A measurement of the hydrogen recombination rate in the diffuse interstellar medium

Abstract

The results of an analysis of H-alpha emission in the diffuse interstellar medium are reported. The analysis is performed on the basis of Fabry-Perot spectrometer observations of H-alpha emission covering a 160 km/s radial velocity interval. It is found that the intensity of the galactic H-alpha background at high galactic latitudes implies an average hydrogen recombination rate of 4 x 10 to the 6th per sec/sq cm of galactic disk for the region within 2 to 3 kpc from the sun. This rate eliminates some possible sources of the ionization for the diffuse interstellar medium. O-type stars and perhaps planetary nebula are the only known sources of ionizing radiation with Lyman continuum fluxes equal to or greater than the hydrogen recombination rate.