Carbon Recombination Lines near 327 MHz I: "Diffuse" CII regions in the Galactic Disk

Abstract

In earlier papers (Roshi & Anantharamaiah 2000, 2001a), we presented extensive surveys (angular resolution -- 2°$\times$ 2°& 2°$\times$ 6\arcmin) of radio recombination lines (RRLs) near 327 MHz in the longitude range $l=$ 332°$\to$ 89°using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between $l=$ 358°$\to$ 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'' \CII regions. The \lv diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from \HII regions indicating that the distribution of the diffuse \CII regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the [CII] 158 micron emission from diffuse \CII regions and find that upto 95 % of the total observed [CII] 158 micron emission can arise in diffuse \CII regions if the temperature of the latter $\sim 80$ K. Our high-resolution survey data shows that the carbon line emitting regions have structures on angular scale $\sim$ 6\arcmin. 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 ($\Delta V\le$ 15 \kms) 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 \CII region in these directions is $>$ 200 pc.

22 pages, 10 figs, Accepted for publication in Astronomy & Astrophysics