Cosmic Static: Dust or Molecules? express
The night sky glows not only with stars. Between them lies a faint microwave radiance that astronomers have noticed for three decades but couldn't explain. One idea: the culprits are polycyclic aromatic hydrocarbons — molecules like tiny soot flakes that spin in space and, like miniature antennas, emit radio waves.
Fresh observations from the DIRBE instrument on the COBE satellite compared this glow with a dust map and a map of the molecules' own emission. It turned out that the microwave signal matches dust much better than molecules, like engine noise coming not from the belt but from the whole body. This means either the molecular "soot" isn't emitting, but other dust grains are, or the molecules glow in microwaves only under special conditions we don't yet understand.
If the mysterious glow originates not in molecules but in ordinary dust, then we've been misreading one of the most common "signatures" in our galaxy. And that matters for all-sky maps used to search for traces of the Big Bang.
Original abstract
as written by the authors arXiv:2601.20545
The physical mechanism producing Anomalous Microwave Emission (AME) has been an unresolved puzzle for close to 30 years. One candidate mechanism is rotational emission from polycyclic aromatic hydrocarbons (PAHs) which can have the necessary electric dipole moment and size distribution to account for the AME in representative interstellar environments. However, previous investigations have found that AME is better correlated with the far-infrared dust emission rather than the PAH emission. In this work we analyze the correlations between the AME and the PAH and far-infrared dust emission using the 3.3 $$\mu$$m PAH emission feature as observed by band 3 of the Diffuse Infrared Background Experiment (DIRBE). This analysis builds on previous work conducted in individual molecular clouds and extends it into fainter, more diffuse structures. In addition, we utilize the COSMOGLOBE DIRBE reduction for this work, building on previous studies that used the original DIRBE data set. We find that the AME is better correlated with far-infrared dust emission ($$\rho\sim$$0.9) than the PAH emission ($$\rho\sim$$0.7) in the central $$|b|\leq$$ 10\degree\ region of the sky. This could indicate either that non-PAH dust grains or an alternative physical emission mechanism is primarily responsible for the AME in the Galactic Plane, or that the excitation conditions for mid-infrared emission and for AME from PAHs differ substantially.
Polycyclic aromatic hydrocarbons are not just cosmic "soot." They form when grilling meat over charcoal and in exhaust fumes, so space and barbecue share the same chemistry.