Formation of Galaxy Clusters

Abstract: 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. We put particular emphasis on examining assumptions and limitations of the widely used self-similar model of clusters. 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.

• The paper demonstrates that galaxy clusters form through hierarchical mergers driven by dark matter dynamics as shown in cosmological simulations.
• Observations reveal self-similar scaling relations between X-ray luminosity, temperature, and mass, emphasizing gravitational dominance in cluster evolution.
• The study highlights challenges in accurately modeling baryonic processes, such as AGN feedback, which affect the intracluster medium and cooling flows.

Insights on the Formation of Galaxy Clusters

The formation and evolution of galaxy clusters are pivotal in understanding the complex interplay between gravitational forces and baryonic processes within the universe. The paper by Andrey V. Kravtsov and Stefano Borgani offers an exhaustive review of the various mechanisms underlying the formation of galaxy clusters, highlighting their importance as both cosmological and astrophysical entities. Galaxy clusters represent the apex of gravitationally bound structures arising from initial density perturbations in the universe, thus serving as powerful tools for analyzing gravitational structure formation and galaxy evolution.

Theoretical Framework and Model Predictions

The formation of galaxy clusters is traditionally explained within the framework of the hierarchical model of structure formation. This model posits that clusters form via the merger and accretion of smaller structures. Within this framework, clusters are often examined through the lens of dark matter (DM)-driven dynamics, and their properties are inferred from cosmological N-body simulations. The paper discusses the success of such simulations in capturing the self-similar nature of cluster formation, where many properties of clusters scale predictably with their mass and redshift. Despite this, certain nuances remain, particularly concerning the interaction between baryonic and non-baryonic components.

Observational Evidence and Scaling Relations

Observations across diverse wavelengths provide crucial insights into galaxy clusters, revealing their complex constituents: luminous galaxies, intracluster plasma, and DM. The paper reviews studies that focus on the scaling relations, such as those between X-ray luminosity, temperature, and total mass, which are instrumental for utilizing clusters in cosmological studies. These relations often align with the self-similar model, particularly beyond the cluster cores, evidencing minimal scatter and highlighting the gravitational dominance in these halos.

Challenges in Modeling Baryonic Processes

One significant challenge addressed is accurately modeling baryonic processes, particularly the feedback from star formation and active galactic nuclei (AGN), which are not as thoroughly understood as gravitational dynamics. This uncertainty in the baryonic physics introduces complexities in the thermodynamic properties of the intracluster medium (ICM). The paper further elaborates on the discrepancy between simulations and observations in the core regions of clusters, where non-gravitational processes significantly alter gas properties, such as cooling flows suppressed by AGN feedback mechanisms.

Non-Standard Models and Beyond

The authors also explore cluster formation within non-standard cosmological models, including those with non-Gaussian initial conditions and modified gravity theories. These models present alternative approaches to understanding cluster abundance and bias, deviating from the predictions of the standard ΛCDM cosmology. They reveal how clusters can potentially constrain deviations in the initial conditions of the universe and modifications to gravity on large scales.

Conclusion and Future Directions

In conclusion, this paper underscores the critical role of galaxy clusters as laboratories for studying fundamental cosmic processes. Their large masses and constituent complexity make them indispensable for testing gravitational theories and exploring the cosmic web's evolution. Despite the progress achieved, further advancements in both observational techniques and computational simulations are necessary to reconcile discrepancies, particularly in baryonic models, and to solidify the role of clusters in constraining fundamental cosmological parameters. Future surveys and high-sensitivity measurements promise to refine our understanding of clusters, contributing to a more cohesive picture of the universe's structure and dynamics.

The research concerning galaxy clusters remains a dynamic field, constantly adapting to incorporate new findings and methodologies, thereby remaining at the forefront of both observational and theoretical astrophysics.