Cold core clusters of galaxies

Sophisticated cosmological simulations finally get to the core. After years of attempts, recent numerical simulations reproduce the thermo-dynamical and chemical characteristics of the cores of galaxy clusters. The long-expected result has been obtained thanks to a detailed model of feedback from black hole accretion and a new hydrodynamic scheme. Astrophysicists from Trieste represent a copious fraction of the Authors' list.

Earlier cosmological simulations followed the gravitational interactions of few tens of particles. After three decades, the technological advancement and the modeling improvement lead to the description of the formation and evolution of large-scale structures through billion of particles and by including the hydro-dynamical forces on top of the gravitational ones. The state of the art of these theoretical representations foresees the inclusion of sub-grid recipes to describe the baryonic cycle, i.e. the formation of stars from cold gas, their evolution, and the feedback released during the supernovae explosion or the accretion around a black hole.

Numerical models, strong of these improvements, are now able to reproduce several of the main characteristics of observed galaxy clusters, including the scaling relations between global quantities, such as the gas mass, the temperature, the Compton parameter, galaxy velocity dispersion, as well as the external profiles of gas density, temperature, pressure, and entropy.

A though challenge remained until recently the description of the thermo-dynamical and chemical properties of the central regions of clusters. Without any exception, simulations produced cores with a peak in metal enrichment and a high entropy level. In other words, numerical models had failed to reproduce the observed difference between cool-core clusters (with low entropy and high central metallicity) and non-cool core clusters (with high entropy and low central metallicity). This theoretical stand off was broken with success only one month ago from the numerical group in Trieste (E. Rasia, S. Borgani, G. Murante, S. Planelles, V. Biffi, and G. L. Granato) in collaboration with the group of Klaus Dolag in Munich.

In a Letter, appeared in The Astrophysical Journal (2015, APJ, 813, L17) and titled "Cool Core Clusters from Cosmological Simulations", the Authors presented the new simulations enriched by a detailed model of feedback from the accretion into a black hole and by a new hydrodynamical scheme. These innovations allowed the distribution of the energy extracted from the supermassive-black that compensates the radiative losses of the central gas at low entropy, leading to a more realistic representation of the central metallicity and entropy.

 

Comparison between the theoretical results for Cool Clusters, green, and for Non-Cool Clusters, red, and the profiles obtained from X-ray data of the Chandra and XMM-Newton satellites. The analysis of the data is of Pratt et al. 2010, A&A, 511, A85 and Ettori et al. 2015, A&A, 578, A46.