Electron distribution in the Galactic disc: results from a non-equilibrium ionization model of the interstellar medium

Abstract

Using 3D non-equilibrium ionization hydrodynamical simulation of the interstellar medium, we study the electron density, ne, in the Galactic disc and compare it with the values derived from dispersion measures (DMs) towards pulsars with known distances located up to 200 pc on either side of the Galactic mid-plane. The simulation results, consistent with observations, can be summarized as follows: (i) the DMs in the simulated disc lie between the maximum and minimum observed values; (ii) the log ne derived from lines of sight crossing the simulated disc follows a Gaussian distribution centred at μ = −1.4 with a dispersion σ = 0.21; thus, the Galactic mid-plane ne = 0.04 ± 0.01 cm−3; (iii) the highest electron concentration bymass (up to 80 per cent) is in the thermally unstable regime (200 < T < 103.9 K); (iv) the volume occupation fraction of the warm ionized medium is 4.9–6 per cent and (v) the electrons have a clumpy distribution along the lines of sight.