Discovery of a Giant Spiral Molecular Cloud in the Galactic Anticenter

Abstract We present wide-field and high-sensitivity CO (1–0) molecular line observations toward the Galactic anticenter region, using the 13.7 m millimeter telescope of the Purple Mountain Observatory. Based on the CO observations, we report the discovery of a giant spiral molecular cloud, which covers the region of 183 . ° 75 ≥ l ≥ 181 . ° 25 and −2 . ° 25 ≤ b ≤ 0 . ° 75, and the velocity range from approximately −16 to −8 km s −1 . The distance of the cloud is measured to be ∼1650 pc, while the derived dimension and gas mass of the cloud are ∼75 pc × 90 pc and ∼8 × 10 4 M ⊙ , respectively. Spiral arms and concentric rings are clearly detected in the cloud, which mimics a typical spiral galaxy observed in the local Universe. The oscillation of radial velocities is observed across the cloud, with a spatial wavelength of ∼4.8–5.3 pc and a velocity amplitude of ∼ 0.11–0.13 km s −1 . The spiral cloud is undergoing slow rotation with an approximately constant velocity gradient (∼2–3 km s −1 deg −1 or ∼ 0.07–0.10 km s −1 pc −1 ). Similar to many spiral galaxies, the observed structure and kinematics of the cloud can be explained by the density wave theory. The discovery provides a unique cloud sample in the solar neighborhood, which may help us to understand the nature of spiral structures in distant galaxies.