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

¹ Sapienza University of Rome, Physics Department, Piazzale Aldo Moro 5, 00185 Rome, Italy
e-mail: federico.radiconi@roma1.infn.it, elia.battistelli@roma1.infn.it
² University of British Columbia, Department of Physics and Astronomy, Vancouver, BC V6T 1Z1, Canada
e-mail: sfatigoni@phas.ubc.ca
³ INFN – Sezione di Roma, Piazzale Aldo Moro 5, 00185 Rome, Italy
⁴ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
⁵ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129 Bologna, Italy
⁶ Instituto de Radioastronomia y Astrofisica, UNAM, Campus Morelia, A.P. 3-72 CP 58089, Mexico
⁷ Instituto de Astrofisica de Canarias, C/Via Lactea s/n, 38205 La Laguna, Tenerife, Spain
⁸ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁹ Infrared Processing Analysis Center, California Institute of Technology, Pasadena, CA 91125, USA

Received: 27 November 2020
Accepted: 20 April 2021

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.