2007MNRAS.379..297D

We have mapped four regions of the southern Galactic plane at 31GHz with the Cosmic Background Imager. From the maps, we have extracted the flux densities for six of the brightest HII regions in the southern sky and compared them with multifrequency data from the literature. The fitted spectral index for each source was found to be close to the theoretical value expected for optically thin free-free emission, thus confirming that the majority of flux at 31GHz is due to free-free emission from ionized gas with an electron temperature of ~7000-8000K.

We also found that, for all six sources, the 31-GHz flux density was slightly higher than the predicted value from data in the literature. This excess emission could be due to spinning dust or another emission mechanism. Comparisons with 100-µm data indicate an average dust emissivity of 3.3±1.7µK (MJysr^–1)^–1, or a 95 per cent confidence limit of <6.1µK (MJysr^–1)^–1. This is lower than that found in diffuse clouds at high Galactic latitudes by a factor of ∼3-4. The most significant detection (3.3σ) was found in G284.3-0.3 (RCW49) and may account for up to ~30 per cent of the total flux density observed at 31GHz. Here, the dust emissivity of the excess emission is 13.6±4.2µK (MJysr^–1)^–1 and is within the range observed at high Galactic latitudes.

Low-level polarized emission was observed in all six sources with polarization fractions in the range 0.3-0.6 per cent. This is likely to be mainly due to instrumental leakage and is therefore an upper limit to the free-free polarization. It corresponds to an upper limit of ∼1 per cent for the polarization of anomalous emission.