The mean flow, velocity dispersion, energy transfer and evolution of rotating and growing dark matter halos

TL;DR

This paper analytically investigates the dynamics and evolution of rotating dark matter halos, revealing how mean flow, velocity dispersion, and energy transfer depend on halo size and stage, with implications for their shape, spin, and energy distribution.

Contribution

It provides an analytical framework for understanding the velocity dispersion, energy transfer, and evolution of rotating and growing dark matter halos, distinguishing between large and small halos.

Findings

Large halos exhibit self-similar radial flow and rotate with angular velocity proportional to Hubble parameter.

Small halos are spherical, isotropic, and rotate with constant angular velocity, with dominant non-spin dispersion.

Halo energy components remain invariant over time, with spin and shape evolving during relaxation.

Abstract

By decomposing velocity dispersion into non-spin and spin-induced, mean flow and dispersion are analytically solved for axisymmetric rotating and growing halos. The polar flow can be neglected and azimuthal flow is directly related to dispersion. The fictitious ("Reynolds") stress acts on mean flow to enable energy transfer from mean flow to random motion and maximize system entropy. For large halos (high peak height $\nu$ at early stage of halo life) with constant concentration, there exists a self-similar radial flow (outward in core and inward in outer region). Halo mass, size and specific angular momentum increase linearly with time via fast mass accretion. Halo core spins faster than outer region. Large halos rotate with an angular velocity proportional to Hubble parameter and spin-induced dispersion is dominant. All specific energies (radial/rotational/kinetic/potential) are time-invariant. Both halo spin ($\sim$0.031) and anisotropic parameters can be analytically derived. For "small" halos with stable core and slow mass accretion (low peak height $\nu$ at late stage of halo life), radial flow vanishes. Small halos rotate with constant angular velocity and non-spin axial dispersion is dominant. Small halos are spherical in shape, incompressible, and isotropic. Radial and azimuthal dispersion are comparable and greater than polar dispersion. Due to finite spin, kinetic energy is not equipartitioned with the greatest energy along azimuthal direction. Different from normal matter, small halos are hotter with faster spin. Halo relaxation from early to late stage involves variation of shape, density, mean flow, momentum, and energy. During relaxation, halo isotopically "stretches" with conserved specific rotational kinetic energy, increasing concentration and momentum of inertial. Halo "stretching" leads to decreasing angular velocity, increasing angular momentum and spin parameter.