A Large-scale ¹²CO, ¹³CO, and C¹⁸O Molecular Cloud Survey in the Outer Galactic Plane over I = [129.°75, 140.°25] and b = [−5.°25, +5.°25]

We present the data of an unbiased J = 1–0 ¹²CO/¹³CO/C¹⁸O survey of molecular clouds in the Galactic plane over l = [129.°75, 140.°25] and b = [−5.°25, +5.°25]. For the full 10.°5 × 10.°5 region sampled at 30″, there are 1,590,120 spectra for each isotopologue. The high sensitivity and large spatial dynamic range of the maps provide valuable insights into the structure, physical properties, and kinematics of the molecular gas. The new data successfully trace the most distant spiral arm, as well as the internal subfeatures of the nearby spiral arms. The combined data set of ¹²CO, ¹³CO,  and C¹⁸O  allows us to make a more accurate inventory of gas column density and mass for molecular gas within this Galactic interval than hitherto attempted. A statistical analysis reveals that a large fraction of the observed molecular gas is emitted from regions with low excitation conditions. As expected, a considerable amount of H₂ mass is found to be confined to the spiral arms. Moreover, we find that the C¹⁸O emission is exclusively seen along spiral arms at current detection limits. The physical properties of molecular gas vary considerably in different spiral arms and inter-arms. We find that the column density probability density functions and the amount of relatively denser gas (traced by ¹³CO or C¹⁸O) appear to be closely related to the level of massive star formation activity within the molecular gas. The high-quality data also reveal that the warp and flare of the Galactic plane become obvious beyond the Perseus arm in this Galactic longitude range.