Low frequency recombination line surveys of the galactic plane

Weak centimetric recombination lines are observed at almost all locations near the galactic plane where the longitude is less than 500. Most of the emission probably comes from extended outer parts of normal HII regions whose dense cores are prominent in the radio continuum. At least one component of the emission, however, comes from a more broadly distributed medium that extends over a few hundred parsecs, and that has a density approximately 1 cm-3 and a temperature of a few thousand Kelvins or less. This gas is not associated with sites of current star formation. There is no evidence that the recombination line observations have detected a truly diffuse, low density, electron component of the interstellar medium. (Auth.)