Spectral properties of Anomalous Microwave Emission in 144 Galactic clouds

¹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Canary Islands, Spain
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
³ Jodrell Bank Centre for Astrophysics, Alan Turing Building, Department of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁴ Imperial College London, Blackett Lab, Prince Consort Road, London SW7 2AZ, UK
⁵ Consejo Superior de Investigaciones Científicas, Madrid, Spain
⁶ Instituto de Física de Cantabria (IFCA), CSIC-Univ. de Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain
⁷ Laboratoire de Physique Subatomique et de Cosmologie, Université Grenoble Alpes, 53 Avenue des Martyrs Grenoble, France
⁸ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Cahill Centre for Astronomy and Astrophysics, California Institute of Technology, Pasadena, CA 91125, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 7 October 2025
Accepted: 1 February 2026

Abstract

Anomalous Microwave Emission (AME) is a diffuse microwave component thought to arise from spinning dust grains, though it remains poorly understood. We analyzed AME in 144 Galactic clouds by combining low-frequency maps from S-PASS (2.3 GHz), C-BASS (4.76 GHz), and QUIJOTE (10–20 GHz) with 21 ancillary maps. Using aperture photometry and parametric spectral energy distribution (SED) fitting via Markov chain Monte Carlo methods without informative priors, we measured AME emissivity, peak frequency, and spectral width. We achieved peak frequency constraints nearly three times tighter than previous work and identify 83 new AME sources. The AME spectra are generally broader than predicted by spinning dust models for a single phase of the interstellar medium, suggesting either multiple spinning dust components along the line of sight or incomplete representation of the grain size distribution in current models. However, the narrowest observed widths match theoretical predictions, supporting the spinning dust hypothesis. The AME amplitude correlates most strongly with the thermal dust peak flux and radiance, showing ∼30% scatter and sublinear scaling, which suggests reduced AME efficiency in regions with brighter thermal dust emission. The AME peak frequency increases with thermal dust temperature in a trend current theoretical models do not reproduce, indicating that spinning dust models must incorporate dust evolution and radiative transfer in a self-consistent framework where environmental parameters and grain properties are interdependent. Polycyclic aromatic hydrocarbon tracers correlate with AME emissivity, supporting a physical link to small dust grains. Finally, a log-Gaussian function provides a good empirical description of the AME spectrum across the sample, given current data quality and frequency coverage.