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MNRAS 513, 5900–5919 (2022) Advance Access publication 2022 May 5

S. E. Harper,1 C. Dickinson,1,2 A. Barr,1 R. Cepeda-Arroita,1 R. D. P. Grumitt,3 H. M. Heilgendorff,4 L. Jew,3 J. L. Jonas,5,6 M. E. Jones,3 J. P. Leahy,1 J. Leech,3 T. J. Pearson,2 M. W. Peel,7,8 A. C. S. Readhead2 and A. C. Taylor3

1 Jodrell Bank Centre for Astrophysics, Alan Turing Building, Department of Physics & Astronomy, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester M13 9PL, UK

2 Cahill Centre for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

3 Sub-department of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK

4 School of Mathematics, Statistics & Computer Science, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban, South Africa

5 Department of Physics and Electronics, Rhodes University, Grahamstown 6139, South Africa

6 South African Radio Astronomy Observatory, 2 Fir Road, Observatory, Cape Town 7925, South Africa

7 Instituto de Astrofı́sica de Canarias, E-38205 La Laguna, Tenerife, Spain

8 Departamento de Astrofı́sica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain

Accepted 2022 April 25. Received 2022 April 25; in original form 2022 February 25

ABSTRACT

The C-Band All-Sky Survey (C-BASS) has observed the Galaxy at 4.76 GHz with an angular resolution of 0°.73 full-width half-maximum, and detected Galactic synchrotron emission with high signal-to-noise ratio over the entire northern sky (δ > −15°). We present the results of a spatial correlation analysis of Galactic foregrounds at mid-to-high (b > 10°) Galactic latitudes using a preliminary version of the C-BASS intensity map. We jointly fit for synchrotron, dust, and free–free components between 20 and 1000 GHz and look for differences in the Galactic synchrotron spectrum, and the emissivity of anomalous microwave emission (AME) when using either the C-BASS map or the 408-MHz all-sky map to trace synchrotron emission. We find marginal evidence for a steepening (<Δβ> = −0.06 ± 0.02) of the Galactic synchrotron spectrum at high frequencies resulting in a mean spectral index of <β> = −3.10 ± 0.02 over 4.76–22.8 GHz. Further, we find that the synchrotron emission can be well modelled by a single power law up to a few tens of GHz. Due to this, we find that the AME emissivity is not sensitive to changing the synchrotron tracer from the 408-MHz map to the 4.76-GHz map. We interpret this as strong evidence for the origin of AME being spinning dust emission.

Key words: radiation mechanisms: non-thermal – radiation mechanisms: thermal – surveys – cosmology: diffuse radiation – radio continuum: ISM.

1 INTRODUCTION

Large-scale diffuse Galactic radio emission, at frequencies 0.1– 50 GHz, is composed of three principal components: synchrotron emission from the propagation of cosmic rays through the Galactic magnetic field (e.g. Strong, Orlando & Jaffe 2011), thermal free–free emission from ionized gas (e.g. Dickinson, Davies & Davis 2003), and spinning dust emission from the rapid rotation of small grains in the interstellar medium (ISM) (e.g. Dickinson et al. 2018). The study of these Galactic components is important both for understanding the astrophysics of our Galaxy, and also for studies of the cosmic microwave background (CMB) (e.g. de Oliveira-Costa et al. 2008; Bonaldi & Ricciardi 2011; Remazeilles et al. 2016).

There are only a limited number of radio surveys that preserve the large-scale structure of Galactic emission. The all-sky 408-MHz map (Haslam et al. 1982) has become the standard map for tracing Galactic synchrotron emission as it has little impact from both synchrotron self-absorption and free–free contamination. However, the 408-MHz map is limited by being total intensity only and having numerous systematics that are challenging to quantify (Remazeilles et al. 2015). Other radio surveys of the large-scale Galactic structure at 820 MHz (Berkhuijsen 1972), 1420 MHz (Reich, Testori & Reich 2001; Calabretta, Staveley-Smith & Barnes 2014), and 2.3 GHz (Reif et al. 1987; Jonas, Baart & Nicolson 1998) are also limited due either to partial sky coverage, systematics, and being in total intensity only (I + Q for the 2.3-GHz map). The S-band Polarization AllSky Survey (S-PASS; Carretti et al. 2019) provided the first map of polarized emission in the Southern sky using the Parkes telescope at 2.3 GHz. New data from the Q-U-I JOint Tenerife (QUIJOTE) experiment at 10–20 GHz will complement these surveys in the Northern hemisphere (Génova-Santos et al. 2015; Guidi & Quijote Collaboration 2020).

The limited number of surveys at the intermediate frequencies between 408 MHz and the lowest frequency Wilkinson Microwave Anisotropy Probe (WMAP) 22.8-GHz band means that capturing © 2022 The Author(s).

Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommonsorg/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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