Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz. (arXiv:2105.10453v1 [astro-ph.GA])

Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz. (arXiv:2105.10453v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Fatigoni_S/0/1/0/all/0/1">S. Fatigoni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Radiconi_F/0/1/0/all/0/1">F. Radiconi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Battistelli_E/0/1/0/all/0/1">E.S. Battistelli</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Murgia_M/0/1/0/all/0/1">M. Murgia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Carretti_E/0/1/0/all/0/1">E. Carretti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Castangia_P/0/1/0/all/0/1">P. Castangia</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Concu_R/0/1/0/all/0/1">R. Concu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bernardis_P/0/1/0/all/0/1">P. de Bernardis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fritz_J/0/1/0/all/0/1">J. Fritz</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Genova_Santos_R/0/1/0/all/0/1">R. Genova-Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Govoni_F/0/1/0/all/0/1">F. Govoni</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Guidi_F/0/1/0/all/0/1">F. Guidi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lamagna_L/0/1/0/all/0/1">L. Lamagna</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Masi_S/0/1/0/all/0/1">S. Masi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Melis_A/0/1/0/all/0/1">A. Melis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Paladini_R/0/1/0/all/0/1">R. Paladini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Perez_Toledo_F/0/1/0/all/0/1">F.M. Perez-Toledo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Piacentini_F/0/1/0/all/0/1">F. Piacentini</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Poppi_S/0/1/0/all/0/1">S. Poppi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rebolo_R/0/1/0/all/0/1">R. Rebolo</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Rubino_Martin_J/0/1/0/all/0/1">J.A. Rubino-Martin</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Surcis_G/0/1/0/all/0/1">G. Surcis</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tarchi_A/0/1/0/all/0/1">A. Tarchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Vacca_V/0/1/0/all/0/1">V. Vacca</a>

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. 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. 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.
Using new 6.6 GHz data and Effelsberg radio telescope ancillary data, we
confirm that, globally, the spectral index is $sim 0.7-0.8$, while in the star
forming regions it decreases to $sim 0.5$. 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 $0.19 pm 0.01$ $M_{odot}$/yr 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 $r=0.93$.

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