Review to be published in 2012 Annual Reviews of Astronomy & Astrophysics
astro-ph/1205.5556

For a PDF version of the article, click here.

FORMATION OF GALAXY CLUSTERS

Andrey Kravtsov 1 and Stefano Borgani 2


1 Department of Astronomy & Astrophysics, Kavli Institute for Cosmological Physics, The University of Chicago, Chicago, IL 60637
2 Dipartimento di Fisica dell Università di Trieste, Sezione di Astronomia, I-34131 Trieste, Italy;
INAF Osservatorio Astronomico di Trieste, Italy;
and INFN Istituto Nazionale di Fisica Nucleare, Trieste, Italy;

Abstract: Formation of galaxy clusters corresponds to the collapse of the largest gravitationally bound overdensities in the initial density field and is accompanied by the most energetic phenomena since the Big Bang and by the complex interplay between gravity-induced dynamics of collapse and baryonic processes associated with galaxy formation. Galaxy clusters are, thus, at the cross-roads of cosmology and astrophysics and are unique laboratories for testing models of gravitational structure formation, galaxy evolution, thermodynamics of the intergalactic medium, and plasma physics. At the same time, their large masses make them a useful probe of growth of structure over cosmological time, thus providing cosmological constraints that are complementary to other probes. In this review, we describe our current understanding of cluster formation: from the general picture of collapse from initial density fluctuations in an expanding Universe to detailed simulations of cluster formation including the effects of galaxy formation. We outline both the areas in which highly accurate predictions of theoretical models can be obtained and areas where predictions are uncertain due to uncertain physics of galaxy formation and feedback. The former includes the description of the structural properties of the dark matter halos hosting cluster, their mass function and clustering properties. Their study provides a foundation for cosmological applications of clusters and for testing the fundamental assumptions of the standard model of structure formation. The latter includes the description of the total gas and stellar fractions, the thermodynamical and non-thermal processes in the intracluster plasma. Their study serves as a testing ground for galaxy formation models and plasma physics. In this context, we identify a suitable radial range where the observed thermal properties of the intra-cluster plasma exhibit the most regular behavior and thus can be used to define robust observational proxies for the total cluster mass. Finally, we discuss the formation of clusters in non-standard cosmological models, such as non-Gaussian models for the initial density field and models with modified gravity, along with prospects for testing these alternative scenarios with large cluster surveys in the near future.

Keywords: Cosmology, galaxy clusters, intra-cluster medium


Table of Contents

INTRODUCTION

THE OBSERVED PROPERTIES OF GALAXY CLUSTERS

UNDERSTANDING THE FORMATION OF GALAXY CLUSTERS
Initial density perturbation field and its linear evolution
Non-linear evolution of spherical perturbations and non-linear mass scale
Spherical collapse model.
The nonlinear mass scale MNL
Nonlinear collapse of real density peaks
Equilibrium
Internal structure of cluster halos
Density Profile
Velocity dispersion profile and velocity anisotropy
Shape
Mass definitions
The Friends-of-Friends mass
The Spherical Overdensity mass
Abundance of halos
The mass function and its universality
Calibrations of halo mass function in cosmological simulations
Clustering of halos
Self-similar evolution of galaxy clusters
Self-similar model: assumptions and basic expectations
The Kaiser model for cluster scaling relations
Extensions of the Kaiser model
Practical implications for observational calibrations of scaling relations
Cluster formation and Thermodynamics of the Intra-cluster gas
Gravitational collapse of the intra-cluster gas
Phenomenological pre-heating models
The role of radiative cooling
Thermodynamics of the intracluster medium with stellar and active galactic nuclei feedback

REGULARITY OF THE CLUSTER POPULATIONS
Characterizing regularity
Scaling relations

CLUSTER FORMATION IN ALTERNATIVE COSMOLOGICAL MODELS
Mass function and bias of clusters in non-Gaussian models
Formation of clusters in modified gravity models

SUMMARY AND OUTLOOK

REFERENCES

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