Work

Hierarchical Dynamics within Molecular Clouds: Type 4 Scaling Relations of Molecular Clouds in the Second Galactic Quadrant

Year: 2026

Type: article

Abstract: We present a statistical study of the hierarchical structure of molecular clouds in the second quadrant of the Galactic midplane ( 139 . ° 75 ≤ l ≤ 159 . ° 75 , − 5 . ° 25 ≤ b ≤ 8 . ° 25 ), based on high-sensitivity 12 CO and 13 CO J = 1 − 0 data from the Milky Way Imaging Scroll Painting survey. Using DBSCAN and a nonbinary dendrogram algorithm, we identify 2912 13 CO clouds. Type 2 (multicloud, single-tracer) scaling relations show arm-dependent trends; the power-law index of the line width–size relation is ∼0.32 in the Local Arm and ∼0.42 in the Perseus and Outer Arms. The classical Keto–Heyer relation is weak in the Local Arm but present in the Perseus and Outer Arms with a slope of ∼0.3. The virial parameter is anticorrelated with cloud size, mass, and surface density, consistent with previous surveys. Type 4 (single-cloud, single-tracer) scaling relations exhibit stronger internal correlations. The line width–size index spans ∼0.3–0.8 and correlates positively with sonic Mach number, indicating that the relation is not universal but depends on cloud compressibility. While substructures do not follow the conventional surface-density-based Keto–Heyer relation, a volume-density formulation restores a clear virial scaling, with evolutionary tracks aligning along lines of constant virial parameter. Moreover, the structural complexity of molecular clouds, as characterized by the relationship between the number of substructures and the min_delta parameter, correlates with surface density rather than sonic Mach number, contrary to results from turbulence-only magnetohydrodynamic simulations. Together, these results are more consistent with a gravity-regulated hierarchical dynamical picture than with a purely turbulence-supported scenario.

Source: The Astrophysical Journal

DOI: 10.3847/1538-4357/aea8f8