Mysterious galactic radiation pinned on nanodiamonds

When electrons and protons formed the first hydrogen atoms some 379,000 years after the Big Bang, they freed up photons to travel the universe. We still see them today as microwave emissions. Astronomers mapping this cosmic microwave background in the 1990s to better understand the universe’s genesis noticed other signals coming from our own galaxy that they couldn’t explain. This anomalous microwave emission (AME) has long been ascribed to polycyclic aromatic hydrocarbons (PAHs). A new study suggests the real source may be nanoscopic diamonds (Nature 2018, DOI: 10.1038/s41550-018-0495-z).

Jane Greaves of Cardiff University was studying dust disks around stars when she spotted AME signals. The three stars the signals were coming from were the only ones in her sample where astronomers had observed signatures of hydrogenated nanodiamonds. These tetrahedral diamonds contain just a few hundred carbon atoms and are about a nanometer or less wide. Their origin isn’t known for certain, but Greaves says they likely form in high-pressure events like collisions or supernovae. Atomic hydrogen—abundant in interstellar space—forms a layer on their surface and collisions with other particles can give them a dipole moment. That, along with their propensity to spin, means they could emit microwave radiation.

Clive Dickinson of the University of Manchester says the results would be stronger if they were based on more observations. So astronomers need to collect more high-resolution observations to determine if diamonds produce AME elsewhere.

Greaves says one of her students is examining published spectra of nanodiamonds to see if the researchers can nail down which species occur around different stars. Whether they are the same or different will help astronomers to determine whether nanodiamonds—including the ones found in meteorites in our own solar system—are produced locally or come from common sources.