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Published July 2021 | Accepted Version + Published
Journal Article Open

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

Abstract

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.

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 (http://www.astropy.org), 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 cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/651/A98

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Accepted Version - 2105.10453.pdf

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Additional details

Created:
August 20, 2023
Modified:
October 23, 2023