On the origin of the Galactic ridge recombination lines.

Radio recombination lines are known to be observable at positions along the galactic ridge which are free of discrete continuum sources. Based on the results of a recent survey of H272 alpha lines it is shown that most of the observed Galactic ridge recombination lines can be explained as emission from outer low-density envelopes of normal H II regions. The distribution of low-density ionized gas and discrete H II regions as a function of the distance from the Galactic center is also derived