Abstract Details
| Name: Arav Bhaskaran Jayaprasad Affiliation: Indian Institute of Astrophysics, Bangalore Conference ID: ASI2025_108 Title : A model of constrained violent relaxation to quasi-stationary states of spherical halos Authors and Co-Authors : Arav Bhaskaran Jayaprasad 1, A. Mangalam 1 Abstract Type : Poster Abstract Category : Galaxies and Cosmology Abstract : Collisionless self-gravitating systems, such as galaxies and dark matter halos, undergo violent relaxation during their formation. The potential oscillations that drive this relaxation damp after a few dynamical times, leading to an incompletely relaxed quasi-stationary state (QSS) that slowly evolves on relaxation time scales. Simulations of dissipationless collapse from cold initial conditions have shown that violently relaxed QSSs have several quasi-universal properties, which might explain the ubiquitous properties of elliptical galaxies. Following a picture of short-lived large amplitude oscillations that lead to a dynamically constrained phase space density, we introduce a class of anisotropic spherical distribution functions (DFs) that depend on energy $E$, pericenter $r_p$, and a free parameter $\xi_s$ which modulates the region of violent relaxation. A DF of these quantities, with a cutoff in pericenter $\phi(r_p, \xi_s)$, chosen to be of the Fermi-Dirac form, generalizes the model of Mangalam et al. (1999) and leads to QSS configurations. Based on the dynamical constraints of incomplete and confined relaxation (Tremaine 1987), we discuss two variants of DFs of this type that suppress orbits with high radial periods with a factor proportional to the radial frequency. These models possess several desirable properties that match well with dissipationless cold collapse simulations, including a density that falls off as $r^{-4}$, velocity dispersions that fall off as $\sigma^2\propto r^{-1}$, an isotropic core with a radially anisotropic halo, pseudo phase space density $\rho/\sigma^3\propto r^{-5/2}$, and an increasing energy distribution with a finite value at $E=0$. In addition, the surface brightness law follows the $R^{1/4}$ law that agrees with observed profiles of elliptical galaxies. We also discuss testing the stability of these models and their extension to axisymmetric systems. |