Observations of 58 compact H ii regions at mid‐infrared wavelengths

We present 3.6, 4.5, 5.8 and 8.0 μm photometry and mapping for 58 compact H II regions identified using the Spitzer Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE). These are used to investigate trends in the colours, magnitudes and structures of the sources, together with spatial variations in flux ratio. It is concluded that most of the sources are remarkably similar, and have comparable variations in flux ratio as a function of position within the shells, together with very narrowly defined ranges of colour index. Systematic variations of surface brightnesses S with harmonic mean size Θ(HARM) also imply that S oc R -β , where β > 0.4; a result which may suggest that the widths of the emission zones are either more or less invariant, or increase with increasing source size. We note that the variation in longer wave flux ratios may imply the presence of polycyclic aromatic hydrocarbon (PAH) emission within large photodissociative regimes, evidence for which is also noted in colour imaging of the sources. The most intense 5.8 and 8 μm emission appears to be concentrated close to the interior ionization fronts, however, where fluorescent excitation of the PAH molecules and the number densities for the grain carriers are both likely to be at their greatest.