Our galaxy, the Milky Way, has several peculiar features whose origins we are only beginning to understand. One such feature is a faint microwave signal throughout the galaxy. For over 20 years, it has been puzzling astronomers, but now researchers have found that the source is nanodiamonds.
The signal is known as an anomalous microwave emission (AME), and astronomers have homed in on three distinct sources of this signal. All three of them are from protoplanetary disks, the ring of gas and dust that surrounds young stars. The discovery is reported in Nature Astronomy.
"This is the first clear detection of anomalous microwave emission coming from protoplanetary disks," co-author David Frayer, an astronomer with the Green Bank Observatory, said in a statement.
The cause of AME was previously believed to be carbon molecules known as polycyclic aromatic hydrocarbons (PAHs). These have been found in protoplanetary disks before, but researchers couldn’t find an AME signal in them. Such signals were clear in the presence of nanodiamonds. In these systems, just 1 to 2 percent of the protoplanetary disk’s carbon was turned into nanodiamonds, but it was enough to create a clear signal.
"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," lead author Jane Greaves, from Cardiff University in Wales, explained. "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."
Nanodiamonds are special because they can emit light just by spinning. They are much smaller than regular dust particles, which allow them to spin incredibly fast so that the emission is quite energetic in the microwave and infrared portion of the electromagnetic spectrum.
"This is a cool and unexpected resolution to the puzzle of anomalous microwave radiation," concluded Greaves. "It's even more interesting that it was obtained by looking at protoplanetary disks, shedding light on the chemical features of early solar systems, including our own."
To probe the AME in greater detail, we’ll need observatories that focus on light at centimeter wavelengths. There are some proposed and some currently being built or upgraded, but having a specific cause is already a huge step forward.