Spatially Resolved Galactic H <scp>ii</scp> Regions Observed by the LAMOST Medium-resolution Spectroscopic Survey of Nebulae (LAMOST MRS-N)

Abstract We present spatially resolved spectroscopic observations of 10 isolated Galactic H II regions using data from the LAMOST Medium-Resolution Spectroscopic Survey of Nebulae (LAMOST MRS-N). The high spatial resolution of the data allows us to investigate the 1D radial profiles of emission-line fluxes (H α , [S II ], and [N II ]), flux ratios ([N II ]/H α , [S II ]/H α , and [S II ]/[N II ]), and radial velocities of these three emission lines. Among these regions, two are ionization bounded, while the remaining eight are matter bounded. The matter-bounded H II regions exhibit shallower slopes in their radial flux profiles compared to the ionization-bounded ones. In most cases, the [N II ]/H α and [S II ]/H α ratios increase with distance from the center of the H II regions, consistent with model predictions that low-ionization emissions dominate the outer zones of these regions. The two ionization-bounded H II regions have kinematic ages t of 0.2 and 0.3 Myr, while the matter-bounded regions span ages from 1 to 12 Myr. For the matter-bounded H II regions, the optical emission flux decreases continuously beyond the photodissociation region (PDR), extending to approximately 1–4 times the radius of the PDR ( r PDR ). The escape fraction f esc of ionizing photons, derived from 1D H α radial flux profiles, is ∼0% for ionization-bounded H II regions, while it ranges from 50% to 90% for the matter-bounded H II regions. The correlation between f esc and t suggests that evolved H II regions (with t &gt; 1 Myr) contribute more significantly to ionizing the surrounding diffuse ionized gas compared to younger, newly formed H II regions.