Electron Distribution in the Galactic Disk - Results From a Non-Equilibrium Ionization Model of the ISM

TL;DR

This study uses 3D non-equilibrium ionization hydrodynamical simulations to analyze electron density distribution in the Galactic disk, aligning well with pulsar dispersion measure observations and revealing a predominantly thermally unstable electron population.

Contribution

It provides the first detailed NEI simulation-based analysis of electron density distribution in the Galactic disk, matching observational data and characterizing the thermally unstable electron component.

Findings

Electron densities from simulations match observed dispersion measures.

Electron density distribution follows a Gaussian centered at log <n_e>=-1.4.

Majority of electrons are in thermally unstable temperature regime.

Abstract

Using three-dimensional non-equilibrium ionization (NEI) hydrodynamical simulation of the interstellar medium (ISM), we study the electron density, $n_{e}$, in the Galactic disk and compare it with the values derived from dispersion measures towards pulsars with known distances located up to 200 pc on either side of the Galactic midplane. The simulation results, consistent with observations, can be summarized as follows: (i) the DMs in the simulated disk lie between the maximum and minimum observed values, (ii) the log <n_e> derived from lines of sight crossing the simulated disk follows a Gaussian distribution centered at \mu=-1.4 with a dispersion \sigma=0.21, thus, the Galactic midplane <n_e>=0.04\pm 0.01$ cm$^{-3}$, (iii) the highest electron concentration by mass (up to 80%) is in the thermally unstable regime (200<T<10^{3.9} K), (iv) the volume occupation fraction of the warm ionized medium is 4.9-6%, and (v) the electrons have a clumpy distribution along the lines of sight.