Radio Recombination Line Emissionfrom the Galactic Plane

166 α Radio recombination line measurements have been made of the distributed electron gas along the galactic ridge with the Mark II radio telescope (beamwidth 31′ × 33′ arc). Spectra taken at 13 points distributed along the galactic ridge between $$l = -1^{\circ.} 0$$ and $$47^{\circ.} 5$$ show that the bulk of the recombination line emission is confined to the region $$l\lesssim 40^\circ (R\lesssim7\enspace \text {kpc})$$ . The electron temperature, T_e , is found to be 6000±1000 K. This result contradicts the assertion by Gottesman & Gordon that $$T_\text e \sim 10^3\enspace \text K$$ . The recombination line and continuum emission originate in elongated H II complexes 3° to 5° in extent whose mean emission measure is 2700 cm ⁻⁶ pc and rms electron density is 1.9 cm ⁻³ . Their physical properties are similar to those of the low density H II regions NGC 2244, NGC 7000 and NGC 7822. It is shown that the density of the electron gas in the central regions of the Galaxy is substantially greater than in the spiral arm regions near the Sun.