Longitude survey of radio recombination lines from the diffuse interstellar medium

Observations of the H137a recombination line (1683.2 MHz), from regions of the galactic plane free of discrete radio sources, show the line radiation to come from a region spatially coincident with the zone of giant H ii regions-a zone hounded by galactic radii of approximately 3 and 9 kpc. Within this zone, the observations determine the quantity 11/T ' 5 to be 5 >c IO- cm K-i.5. If the ionization is in statistical equilibrium, the net ionization rate appears to be at least I0- i, compared to the value of tO- 1 measured outside the zone by other methods. However, the ionization might also be produced by supernova events, in whkh case a given volume of gas should be exposed to the ultraviolet flash at least every tO years.