Low density ionized gas in the inner Galaxy - interpretation of recombination line observations at 325 MHz.

Abstract

The properties of diffuse ionized gas in the inner Galaxy are derived from a recent survey of the H 272-alpha recombination line (324.99 MHz) in the direction of 34 H II regions, 12 SNRs, and six regions of continuum minimum in the Galactic plane. The derived properties of the gas are an electron density of 0.5-6/cu cm, an electron temperature of 3000-8000 K, and emission measures of 500-3000 pc/cm exp 6. It is suggested that the low-frequency recombination lines arise in low-density envelopes of the H II regions; it is found that these envelopes should have electron densities in the range 1-10/cu cm and linear sizes of 30-300 pc in order to produce the observed H 272-alpha lines.