NRO/CSO/ASTE Galactic Plane CO Survey

Abstract

How the gas dynamics around the spiral arms (compression, shock, expansion) affects the internal structure of molecular clouds and physical conditions of the gas? We are carrying out a CO survey of the Galactic plane. The distribution of the molecular gas in the Galactic spiral arms and interarm is observed in CO and CO at three rotational transitions (J = 1–0, 2–1, and 3–2) using the Nobeyama Radio Observatory (NRO) 45-m, the Caltech Submillimeter Observatory (CSO) 10-m, and the Atacama Submillimeter Telescope Experiment (ASTE) 10-m telescopes. A pilot region (0. ◦8×0. ◦8 in the J = 1–0, 0. ◦3×0. ◦5 in the 2–1 and 3–2) at l 38◦ has been observed at a spatial resolution of 15–30′′ (0.5–1 pc at 8 kpc). The arm and interarm regions are simultaneously sampled at different radial velocities in the line of sight. We find that the molecular gas in the arm is confined mostly in dense clumps and filaments, while the interarm gas shows a featureless, extended distribution. The extension of the interarm gas is greater than the survey coverage, 90 pc × 90 pc, exceeding the size of the largest giant molecular cloud. Additionally, the interarm gas has no small-scale, internal structure over the large extension, indicating a new molecular gas component in the Galaxy. The observed CO J = 3–2/CO J = 1–0 intensity ratio, R3–2/1–0( CO), is higher in the arm (0.37 ± 0.03) than in the interarm (0.28±0.02). The arm gas, even the ambient gas between clumps, exhibits higher line ratio than the interarm gas. The large-velocity-gradient model indicates that R3–2/1–0( CO) tightly correlates with the kinetic temperature of the gas: the arm molecular gas is warm (warmer than 10 K, up to several × 10 K), while the interarm gas is cooler than 10 K.