A multi-wavelength study of Galactic H II regions with extended emission

¹ Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Trivandrum, Kerala 695547, India
² National Centre for Radio Astrophysics–Tata Institute of Fundamental Research, Post Box 3, Ganeshkhind PO, Pune 411007, India
³ Max Planck Institute for Radioastronomy (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁴ Department of Physics, Indian Institute of Science, Bengaluru 560012, India
⁵ German Aerospace Center, Scientific Information, 51147 Cologne, Germany

Received: 20 March 2024
Accepted: 28 June 2024

Abstract

Context. H II regions are the signposts of massive (M ≥ 8 M_⊙) star-forming sites in our Galaxy. It has been observed that the ionizing photon rate inferred from the radio continuum emission of H II regions is significantly lower (~90%) than that inferred from far-infrared fluxes measured by the Infrared Astronomical Satellite.

Aims. This discrepancy in the ionizing photon rates may arise due to there being significant amounts of dust within the H II regions or the presence of extended emission that is undetected by high-resolution radio interferometric observations. Here, we study a sample of eight compact and ultracompact H II regions with extended emission to explore its role in resolving the discrepancy.

Methods. We have used observations at the upgraded Giant Metrewave Radio Telescope (1.25–1.45 GHz) and data from the GLOSTAR survey (4–8 GHz) to estimate the ionizing photon rate from the radio continuum emission. We have also estimated the ionizing photon rate from the infrared luminosity by fitting a spectral energy distribution function to the infrared data from the Spitzer-GLIMPSE, MIPSGAL, and Hi-GAL surveys. The excellent sensitivity of the radio observations to extended emission allows us to investigate the actual fraction of ionizing photons that are absorbed by dust in compact and ultracompact H II regions.

Results. Barring one source, we find a direct association between the radio continuum emission from the compact and diffuse components of the H II region. Our study shows that the ionizing photon rates estimated using the radio and infrared data are within reasonable agreement (5–28%) if we include the extended emission. We also find multiple candidate ionizing stars in all our sources, and the ionizing photon rates from the radio observations and candidate stars are in reasonable agreement.