Marco Merafina

Dark energy and the structure of the Coma cluster of galaxies

Context. We consider the Coma cluster of galaxies as a gravitationally bound physical system embedded in the perfectly uniform static dark energy background as implied by theCDM cosmology. Aims. We ask if the density of dark energy is high enough to affect the structure of a large rich cluster of galaxies? Methods. We use recent observational data on the cluster together with our theory of local dynamical effects of dark energy, including the zero-gravity radius RZG of the local force field as the key parameter. Results. 1) Three masses are defined which characterize the structure of a regular cluster: the matter mass MM, the dark-energy effective mass MDE ( 14 Mpc and should be taken into account when its total mass is derived.

STABILITY OF DENSE STELLAR CLUSTERS AGAINST RELATIVISTIC COLLAPSE. II. MAXWELLIAN DISTRIBUTION FUNCTIONS WITH DIFFERENT CUTOFF PARAMETERS

We investigate the stability of dense stellar clusters against relativistic collapse by approximate methods described in the previous paper in this series. These methods, together with the analysis of the fractional binding energy of the system, have been applied to sequences of equilibrium models, with cutoff in the distribution function, which generalize those studied by Zeldovich & Podurets. We show the existence of extreme configurations, which are stable all the way up to infinite values of the central redshift.

Galactic cluster winds in presence of a dark energy

We obtain a solution for the hydrodynamic outflow of the polytropic gas from the gravitating center, in presence of the uniform Dark Energy (DE). The antigravity of DE is enlightening the outflow and make the outflow possible at smaller initial temperature, at the same density. The main property of the wind in presence of DE is its unlimited acceleration after passing the critical point. In application of this solution to the winds from galaxy clusters we suggest that collision of the strongly accelerated wind with another galaxy cluster, or with another galactic cluster wind could lead to the formation of a highest energy cosmic rays.

Self-Gravitating Gas Spheres in a Box and Relativistic Clusters: Relation between Dynamical and Thermodynamical Stability

We derive a variational principle for the dynamical stability of a cluster as a gas sphere in a box. Newtonian clusters are always dynamically stable, and for relativistic clusters, the relation between dynamical and thermodynamical instabilities is analyzed. The boundaries between dynamically and thermodynamically stable and unstable models are found numerically for relativistic stellar systems with different cutoff parameters. A criterion based on the binding energy curve is used for determination of the boundary of dynamical stability.

Galaxy clusters in presence of dark energy: a kinetic approach

Context. The external regions of galaxy clusters may be under strong influence of the dark energy, which was discovered by observations of supernovae Ia at redshift z < 1. The presence of the dark energy in the gravitational equilibrium equation, with the Einstein Λ term, balances the gravity, and extends the equilibrium configuration more in radius. Aims. We investigate the features of the equilibrium configurations to analyse how the presence of the dark energy affects the density profiles and radial extension by specifying the conditions for which the gravitational equilibrium begins. Methods. We derived the kinetic equation for an equilibrium configuration in presence of dark energy and solved the gravitational equilibrium equation by considering a Maxwell-Boltzmann distribution function with a cut-off in the framework of the Newtonian regime, because the observed velocities of galaxies inside a cluster are much lower than the velocity of light. Results. The prevalence of dark energy effects on the gravity shows a wide region in the W0‐ρΛ diagram where equilibrium solutions are not possible. In these particular conditions, the galaxies located in the external regions of a cluster can flow out, following the accelerating expansion of the Universe.

SPHERICALLY SYMMETRIC STELLAR CLUSTERS WITH ANISOTROPY AND CUTOFF ENERGY IN MOMENTUM DISTRIBUTION. I. THE NEWTONIAN REGIME

We construct numerical models of spherically symmetric Newtonian stellar clusters with anisotropic distribution functions. These models generalize solutions obtained earlier for isotropic Maxwellian distribution functions with an energy cutoff and take into account distributions with different levels of anisotropy.

Relativistic Collisionless Clusters with Arbitrarily Large Central Redshift

Some previous results obtained by Rasio, Shapiro, & Teukolsky are analyzed within the theoretical framework developed by Bisnovatyi-Kogan et al. The Rasio et al. configurations are shown to belong to a special family of configurations of equilibrium for relativistic clusters with a constant value of the central cutoff energy W0 114. A new classification of the configurations of equilibrium into three different regimes in the central redshift versus temperature diagram is proposed. The possible confrontation of these results with observational parameters is outlined briefly.

Systems of self-gravitating bosons with a cut-off in their distribution function. Newtonian treatment

SummaryWe examine the Newtonian equilibrium configurations of a system of bosons undergoing quantum condensation, with a distribution function with a cutoff in the momentum space. Bounded configurations with a core of condensed particles surrounded by an uncondensed phase are obtained. The results are compared and contrasted with the ones in which the spatial divergences are removed by a cut-off in density. The well-known solution corresponding to fully condensed configurations is obtained for suitable values of the central density.

Data Analysis of Globular Cluster Harris Catalogue in View of the King Models and Their Dynamical Evolution. II. Observational Evidences

We summarize some observational comparison concerning the features of globular clusters (GCs) population in connection to the evolution of King models. We also make a comparison with some extragalactic GCs systems, in order to underline the effects of the main body on the dynamical evolution.

Data Analysis of Globular Cluster Harris Catalogue in View of the King Models and Their Dynamical Evolution. I. Theoretical model

We discuss the possibility to analyze the problem of gravothermal catastrophe in a new way, by obtaining thermodynamical equations to apply to a selfgravitating system. By using the King distribution function in the framework of statistical mechanics we treat the globular clusters evolution as a sequence of quasi-equilibrium thermodynamical states.