2019A&A...623A.105G

Context. Large-scale radio continuum surveys provide data to get insights into the physical properties of radio sources. HII regions are prominent radio sources produced by thermal emission of ionised gas around young massive stars.
Aims. We identify and analyse HII regions in the Sino-German λ6cm polarisation survey of the Galactic plane.
Methods. Objects with flat radio continuum spectra together with infrared and/or Hα emission were identified as HII regions. For HII regions with small apparent sizes, we cross-matched the λ6cm small-diameter source catalogue with the radio HII region catalogue compiled by Paladini and the infrared HII region catalogue based on the WISE data. Effelsberg λ21 cm and λ11cm continuum survey data were used to determine source spectra. High angular resolution data from the Canadian Galactic Plane Survey and the NRAO VLA Sky Survey were used to solve the confusion when low angular resolution observations were not sufficient. Extended HII regions were identified by eye by overlaying the Paladini and the WISE HII regions onto the λ6cm survey images for coincidences. The TT-plot method was employed for spectral index verification.
Results. A total of 401 HII regions were identified and their flux densities were determined with the Sino-German λ6cm survey data. In the surveyed area, 76 pairs of sources are found to be duplicated in the Paladini HII region catalogue, mainly due to the non-distinction of previous observations with different angular resolutions and 78 objects in their catalogue are misclassified as HII regions, being actually planetary nebulae, supernova remnants, or extragalactic sources that have steep spectra. More than 30 HII regions and HII region candidates from our λ6 cm survey data, especially extended ones, do not have counterparts in the WISE HII region catalogue, of which 9 are identified for the first time. Our results imply that some more Galactic HII regions still await to be discovered and the combination of multi-domain observations is important for HII region identification. Based on the newly derived radio continuum spectra and the evidence of infrared emission, the previously identified SNRs G11.1-1.0, G20.4+0.1 and G16.4-0.5 are believed to be HII regions.