Carbon recombination lines near 327 MHz

I. “Diffuse” regions in the Galactic Disk

¹ National Radio Astronomy Observatory, Green Bank, WV 24944, USA (The National Radio Astronomy Observatory is operated by associated Universities, Inc., under agreement with the National Science Foundation.)
² National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Pune, India
³ Raman Research Institute, Bangalore, India

Corresponding author: D. Anish Roshi, aroshi@nrao.edu

Received: 30 January 2002
Accepted: 13 June 2002

Abstract

In earlier papers (Roshi & Anantharamaiah [CITE], [CITE]), we presented extensive surveys (angular resolution – 2°  2° and 2°  6´) of radio recombination lines (RRLs) near 327 MHz in the longitude range 332°  89° using the Ooty Radio Telescope. These surveys have detected carbon lines mostly between 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”  regions. The  diagram and radial distribution constructed from our carbon line data shows similarity with that obtained from hydrogen recombination lines at 3 cm from  regions indicating that the distribution of the diffuse  regions in the inner Galaxy resembles the distribution of the star-forming regions. We estimated the ]  158 μm emission from diffuse  regions and find that upto 95% of the total observed ]  158 μm emission can arise in diffuse  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 139 and toward the longitude range 175 to 675 shows the presence of narrow ( 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  region in these directions is >200 pc.