MSX mid‐infrared imaging of massive star birth environments – II. Giant H ii regions

We conduct a Galactic census of giant H  ii (GH  ii ) regions, based on the all‐sky 6‐cm data set of Kuchar & Clark, plus the kinematic distances obtained by Russeil. From an inspection of mid‐infrared (MIR) Mid‐course Space Experiment ( MSX ) and far‐IR IRAS Sky Survey Atlas images, we identify a total of 56 GH  ii regions in the Milky Way, of which 15 per cent (65 per cent) can be seen at optical (near‐IR) wavelengths. The mid to far‐IR fluxes from each GH  ii region are measured, and sample the thermal emission from the ubiquitous dust present within the exciting clusters of OB stars, arising from the integrated luminosity of the hot stars heating the cluster dust, for which we obtain log L (IR) = 5.5–7.3 L ⊙ . The MIR 21‐μm spatial morphology is presented for each GH  ii region, and often indicates multiple emission sources, suggesting complicated cluster formation. IR colour–colour diagrams are presented, providing information concerning the temperature distribution and the optical depth of the dust. For the clusters of our study, the dust is not optically thick to all stellar radiation, thus the measured infrared luminosity is lower than L bol . As the dust environment of a cluster begins to dissipate, the thermal emission and its optical depth ought to decrease even before the stars evolve appreciably. We see evidence of this in our empirical relationship between the integrated IR and Lyman continuum luminosities.