A team of astronomers led by Lucie Correia just brought a new perspective on galaxies in the millimeter–centimeter range with archival data, in a recent paper published in Astronomy and Astrophysics.
The team has achieved a detailed characterization of how nearby galaxies emit light in the little-explored millimeter–centimeter (mm–cm) wavelength range. By combining archival data from four
major all-sky space missions (COBE-DIRBE, IRAS, Planck, and WMAP), the researchers shed new light on this intermediate part of the spectrum, where dust and gas processes overlap.
While the infrared and radio emissions of galaxies have been extensively studied, the intermediate region in the mm-cm domain (from 300 to 30 GHz) has been less observed. In this domain, several forms of emission overlap: thermal emission from dust grains, gas surrounding young stars, radiation from high-energy particles moving through magnetic fields, and a mysterious emission attributed to rotating small dust grains. To explore these processes in detail, the team focused on six of the brightest and closest galaxies: the Large and Small Magellanic Clouds (LMC, SMC), Andromeda (M31), M33, NGC 253, and NGC 4945.
All observations were processed to a common resolution of 1°, allowing a consistent comparison across 18 wavelength bands from the far-infrared to 1.3 cm. Working with nearby objects at such coarse resolution required careful treatment to avoid contamination from foreground and background sources.
of 1°, at 350 μm. Around the galaxy, in the foreground and background, various emission sources overlap
and mix with that of M31: dust from the Milky Way, nearby point sources (represented by red circles), more
distant unresolved galaxies, and cosmological microwave background fluctuations. Credit L. Correia.
After carefully removing the effects of Milky Way dust, bright foreground objects, and distant faint galaxies, the researchers were able to reconstruct the total light coming from each target galaxy. The various overlapping emissions, however, remain difficult to distinguish individually. Interestingly, the team found no strong sign of the small-grain microwave signal that is seen in the Milky Way. This suggests that, in other galaxies, this emission may be concentrated in small regions and is diluted when viewed across the entire galaxy.
Another intriguing finding is that five of the six galaxies lie in regions where the cosmic microwave background (CMB), the faint light left over from the Big Bang, is slightly brighter than average. While this might look like a potential bias, these fluctuations are actually consistent with what is normally expected in the CMB at this scale. It shows that the universe’s earliest light is always present, even when we study our closest galactic neighbors.
This study represents an important step toward understanding the astrophysics of galaxies in the mm–cm range, where cosmic processes intersect. By building a consistent picture across a wide range of wavelengths, the researchers provide new insights into dust properties, and the complex nature of microwave emissions in galaxies.
This work demonstrates the power of re-analyzing archival data across a wide range of wavelengths and highlights the need for future high-resolution instruments at mm and cm wavelengths. Such facilities will be essential to better constrain dust properties, identify or rule out faint microwave signals in external galaxies, clearly separating galaxy emissions from the cosmic microwave background, and, more broadly, explore how the interstellar medium evolves across cosmic time.
Above: Visualisation using Aladin Lite to compare Planck data revealing galactic emission (at frequencies from 10 to 90GHz) with the cosmic microwave background.
Article : Far-infrared to centimeter emission of very nearby galaxies with archival data, Correia et al. 2026, Astronomy & Astrophysics, 705, A258.
Scientific contact : Lucie Correia, lucie.correia@astro.unistra.fr
For more information : Savoir(s) article (fr).