Carbon recombination lines near 327 MHz. I. ``Diffuse'' C II regions in the Galactic Disk

Abstract

In earlier papers (Roshi & Anantharamaiah \cite{ra00}, \cite{ra01a}), we presented extensive surveys (angular resolution - 2o x 2o and 2o x 6 arcmin) of radio recombination lines (RRLs) near 327 MHz in the longitude range l = 332o -〉 89o using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between l = 358o -〉 20o and in a few positions at other longitudes. This paper presents the observed carbon line parameters in the high-resolution survey and a study of the galactic distribution and angular extent of the line emission observed in the surveys. The carbon lines detected in the surveys arise in ``diffuse'' C II regions. The l-v diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from H II regions indicating that the distribution of the diffuse C II regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the [C II] 158 mu m emission from diffuse C II regions and find that upto 95% of the total observed [C II] 158 mu m emission can arise in diffuse C II regions if the temperature of the latter ~ 80 K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale ~ 6 arcmin. Analysis of the dual-resolution observations toward a 2o wide field centered at l = 13.9 deg and toward the longitude range l = 1.75 deg to 6.75 deg shows the presence of narrow (Delta V <= 15 km s-1) carbon line emitting regions extending over several degrees in l and b. The physical size perpendicular to the line-of-sight of an individual diffuse C II region in these directions is 〉200 pc.