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

Abstract

We present the results of a low-resolution (2° × 2°) survey of radio recombination lines (RRLs) near 327 MHz in the Galactic plane made with the Ooty Radio Telescope (ORT). Although the angular resolution is coarse, these observations represent the first contiguous survey of low-frequency RRL emission in the longitude range l = 330°-0°-89° (inner Galaxy). Hydrogen RRLs were detected in almost all directions in the inner Galaxy and carbon lines in several positions. In the outer Galaxy (l = 172°-252°), an unbiased set of 14 positions were observed 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 Galactic latitude at two specific longitudes (l = 0°.0 and 13°.9). RRLs were detected up to b = ±3°. The l-ν diagram and the radial distribution, obtained from RRL emission near 327 MHz, show good similarity with that of RRL emission near 1.4 GHz, "intense" ¹²CO emission and to some extent with the RRLs observed near 3 cm from normal H II region. These distributions are distinctly different from those of Hα and H I emission from the Galactic disk. Based on a comparison of the radial distribution of different components in the Galactic disk, we conclude that the diffuse RRL emission is associated with star-forming regions and possibly with a low-density component of known H II regions in the inner Galaxy.