2001ApJ...557..226R

We present constraints on the physical properties of the ionized gas responsible for hydrogen radio recombination lines (RRLs) near 327 MHz detected in a recent Galactic plane survey made with the Ooty Radio Telescope. To obtain these constraints, we combined the data at 327 MHz with previously published RRL observations near 1.4 GHz. The density of the ionized gas is well constrained and is in the range of 1 to 10 cm^–3. The data implies upper limits to the temperature and size of the line emitting regions of ∼12,000 K and ∼500 pc, respectively. Assuming an electron temperature of 7000 K, the derived path lengths of the line emitting region are in the range of 20 to 200 pc. The derived properties of the ionized gas responsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 µm emission and a considerable fraction of the [C II] 158 µm emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The Hα emission from these ionized gases is mostly undetected in the existing Hα surveys because of large interstellar extinction. About 50% of the free-free absorption of the Galactic nonthermal radiation observed at frequencies less than 100 MHz can be accounted for by the same ionized gas. We also discuss the origin of this low-density ionized gas in the inner Galaxy. The derived low line-of-sight filling factor (<1%) for this ionized gas indicates that it does not form a pervasive medium. On the basis of the similarity of the distribution of this gas in the Galactic disk with that of the star-forming regions and the range of derived physical properties, we support the earlier suggestion that the low-frequency RRL emission originates from low-density ionized gas, which forms envelopes of normal H II regions.