Fluid-like entropy and equilibrium statistical mechanics of self-gravitating systems

TL;DR

This paper advances the understanding of self-gravitating systems by using a fluid-like entropy approach with improved extremizing methods, resulting in realistic density profiles consistent with observations and proposing a hybrid entropy model.

Contribution

It introduces an improved extremizing process for fluid-like entropy in self-gravitating systems, producing finite mass and energy density profiles aligned with observational data.

Findings

Derived density profiles with finite mass and energy

Profiles consistent with elliptical galaxy NGC 3379 observations

Proposed hybrid entropy model combining Boltzmann-Gibbs and Tsallis entropy

Abstract

The statistical mechanics of self-gravitating systems has not been well understood, and still remains an open question so far. In a previous study by Kang & He, we showed that the fluid approximation may give a clue to further investigate this problem. In fact, there are indeed many dynamical similarities between self-gravitating and fluid systems. Based on a fluid-like entropy, that work explained successfully the outer density profiles of dark matter halos, but there left some drawbacks with the calculation concerning extremizing process of the entropy. In the current paper, with the improved extremizing calculation -- including an additional differential constraint of dynamical equilibrium and without any other assumptions, we confirm that statistical-mechanical methods can give a density profile with finite mass and finite energy. Moreover, this density profile is also consistent with the observational surface brightness of the elliptical galaxy NGC 3379. In our methods, the density profile is derived from the equation of state, which is obtained from entropy principle but does not correspond to the maximum entropy of the system. Finally, we suggest an alternative entropy form, a hybrid of Boltzmann-Gibbs and Tsallis entropy, whose global maximum may give rise to this equation of state.