The Size–Velocity Dispersion Relationship of Galactic H ii Regions

Abstract The size–velocity dispersion ( σ ) relation, while well established for giant H ii regions, remains uncertain for their smaller counterparts (physical radii R < 20 pc). Thanks to the Large Sky Area Multi-Object Fiber Spectroscopic Telescope MRS-N dataset’s large sky coverage and high spatial/spectral resolution, we examined this relationship using 10 isolated Galactic H ii regions with R < 20 pc. Our results reveal two key findings: (1) these small-size H ii regions remarkably follow the same size– σ relation as giant H ii regions, suggesting this correlation could serve as a novel distance indicator for Galactic H ii regions; and (2) we find distinct dynamical behaviors between younger and older H ii regions. Specifically, in younger (0.5 Myr), matter-bounded H ii regions, a clear correlation emerges, implying that expansion-driven processes begin to play a significant role in generating turbulence. We therefore propose an evolutionary transition in the primary turbulence mechanisms—from being dominated by stellar winds and radiation to being increasingly influenced by expansion-driven dynamics—during the evolution of H ii regions. Considering the small sample size used in this work—particularly the inclusion of only two young H ii regions, which also have large uncertainties in their expansion velocities—further confirmation of this interpretation will require higher-resolution 2D spectroscopy to resolve blended kinematic components along the line of sight for more accurate estimation of expansion velocities, along with an expanded sample that specifically includes more young H ii regions.