Hydrogen recombination lines near 327 MHz. I. Distribution of low-density ionized gas in the Galactic disk

We present the results of a low-resolution (2 degrees x 2 degrees) survey o f radio recombination lines (RRLs) near 327 MHz in the Galactic plane made with the Ooty Radio Telescope (ORT). Although the angular resolution is coa rse, these observations represent the first contiguous survey of low-freque ncy RRL emission in the longitude range l = 330 degrees-0 degrees-89 degree s (inner Galaxy). Hydrogen RRLs were detected in almost all directions in t he inner Galaxy and carbon lines in several positions. In the outer Galaxy (l = 172 degrees-252 degrees), an unbiased set of 14 positions were observe d and lines were detected toward three of them. To study the extent of the ionized gas above the Galactic disk, we have observed RRLs along the Galact ic latitude at two specific longitudes (I = 0 degrees.0 and 13.degrees 9). RRLs were detected up to b = +/-3 degrees. The l-v diagram and the radial d istribution, obtained from RRL emission near 327 MHz, show good similarity with that of RRL emission near 1.4 GHz, "intense" (CO)-C-12 emission and to some extent with the RRLs observed near 3 cm from normal H II region. Thes e distributions are distinctly different from those of H alpha and H I emis sion from the Galactic disk. Based on a comparison of the radial distributi on of different components in the Galactic disk, we conclude that the diffu se RRL emission is associated with star-forming regions and possibly with a low-density component of known H II regions in the inner Galaxy.