CBI limits on 31 GHz excess emission in southern H II regions
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 H ii 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 31 GHz is due to free–free emission from ionized gas with an electron temperature of ≈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-μm data indicate an average dust emissivity of 3.3 ± 1.7 μK (MJy sr^(−1))^(−1) , or a 95 per cent confidence limit of <6.1 μK (MJy sr^(−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 (RCW 49) and may account for up to ≈30 per cent of the total flux density observed at 31 GHz. Here, the dust emissivity of the excess emission is 13.6 ± 4.2 μK (MJy sr^(−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.
Additional Information© 2007 The Authors. Journal compilation © 2007 RAS. Accepted 2007 May 4. Received 2007 May 1; in original form 2007 February 22. CD thanks Barbara and Stanley Rawn Jr for funding a fellowship at the California Institute of Technology for part of this work. We thank the staff and engineers at the Chajnantor Observatory for their hard work and continuing support. In particular, we thank Cristobal Achermann, Ricardo Bustos, Rodrigo Reeves and Nolberto Oyarace. SC acknowledges support from FONDECYT grant 1060827. SC and LB acknowledge support from the Chilean Center for Astrophysics FONDAP 15010003. Part of the research described in this paper was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
Published - DICmnras07.pdf