galaxy clusters where old galaxies sit and within which only a few new stars are born. Follow-up studies with observations at different wavelengths allowed a more complete view of intergalactic matter. Nonetheless, all NEPAL results confirm the importance of dynamical processes in the evolution of galaxy clusters. In particular, extended spiral flows are intimately related to the clusters' core structure. Research work is still ongoing to develop a self-consistent model of the cool cores based on new analytical and numerical tools.
Keywords
Galaxy clusters, intracluster medium, shock waves, NEPAL, Kelvin-Helmoholtz instabilities, computer simulations