Molecular Clouds in the Extreme Outer Galaxy between l = 34.°75 to 45.°25

Abstract

Abstract We present the results of an unbiased CO survey in the Galactic range of 34.°75 ≤ l ≤ 45.°25 and −5.°25 ≤ b ≤ 5.°25, and the velocity range beyond the Outer arm. A total of 168 molecular clouds (MCs) are identified within the Extreme Outer Galaxy (EOG) region, and 31 of these MCs are associated with 13CO emission. However, none of them show significant C18O emission under the current detection limit. The typical size and mass of these MCs are 5 pc and 3 × 103 M ⊙, implying a lack of large and massive MCs in the EOG region. Similar to MCs in the outer Galaxy, the velocity dispersions of EOG clouds are also correlated with their sizes; however, they are well displaced below the scaling relationship defined by the inner Galaxy MCs. These MCs with a median Galactocentric radius of 12.6 kpc show very different distributions from those of the MCs in the Outer arm published in our previous paper, while roughly following the Outer Scutum–Centaurus arm defined by Dame & Thaddeus. This result may provide robust evidence for the existence of the Outer Scutum–Centaurus arm. The lower limit of the total mass of this segment is about 2.7 × 105 M ⊙, which is about one magnitude lower than that of the Outer arm. The mean thickness of the gaseous disk is about 1.°45 or 450 pc, and the scale height is about 1.°27, or 400 pc above the b = 0° plane. The warp traced by CO emission is very obvious in the EOG region and its amplitude is consistent with the predictions by other warp models using different tracers, such as dust, H i, and stellar components of our Galaxy.