C-Band All-Sky Survey: A First Look at the Galaxy

We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary intensity data taken with the C-Band All Sky Survey (C-BASS) northern instrument at the Owens Valley Radio Observatory, California. Combining C-BASS maps with ancillary data to make temperature-temperature plots we find synchrotron spectral indices of β = −2.65±0.05 between 0.408 GHz and 5 GHz and β = −2.72 ± 0.09 between 1.420 GHz and 5 GHz for −10◦ < |b| < −4◦, 20◦ < l < 40◦. Through the subtraction of a radio recombination line (RRL) free-free template we determine the synchrotron spectral index in the Galactic plane (|b| < 4◦) to be β = −2.56± 0.07 between 0.408 GHz and 5 GHz, with a contribution of 53± 8 per cent from free-free emission at 5GHz. These results are consistent with previous low frequency measurements in the Galactic plane. By including C-BASS data in spectral fits we demonstrate the presence of anomalous microwave emission (AME) associated with the Hii complexes W43, W44 and W47 near 30 GHz, at 4.4 σ, 3.1 σ and 2.5 σ respectively. The CORNISH VLA 5GHz source catalogue rules out the possibility that the excess emission detected around 30 GHz may be due to ultra-compact Hii regions. Diffuse AME was also identified at a 4σ level within 30◦ < l < 40◦, −2◦ < b < 2◦ between 5 GHz and 22.8 GHz.