Abstract

Hydrogen radio recombination lines (RRLs) are one of the major diagnostics of the physical properties of HII regions. We use RRL H40?, He40?, and 3 mm continuum emission to investigate the properties of a large sample of resolved UC HII regions identified in the ATOMS survey. In total, we identify 94 UC HII regions from H40? emission. The basic parameters for these UC HII regions, such as electron density, emission measure, electron temperature, ionic abundance ratio (nHe+/nH+), and line width are derived. The median electron density and the median nHe+/nH+ ratio of these UC HII regions derived from RRLs are ?9000 cm?3 and 0.11, respectively. Within UC HII regions, the nHe+/nH+ ratios derived from the intensity ratio of the He40? and H40? lines seems to be higher in the boundary region than in the centre. The H40? line width is mainly broadened by thermal motion and microturbulence. The electron temperature of these UC HII regions has a median value of ?6700 K, and its dependence on galactocentric distance is weak. © 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.