Far-infrared to centimeter emission of very nearby galaxies with archival data

¹ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
² Sterrenkundig Observatorium, Ghent University, Krijgslaan 281-S9, 9000 Gent, Belgium
³ IPAC, California Institute of Technology, Pasadena, CA 91125, USA
⁴ DLR Galileo Competence Center, Oberpfaffenhofen, 82234 Wessling, Germany
⁵ Institut de Recherche en Astrophysique et Planétologie, Toulouse, France

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

Received: 11 September 2025
Accepted: 20 November 2025

Abstract

Compared to the well-studied infrared and radio domains, galaxy emission in the range from a few millimeters (mm) to centimeters (cm) (300−30 GHz) has been observed less frequently. In this domain, galaxy emission consists of thermal dust emission that is superimposed on free-free and synchrotron emission, with a possible additional contribution from anomalous microwave emission (AME) that peaks near 1 cm. We accurately characterize the integrated spectral energy distribution (SED) of galaxies in the mm−cm range. We used COBE-DIRBE, IRAS, Planck, and WMAP all-sky surveys brought to the same spatial resolution of ∼ 1^° to cover 18 photometric bands from 97 μm to 1.3 cm. Because of the low angular resolution and mixing with foreground and background emission that hampers the detection of the galaxy, our sample consists of six of the brightest nearby galaxies: the Large and Small Magellanic Clouds (LMC and SMC), M31, M33, NGC 253, and NGC 4945. We subtracted Milky Way dust emission, distant unresolved galaxies, and foreground point sources in the field. We fit each integrated SED with a model of thermal dust, free-free, synchrotron, AME, and cosmic microwave background (CMB) temperature fluctuations. The integrated SEDs of our sample of galaxies are well fit by the model within the uncertainties, although degeneracies between the different components contributing to the mm−cm emission complicate the estimation of their individual contributions. We did not clearly detect AME in any of our target galaxies, and the AME emissivity upper limits are weak compared to Galactic standards. This suggests that the AME signal might be diluted at the scale of a whole galaxy. We inferred positive CMB fluctuations in the background of five out of our six galaxies. This effect might be related to the degeneracy between the dust emissivity index and CMB fluctuations in the background, or it might be linked to the specific spatial distribution of CMB fluctuations coupled with our 1^° resolution and small number statistics.