The size-velocity dispersion relationship of Galactic HII regions

TL;DR

The study tests whether small Galactic HII regions with $R<20$ pc share the giant-region size–$\sigma$ relation, using 10 isolated HII regions from the LAMOST MRS-N dataset and four data-processing pipelines. After correcting for instrumental, thermal, and fine-structure broadening, the authors find that these regions largely obey the relation $\sigma ∝ R^{0.406}$, consistent with previous work on larger regions, and thus support a universal size–$\sigma$ scaling that could serve as a distance indicator. A key result is the distinct $\sigma$–$V_{exp}$ behavior: eight older, matter-bounded regions show a strong correlation between velocity dispersion and expansion velocity, while the two younger ionization-bounded regions do not, implying a transition from winds/ionization-driven turbulence to expansion-driven turbulence during evolution. The work highlights the need for larger, more diverse samples and higher-resolution 2D spectroscopy to confirm the interpretation and to robustly quantify the role of expansion in shaping turbulence across HII-region evolution.

Abstract

The size-velocity dispersion ($σ$) relation, while well established for giant HII regions, remains uncertain for their smaller counterparts (physical radii R < 20 pc). Thanks to the LAMOST MRS-N dataset's large sky coverage and high spatial/spectral resolution, we examined this relationship using 10 isolated Galactic HII regions with R < 20 pc. Our results reveal two key findings: (1) these small-size HII regions remarkably follow the same size-$σ$ relation as giant HII regions, suggesting this correlation could serve as a novel distance indicator for Galactic HII regions; and (2) we find distinct dynamical behaviors between younger and older HII regions. Specifically, in younger (< 0.5 Myr), ionization-bounded HII regions, the velocity dispersion shows no correlation with expansion velocity, indicating that turbulence is driven primarily by stellar winds and ionization processes. In contrast, in older (> 0.5 Myr), matter-bounded HII 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 HII regions. Considering the small sample size used in this work, particularly the inclusion of only two young HII 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 HII regions.

Figures (7)

• Figure 1: A representative spectrum from LAMOST MRS-N. The upper panel displays the spectrum reduced by subtracting the geocoronal H$\alpha$ emission (method S1), while the lower panel shows the result of subtracting all the skylines and the foreground/background emission from DIG (method S2). Insets provide a detailed view of the nebular emission lines, including H$\alpha$, [N II] and [S II] . The results of a single Gaussian fit are overplotted in red. As the spectrum is not flux-calibrated, the flux is given in Analog-to-Digital Units (ADU).
• Figure 2: Fiber positions and integrated spectra. The data for each region (labeled with its ID, WISE name, and fiber count within 1R region) is displayed across two rows. The first row presents the result of the original reduction method, which only subtracts the geocoronal H$\alpha$ emission. The second row shows the result of the second method, which additionally subtracts the foreground and background diffuse emissions. Each row contains six panels: the first shows the fiber positions (crosses) color-coded by H$\alpha$ flux, overlaid with $3'\times3'$ grids; the second and third panels present the observed and velocity-shifted H$\alpha$ spectra, respectively, both displaying individual fibers (black), the co-added integrated spectrum (blue), and the best-fit Gaussian (green), with the latter also marking the H$\alpha$ vacuum wavelength centroid (red dashed line); panels four to six follow the same format for the corresponding [N II] data.
• Figure 2: (Continued)
• Figure 2: (Continued)
• Figure 3: Size-$\sigma$ relationship of regions. Panels (a) through (d) display the results from the S1-1, S1-2, S2-1, and S2-2 methods, respectively. The open circles represent the 10 regions studied in this work, each labeled with its corresponding ID number, while the remaining data points are from previous studies for comparison. The dashed line shows the size-$\sigma$ relation fit from Cosens-2022, with the shaded region indicating the uncertainties of the linear regression.