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Study of the thermal and nonthermal emission components in M 31: the Sardinia Radio Telescope view at 6.6 GHz

Fatigoni, S. and Radiconi, F. and Battistelli, E. S. and Murgia, M. and Carretti, E. and Castangia, P. and Concu, R. and de Bernardis, P. and Fritz, J. and Genova-Santos, R. and Govoni, F. and Guidi, F. and Lamagna, L. and Masi, S. and Melis, A. and Paladini, R. and Perez-Toledo, F. M. and Piacentini, F. and Poppi, S. and Rebolo, R. and Rubino-Martin, J. A. and Surcis, G. and Tarchi, A. and Vacca, V. (2021) Study of the thermal and nonthermal emission components in M 31: the Sardinia Radio Telescope view at 6.6 GHz. Astronomy and Astrophysics, 651 . Art. No. A98. ISSN 0004-6361. doi:10.1051/0004-6361/202040011.

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Context. The Andromeda galaxy is the best-known large galaxy besides our own Milky Way. Several images and studies exist at all wavelengths from radio to hard X-ray. Nevertheless, only a few observations are available in the microwave range where its average radio emission reaches the minimum. Aims. In this paper, we want to study the radio morphology of the galaxy, decouple thermal from nonthermal emission, and extract the star formation rate. We also aim to derive a complete catalog of radio sources for the mapped patch of sky. Methods. We observed the Andromeda galaxy with the Sardinia Radio Telescope at 6.6 GHz with very high sensitivity and angular resolution, and an unprecedented sky coverage. Results. Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, we confirm that, globally, the spectral index is ∼0.7−0.8, while in the star forming regions it decreases to ∼0.5. By disentangling (gas) thermal and nonthermal emission, we find that at 6.6 GHz, thermal emission follows the distribution of HII regions around the ring. Nonthermal emission within the ring appears smoother and more uniform than thermal emission because of diffusion of the cosmic ray electrons away from their birthplaces. This causes the magnetic fields to appear almost constant in intensity. Furthermore, we calculated a map of the star formation rate based on the map of thermal emission. Integrating within a radius of R_(max) = 15 kpc, we obtained a total star formation rate of 0.19 ± 0.01 M_⊙ yr⁻¹ in agreement with previous results in the literature. Finally, we correlated our radio data with infrared images of the Andromeda galaxy. We find an unexpectedly high correlation between nonthermal and mid-infrared data in the central region, with a correlation parameter r = 0.93. Finally, by computing the logarithmic 24_(μm)/21_(cm) ratio q_(24 μm), we find a decreasing trend with increasing galactocentric distance and an increasing dispersion. The logarithmic far-infrared-to-radio ratio is found to be 2.41 ± 0.04.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Fatigoni, S.0000-0002-3790-7314
Radiconi, F.0000-0002-1669-6853
Battistelli, E. S.0000-0001-5210-7625
Murgia, M.0000-0002-4800-0806
Carretti, E.0000-0002-3973-8403
Castangia, P.0000-0002-2318-8531
de Bernardis, P.0000-0001-6547-6446
Fritz, J.0000-0002-7042-1965
Genova-Santos, R.0000-0001-5479-0034
Govoni, F.0000-0002-2831-7603
Guidi, F.0000-0001-7593-3962
Lamagna, L.0000-0002-2978-1352
Masi, S.0000-0001-5105-1439
Melis, A.0000-0002-6558-1315
Paladini, R.0000-0002-5158-243X
Perez-Toledo, F. M.0000-0001-6265-2248
Piacentini, F.0000-0002-5444-9327
Poppi, S.0000-0002-4698-2607
Rebolo, R.0000-0003-3767-7085
Rubino-Martin, J. A.0000-0001-5289-3021
Surcis, G.0000-0003-2775-442X
Tarchi, A.0000-0001-8540-3500
Vacca, V.0000-0003-1997-0771
Additional Information:© ESO 2021. Received: 27 November 2020 | Accepted: 20 April 2021. We acknowledge the support by 2016 Sapienza Ateneo project. The Sardinia Radio Telescope (SRT) is funded by the Ministry of University and Research (MIUR), Italian Space Agency (ASI), and the Autonomous Region of Sardinia (RAS), the European Union (EU) and is operated for the National Institute for Astrophysics (INAF) by Cagliari Observatory (OAC). This research made use of Astropy (, a community-developed core Python package for Astronomy (Astropy Collaboration 2013, 2018). This research made use of Montage. It is funded by the National Science Foundation under Grant Number ACI-1440620, and was previously funded by the National Aeronautics and Space Administration’s Earth Science Technology Office, Computation Technologies Project, under Cooperative Agreement Number NCC5-626 between NASA and the California Institute of Technology. J.F. acknowledges financial support from the UNAM- DGAPA-PAPIIT IN111620 grant, México. We thank the referee for the useful comments that helped us improving the paper. We thank Mark Halpern for useful suggestions which improved the paper. Table A.1 and the 6.6 GHz SRT map are only available at the CDS via anonymous ftp to ( or via
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Sapienza AteneoUNSPECIFIED
Ministero dell'Istruzione, dell'Università e della Ricerca (MIUR)UNSPECIFIED
Agenzia Spaziale Italiana (ASI)UNSPECIFIED
Autonomous Region of SardiniaUNSPECIFIED
Dirección General Asuntos del Personal Académico (DGAPA)IN111620
Subject Keywords:galaxies: individual: Messier 31 / radio continuum: galaxies / ISM: general / radiation mechanisms: non-thermal / radiation mechanisms: thermal
Record Number:CaltechAUTHORS:20210813-181156560
Persistent URL:
Official Citation:Study of the thermal and nonthermal emission components in M 31: the Sardinia Radio Telescope view at 6.6 GHz S. Fatigoni, F. Radiconi, E. S. Battistelli, M. Murgia, E. Carretti, P. Castangia, R. Concu, P. de Bernardis, J. Fritz, R. Genova-Santos, F. Govoni, F. Guidi, L. Lamagna, S. Masi, A. Melis, R. Paladini, F. M. Perez-Toledo, F. Piacentini, S. Poppi, R. Rebolo, J. A. Rubino-Martin, G. Surcis, A. Tarchi and V. Vacca A&A, 651 (2021) A98 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:110240
Deposited By: George Porter
Deposited On:16 Aug 2021 17:20
Last Modified:16 Aug 2021 17:20

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