Study of the thermal and nonthermal emission components in M31: 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 Rmax = 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 24μm/21cm ratio q24 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.

New Continuum Observations of the Andromeda galaxy M31 with FAST

“…The asymmetry of the non-thermal spectral index between the northern and southern part of M31 was also found by Fatigoni et al (2021) based on the observations at frequencies between 1.5 and 6.6 GHz. The star formation rate is also higher toward the north (Fatigoni et al 2021).…”

“…The asymmetry of the non-thermal spectral index between the northern and southern part of M31 was also found by Fatigoni et al (2021) based on the observations at frequencies between 1.5 and 6.6 GHz. The star formation rate is also higher toward the north (Fatigoni et al 2021). A higher star formation rate results in a larger number of stars and consequently a larger number of supernovae and supernova remnants that can produce more cosmic ray electrons.…”

“…Beck et al (2020) derived a total intensity spectral index of −0.71 ± 0.02, and a synchrotron emission spectral index of −0.81 ± 0.03 using the observations at 2.645 GHz,4.85 GHz,and 8.35 GHz. The spectral index toward the ring increases to −0.5 because the thermal fraction is large and the spectrum thus tends to be flat (Fatigoni et al 2021).…”

Weighted Stacking of Radio Images Affected by Noise and Interfering Signals

“…We apply the algorithm to calibrated data. For data analysis details, we refer to the work carried out by Fatigoni et al (2021) and Battistelli et al (2019). Regarding the application presented in this work, it is relevant to point out that data are calibrated for bandpass and flux density.…”

“…In the works by Fatigoni et al (2021) and Battistelli et al (2019), RFI signals were removed with automated flagging methods. Overall, approximately 30% of the total data was removed due to RFIs.…”