Volume Filling Factors of the DIG in M 31

The volume filling factor f _v of the diffuse ionized gas in the bright emission ring of M 31 is derived from radio continuumobservations. The dependence of f _v on the local mean electron density n _e is a power law, f _v(n _e) = a n _e ^-bwith a = 0.011± 0.003 and b = -1.2± 0.3, where n _e is in cm^-3. The same power law was recently found for the DIG in the solar neighbourhood from pulsar data.