Cloud-to-cloud Velocity Dispersions Across a Local Arm Segment

Using a large sample of 9617 molecular clouds (MCs) from the Milky Way Imaging Scroll Painting survey, we mainly measure one-dimensional cloud-to-cloud velocity dispersions across a 450 deg 2 segment of the Local Arm in the Galactic second quadrant. We define the cloud-to-cloud velocity dispersion using two metrics: the standard deviation ( σ bin ) and flux-weighted rms value ( σ bin,w ) of the centroid velocities of 12 CO-detected MCs within spatial bins. The typical values of σ bin and σ bin,w are 7.5 ± 0.5 km s −1 and 6.2 ± 0.5 km s −1 , respectively. After categorizing clouds by sizes into three types: Type S (0.15–1.2 pc), Type M (1.2–4.8 pc), and Type L (≳4.8 pc), we find that the spatial distribution of Type S and M MCs projected onto the Galactic longitude–latitude ( l – b ) plane is generally uniform. Additionally, the cloud-to-cloud velocity dispersion among Type S clouds (∼7.6 and 7.4 km s −1 for σ bin and σ bin,w , respectively) is systematically greater than that among Type M clouds (∼6.7 and 6.0 km s −1 for σ bin and σ bin,w , respectively), with differences of 0.9–1.4 km s −1 . From these measurements, we estimate merger timescales between MCs to be approximately 0.3–0.9 Myr, which is shorter than their internal crossing timescales (∼1 Myr for Type S , ∼2 Myr for Type M , and ≳5 Myr for Type L ). This disparity, particularly pronounced for larger Type L clouds, suggests that MCs are dynamically transient structures, with their gas content changing due to frequent interactions with neighboring clouds.