Strong evidence of Anomalous Microwave Emission from the flux density spectrum of M31

Abstract

We have observed the Andromeda galaxy, Messier 31 (M31), at 6.7GHz with the Sardinia Radio Telescope. We mapped the radio emission in the C-band, re-analyzed WMAP and Planck maps, as well as other ancillary data, and we have derived an overall integrated flux density spectrum from the radio to the infrared. This allowed us to estimate the emission budget from M31. Integrating over the whole galaxy, we found strong and highly significant evidence for anomalous microwave emission (AME), at the level of (1.45+0.17-0.19)Jy at the peaking frequency of ~25GHz. Decomposing the spectrum into known emission mechanisms such as free-free, synchrotron, thermal dust, and AME arising from electric dipole emission from rapidly rotating dust grains, we found that the overall emission from M31 is dominated, at frequencies below 10GHz, by synchrotron emission with a spectral index of -1.10+0.10-0.08, with subdominant free-free emission. At frequencies >10GHz, AME has a similar intensity to that of synchrotron and free-free emission, overtaking them between 20GHz and 50GHz, whereas thermal dust emission dominates the emission budget at frequencies above 60GHz, as expected.

9 pages, 3 figure, accepted for publication in ApJL

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• QUIJOTE scientific results -- X. Spatial variations of Anomalous Microwave Emission along the Galactic plane
• Study of the thermal and nonthermal emission components in M31: the Sardinia Radio Telescope view at 6.6 GHz
• QUIJOTE Scientific Results -- VII. Galactic AME sources in the QUIJOTE-MFI Northern Hemisphere Wide-Survey
• Searching for Anomalous Microwave Emission in nearby galaxies. K-band observations with the Sardinia Radio Telescope
• QUIJOTE Scientific Results -- XVII. Studying the Anomalous Microwave Emission in the Andromeda Galaxy with QUIJOTE-MFI
• CHANG-ES XXXII: Spatially Resolved Thermal/Nonthermal Separation from Radio Data Alone -- New Probes into NGC 3044 and NGC 5775
• Where's the Dust?: The Deepening Anomaly of Microwave Emission in NGC 4725 B
• The C-Band All-Sky Survey (C-BASS): New Constraints on the Integrated Radio Spectrum of M 31
• QUIJOTE scientific results -- XVIII. New constraints on the polarization of the Anomalous Microwave Emission in bright Galactic regions: $\rho$\,Ophiuchi, Perseus and W43
• Sardinia Radio Telescope observations of Local Group dwarf galaxies -- I. The cases of NGC6822, IC1613, and WLM
• Spinning Nanoparticles Impacted by C-shock: Implications for Radio-millimeter Emission from Star-forming Regions
• The mm-to-cm SED of spiral galaxies. Synergies between NIKA2 and SRT instruments
• Radio-to-Submillimetre Spectral Energy Distributions of NGC 1365
• Far-infrared to centimeter emission of very nearby galaxies with archival data
• Weighted Stacking of Radio Images Affected by Noise and Interfering Signals