Clusters of galaxies
a Space Research Institute, Russian Academy of Sciences, ul. Profsoyuznaya 84/32, Moscow, 117997, Russian Federation
b Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, Massachusetts, 02138, USA
c The University of Chicago, 5801 South Ellis Ave., Chicago, Illinois, 60637, USA
d NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
e Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, Garching, 85741, Germany
Galaxy clusters form through nonlinear growth of primordial density fluctuations and are the most massive gravitationally-bound objects in the present Universe. Their number density at different epochs and their properties depend strongly on the properties of Dark Matter and Dark Energy, making clusters a powerful tool for observational cosmology. Observations of the hot gas filling the gravitational potential well of a cluster enable us to study gas-dynamical and plasma effects, and how the supermassive black holes affect heating and cooling of the gas on cluster scales. The work of Yakov Borisovich Zeldovich has had a profound impact on virtually all cosmological and astrophysical studies of galaxy clusters, introducing such concepts as the Harrison—Zeldovich spectrum, the Zeldovich approximation, baryon acoustic peaks, and the Sunyaev—Zeldovich effect. Here we review the most basic properties of clusters and their role in modern astrophysics and cosmology.