Astroboffins trace mysterious noise from hard rock in space

Signs in the sky from diamonds

An artist's impression of diamonds emitting radiation around a protostar. Image credit : S. Dagnello, NRAO/AUI/NSF

The source of unusual faint pockets of microwave radiation found only in a few regions of the Milky Way has been traced back to tiny specks of nanodiamond dust.

A paper published in Nature Astronomy on Monday cracks the mystery of anomalous microwave emission (AME). It was first discovered in the late twentieth and early twenty-first century when scientists began probing the cosmic microwave background left over from the Big Bang.

They discovered a signal emanating at a frequency of 10-60 GHz, and considered it as a form of noise contaminating their measurements. At first, they thought the radiation came from spinning polycyclic aromatic hydrocarbon (PAH) molecules scattered throughout space.

"Though we know that some type of particle is responsible for this microwave light, its precise source has been a puzzle since it was first detected nearly 20 years ago," said Jane Greaves, lead author of the paper and an astronomer at Cardiff University.

But researchers believe they have now solved the problem. Instead of PAHs, it’s grains of spinning hydrogenated nanodiamonds that are so tiny, they cannot be seen by most microscopes.

The carbon and hydrogen bonds at the surface carry a slight negative and positive charge creating electric-dipole moments. As they spin, they emit electromagnetic radiation. Their tiny size means they can rotate incredibly quickly to give off microwave radiation.

Tiny objects, bigger sounds

The team turned to the National Science Foundation's Green Bank Telescope in the United States, and the Australia Telescope Compact Array to observe the environment around 14 protostars in the Milky Way. Only three of the protostar disks contained AME.

“This is the first clear detection of anomalous microwave emission coming from protoplanetary disks," said David Frayer, coauthor on the paper, and a astronomer with the Green Bank Observatory.

Some disks contained clear infrared signals from PAHs but no AMEs. The systems that did have AMEs were the only ones to host hydrogenated nanodiamonds.


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“In a Sherlock Holmes-like method of eliminating all other causes, we can confidently say the best candidate capable of producing this microwave glow is the presence of nanodiamonds around these newly formed stars," said Greaves.

Nanodiamonds are very rare, it is estimated that only one to two per cent of carbon in these protoplanetary disks is used to create the molecules. No other young stars have been observed to emit this kind of radiation before. The nanodiamonds are created in the vapor of superheated, pressurized carbon atoms that have been energized from these hot stellar nurseries.

"This is good news for those who study polarization of the cosmic microwave background, since the signal from spinning nanodiamonds would be weakly polarized at best," said Brian Mason, co-author fo the paper and an astronomer at the National Radio Astronomy Observatory.

"This means that astronomers can now make better models of the foreground microwave light from our galaxy, which must be removed to study the distant afterglow of the Big Bang." ®

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