Relative Velocities between 13CO Structures within 12CO Molecular Clouds

Velocity fields of molecular clouds (MCs) can provide crucial information on the merger and split between clouds, as well as their internal kinematics and maintenance, energy injection and redistribution, and even star formation within clouds. Using the CO spectral lines data from the Milky Way Imaging Scroll Painting survey, we measure the relative velocities along the line of sight (ΔVLOS) between 13CO structures within 12CO MCs. Emphasizing MCs with double and triple 13CO structures, we find that approximately 70% of ΔVLOS values are less than ∼1 km s−1, and roughly 10% of values exceed 2 km s−1, with a maximum of ∼5 km s−1. Additionally, we compare ΔVLOS with the internal velocity dispersion of 13CO structures (σ13CO,in) and find that about 40% of samples in either double or triple regime display distinct velocity discontinuities, i.e., the relative velocities between 13CO structures are larger than the internal line widths of 13CO structures. Among these 40% samples in the triple regime, 33% exhibit signatures of combinations through the two-body motion, whereas the remaining 7% show features of configurations through the multiple-body motion. The ΔVLOS distributions for MCs with double and triple 13CO structures are similar, as well as their ΔVLOS/σ13CO,in distributions. This suggests that relative motions of 13CO structures within MCs are random and independent of cloud complexities and scales.