Filling factors and scale heights of the DIG in the Milky Way

Abstract

The combination of dispersion measures of pulsars, distances from the model of Cordes and Lazio (2002) and emission measures from the WHAM survey enabled a statistical study of electron densities and filling factors of the diffuse ionized gas (DIG) in the Milky Way. The emission measures were corrected for absorption and contributions from beyond the pulsar distance. For a sample of 157 pulsars at |b| > 5 degrees, mainly located in interarm regions within about 3 kpc from the Sun, we find that: (1) The average volume filling factor along the line of sight is inversely proportional to the mean electron density in clouds. (2) The average volume filling factor increases towards larger distances from the Galactic plane. (3) The local volume filling factor may reach a maximum near |z| = 0.9 kpc, whereas the local electron density continues to decrease at higher |z|, thus causing the observed flattening in the distribution of dispersion measures perpendicular to the plane above this height. (4) The scale heights of the electron density, the volume filling factor and the emission measure are the same and in the range 250-500 pc.