Atomic carbon in the southern Milky Way

Abstract

We present a coarsely sampled longitude-velocity (l-V) map of the region l = 300 degrees - 354 degrees, b = 0 degrees in the 492 GHz fine-structure transition of neutral atomic carbon (C-0 P-3(1) - P-3(0); [C I]), observed with the Portable 18 cm Submillimeter-wave Telescope (POST18). The l-V distribution of the [ C I] emission resembles closely that of the CO J = 1-0 emission, showing a widespread distribution of atomic carbon on the Galactic scale. The ratio of the antenna temperatures, R-C I/CO, concentrates on the narrow range from 0.05 to 0.3. A large velocity gradient (LVG) analysis shows that the [C I] emission from the Galactic disk is dominated by a population of neutral gas with high C-0/CO abundance ratios and moderate column densities, which can be categorized as diffuse translucent clouds. The ratio of bulk emissivity, J(C I)/J(CO), shows a systematic trend, suggesting the bulk C-0/CO abundance ratio increasing with the Galactic radius. A mechanism related to kiloparsec-scale structure of the Galaxy may control the bulk C-0/CO abundance ratio in the Galactic disk. Two groups of high-ratio (R-C I/CO > 0.3) areas reside in the l-V loci several degrees inside of tangential points of the Galactic spiral arms. These could be gas condensations just accumulated in the potential well of spiral arms and be in the early stages of molecular cloud formation.