Magnetized galactic halos and velocity lags

TL;DR

This paper develops an analytic model of magnetized galactic halos to explain observed magnetic field structures and velocity lags, highlighting magnetic influence on angular momentum transport and halo dynamics.

Contribution

It introduces a steady-state, scale-free analytic model of magnetized galactic halos with anisotropic equipartition, linking magnetic fields to velocity lag profiles.

Findings

Reproduces observed X-shaped magnetic field structures.

Provides an analytic formula for velocity lag decline with height.

Predicts how lag varies with radius.

Abstract

We present an analytic model of a magnetized galactic halo surrounding a Mestel gravitating disc. The magnetic field is taken to be in energy equipartition with the pressure dominant rotating halo gas ({\it not} with the cosmic rays), and the whole system is in a steady state. A more flexible `anisotropic equipartition' model is also explored. A definite pressure law is required to maintain the equilibrium, but the halo density is constant. The velocity/magnetic system is scale-free. The objective is to find the rotational velocity lag in such a halo. The magnetic field is not force-free so that angular momentum may be transported from the halo to the intergalactic medium. We find that the `X'-shaped structure observed for halo magnetic fields can be obtained together with a simple analytic formula for the rate of decline of the velocity with height $z$. The formula also predicts the change in lag with radius, $r$.