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

TL;DR

This paper analyzes the diffuse Galactic emission at 5 GHz using C-BASS data, revealing synchrotron spectral indices, free-free contributions, and evidence of anomalous microwave emission associated with HII regions and diffuse areas.

Contribution

First detailed analysis of 5 GHz Galactic emission combining C-BASS data with ancillary data, identifying synchrotron spectral indices, free-free contributions, and detecting anomalous microwave emission in the Galactic plane.

Findings

Synchrotron spectral indices of -2.65 and -2.72 between 0.408-5 GHz and 1.420-5 GHz.

Free-free emission contributes 53% at 5 GHz in the Galactic plane.

Detection of anomalous microwave emission near 30 GHz in specific HII complexes and diffuse regions.

Abstract

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 $\beta = -2.65 \pm 0.05$ between 0.408 GHz and 5 GHz and $ \beta = -2.72 \pm 0.09$ between 1.420 GHz and 5 GHz for $-10^{\circ} < |b| < -4^{\circ}$, $20^{\circ} < l < 40^{\circ}$. Through the subtraction of a radio recombination line (RRL) free-free template we determine the synchrotron spectral index in the Galactic plane ($ |b| < 4^{\circ}$) to be $\beta = -2.56 \pm 0.07$ between 0.408 GHz and 5 GHz, with a contribution of $53 \pm 8$ per cent from free-free emission at 5\,GHz. 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 sigma, 3.1 sigma and 2.5 sigma respectively. The CORNISH VLA 5 GHz 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 4 sigma level within $30^{\circ} < l < 40^{\circ}$, $-2^{\circ} < b < 2^{\circ}$ between 5 GHz and 22.8 GHz.