Accounting for the absence of anomalous microwave emission in the M 31 halo

¹ Department of Mathematics and Physics “Ennio De Giorgi”, University of Salento, Via per Arnesano, I-73100 Lecce, Italy
² INFN, Sezione di Lecce, Via per Arnesano, I-73100 Lecce, Italy
³ INAF, Sezione di Lecce, Via per Arnesano, I-73100 Lecce, Italy
⁴ Department of Physics and Astronomy, School of Natural Sciences, National University of Sciences and Technology, H-12, 44000 Islamabad, Pakistan

^⋆ Corresponding author: francesco.depaolis@le.infn.it

Received: 8 April 2025
Accepted: 14 October 2025

Abstract

The discovery of a temperature asymmetry in the cosmic microwave background (CMB) data towards various galaxies is enabling a deeper comprehension of galactic halos. Estimating the fraction of missing baryons in the halos is a crucial step forward, but it relies on understanding the real cause of the observed CMB temperature asymmetry, to which many effects might contribute. We analysed the contribution played by the anomalous microwave emission (AME) from halo dust grains in the halo of the M 31 galaxy. Assuming dust grains to be either amorphous carbon or silicates, with sizes ranging from 0.01 μm to about 0.3 μm and mass in the range of 10⁻¹⁴ − 10⁻¹³ g, we estimated the total mass, distribution, and diffuse emission in the 100 μm band of the infrared astronomical satellite (IRAS). We then estimated the temperature asymmetry induced by the rotation of the M 31 halo and compared the obtained values with the Planck’s spectral matching independent component analysis (SMICA)-processed data. We find that the AME cannot account for the measured CMB temperature asymmetry, with its contribution constrained to ≲7%, thereby indicating that additional physical mechanisms must be responsible for the observed signal.