Hydrogen recombination lines near 327 MHz. III. Physical properties and origin of the low-density ionized gas in the inner galaxy

We present constraints on the physical properties of the ionized gas respon sible for hydrogen radio recombination lines (RRLs) near 327 MHz detected i n a recent Galactic plane survey made with the Ooty Radio Telescope. To obt ain these constraints, we combined the data at 327 MHz with previously publ ished 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 similar to 12,000 K and similar to 500 pc, respectively. Assuming an electron tempe rature 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 res ponsible for the RRL emission near 327 MHz suggest that most of the [N II] 205 mum emission and a considerable fraction of the [C II] 158 mum emission observed in the Galactic plane by the COBE satellite could also originate in the same gas. The H alpha emission from these ionized gases is mostly un detected in the existing H alpha surveys because of large interstellar exti nction. About 50% of the free-free absorption of the Galactic nonthermal ra diation observed at frequencies less than 100 MHz can be accounted for by t he 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 Galac tic disk with that of the star-forming regions and the range of derived phy sical 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.