Carbon recombination lines near 327 MHz I. "Diffuse" C II regions in the galactic disk

Abstract

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution - 2°× 2° and 2° 6') of radio recombination lines (RRLs) near 327 MHz in the longitude range l =332°→ 89° using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between l = 358°→ 20° 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-ν 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 µm emission from diffuse C _II regions and find that upto 95% of the total observed [C _II] 158µ 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'. Analysis of the dual-resolution observations toward a 2 wide field centered at l =13° .9 and toward the longitude range l = 1°.75 to 6°.75 shows the presence of narrow ( ΔV≤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.