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Published August 2023 | Published
Journal Article Open

The C-Band All-Sky Survey (C-BASS): new constraints on the integrated radio spectrum of M 31


The Andromeda galaxy (M 31) is our closest neighbouring spiral galaxy, making it an ideal target for studying the physics of the interstellar medium in a galaxy very similar to our own. Using new observations of M 31 at 4.76 GHz by the C-Band All-Sky Survey (C-BASS), and all available radio data at 1° resolution, we produce the integrated spectrum and put new constraints on the synchrotron spectral index and anomalous microwave emission (AME) from M 31. We use aperture photometry and spectral modelling to fit for the integrated spectrum of M 31, and subtract a comprehensive model of nearby background radio sources. The AME in M 31 is detected at 3σ significance with a peak near 30 GHz and flux density 0.27 ± 0.09 Jy. The synchrotron spectral index of M 31 is flatter than our own Galaxy at α =−0.66 ± 0.03 with no strong evidence of spectral curvature. The emissivity of AME averaged over the total emission from M 31 is lower than typical AME sources in our Galaxy, implying that AME is not uniformly distributed throughout M 31 and instead is likely confined to sub-regions – this will need to be confirmed using future higher resolution observations around 20–30 GHz.

Additional Information

© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) The C-BASS project (http://cbass.web.ox.ac.uk) is a collaboration between Oxford and Manchester Universities in the UK, the California Institute of Technology in the USA, Rhodes University, UKZN and the South African Radio Observatory in South Africa, and the King Abdulaziz City for Science and Technology (KACST) in Saudi Arabia. It has been supported by the NSF awards AST-0607857, AST-1010024, AST-1212217, and AST-1616227, and NASA award NNX15AF06G, the University of Oxford, the Royal Society, STFC, and the other participating institutions. SEH, CD, and JPL acknowledge support from an STFC Consolidated Grant (ST/P000649/1). This research was also supported by the South African Radio Astronomy Observatory, which is a facility of the National Research Foundation, an agency of the Department of Science and Technology. We would like to thank Russ Keeney for technical help at OVRO. We use the HEALPIX package (Górski et al. 2005), IDL astronomy library, and Python ASTROPY (Astropy Collaboration 2013, 2018), MATPLOTLIB (Hunter 2007), NUMPY (Harris et al. 2020), HEALPY (Zonca et al. 2019), CORNER (Foreman-Mackey 2016), and SCIPY (Virtanen et al. 2020) packages. This research has used the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has also used data from the OVRO 40-m monitoring program (Richards et al. 2011), supported by private funding from the California Institute of Technology and the Max Planck Institute for Radio Astronomy, and by NASA grants NNX08AW31G, NNX11A043G, and NNX14AQ89G and NSF grants AST-0808050 and AST-1109911. Some of the presented results are based on observations obtained with the QUIJOTE experiment (http://research.iac.es/proyecto/quijote). DATA AVAILABILITY. All model fits for M 31 and radio sources can be made available upon request. Ancillary data sets described in Section 2.2 are available via from the papers cited, the NASA LAMBDA website, or the Planck legacy archive. The C-BASS map is not currently available but will be published (Taylor et al., in preparation) and released in the near future.

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August 22, 2023
October 20, 2023