CBI limits on 31 GHz excess emission in southern HII regions

Abstract

We have mapped four regions of the southern Galactic plane at 31 GHz 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 multi-frequency 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 31 GHz is due to free-free emission from ionised gas with an electron temperature of $\approx 7000-8000$ K. 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 $100\mu$m data indicate an average dust emissivity of $3.3\pm 1.7\mu$K (MJy/sr)${}^{-1}$, or a 95 per cent confidence limit of $<6.1\mu$K (MJy/sr)${}^{-1}$. This is lower than that found in diffuse clouds at high Galactic latitudes by a factor of $\sim 3-4$. The most significant detection ($3.3\sigma$) was found in $G284.3-0.3$ (RCW49) and may account for up to $\approx 30$ per cent of the total flux density observed at 31 GHz. Here, the dust emissivity of the excess emission is $13.6\pm 4.2\mu$K (MJy/sr)${}^{-1}$ and is within the range observed at high Galactic latitudes. Low level polarised emission was observed in all six sources with polarisation fractions in the range $0.3-0.6$ per cent. This is likely to be mainly due to instrumental leakage and is therefore upper an upper limit to the free-free polarisation. It corresponds to an upper limit of $\sim 1$ per cent for the polarisation of anomalous emission.

Accepted in MNRAS. 12 pages, 10 figures, 5 tables

References in corpus (4)

• Polarization Observations with the Cosmic Background Imager
• Polarization Observations of the Anomalous Microwave Emission in the Perseus Molecular Complex with the Cosmosomas Experiment
• An upper limit on anomalous dust emission at 31 GHz in the diffuse cloud [LPH96]201.663+1.643
• Constraints on spinning dust towards Galactic targets with the VSA: a tentative detection of excess microwave emission towards 3C396

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• AMI limits on 15 GHz excess emission in northern HII regions
• New radio observations of anomalous microwave emission in the HII region RCW175
• On the Limitations of the Anomalous Microwave Emission Emissivity
• COMAP Early Science: VI. A First Look at the COMAP Galactic Plane Survey
• Resolved observations at 31 GHz of spinning dust emissivity variations in $\rho$ Oph