If you’ve been out in the wee hours to watch the rise of Matariki, you might have also been awed by our entire galaxy. The Milky Way is best viewed on moonless nights during the winter months, when it can be seen as a hazy band of stars stretching across the night sky.
But now, astronomers have delivered a new map of our galactic home, made not by light but by matter. They used an extraordinary particle catcher buried deep in the ice near the South Pole, known as the IceCube Neutrino Observatory. It is literally a giant ice cube – a cubic kilometre of ice studded with a 3D grid of strings beaded with more than 5000 exquisitely sensitive light detectors ready to catch neutrinos.
Neutrinos are ubiquitous but elusive. Each second, trillions of these ghost-like cosmic messengers hurtle through the Earth, but they remain undetected unless they collide with another particle within this giant frozen detector. “Many neutrinos pass completely unimpeded through the Earth,” says Jenni Adams, an astrophysicist at the University of Canterbury and a member of the 350-strong international IceCube collaboration.
“They do occasionally interact with water molecules, creating particle by-products called muons that we witness as flashes of light inside the detector’s sensors. From the patterns these flashes make, we can reconstruct the energy, and sometimes the sources, of the neutrinos.”
For astronomers, neutrinos open a window to the most extreme environments in the universe. They are produced in places that emit high-energy cosmic rays, but these rays are difficult to trace because they are charged and therefore zig-zag through the universe deflected by magnetic fields.
In contrast, neutrinos are neutral, as their name suggests, and travel in straight lines.
“So, if we can detect the path of neutrinos arriving at Earth, this will point back to where they were created,” Adams says.
The IceCube team has been looking for neutrinos aligned with the Milky Way for some time. But it’s only now, with the help of machine learning, that they could be teased apart reliably from other high-energy neutrinos arriving from outside our own galaxy.
“We detected extra-galactic neutrinos first,” Adams says. They carry the highest energy in the universe and the team has been able to pinpoint two of their sources as supermassive black holes at the centre of galaxies.
The source of the neutrinos that originate within the Milky Way remains as yet unclear, but Adams says the most likely places are supernova remnants. “This is when a massive star finishes burning everything inside. The supernova event happens relatively quickly. But then the material expands out into the interstellar medium over thousands of years. And it’s like a shock front, a sort of explosion, and you see that cloud going out from it.”
In our galaxy, a supernova happens roughly every 100 years. The Milky Way neutrino map now helps astronomers to determine the amount of energy associated with these sources and to figure out the highest energy our galaxy is capable of producing. “How high energy supernova remnants can get is one of the big questions we would like to answer with this kind of research. If we see a peak in the galactic neutrinos, then we can say that’s the peak our galaxy is capable of accelerating cosmic rays to.”
The galactic neutrino discovery will also help with modelling the beginning of the Milky Way because cosmic rays are fundamental to understanding how galaxies are formed. “To understand our own galaxy, we need to understand the cosmic rays in it: how they got the energy and when they were first introduced into the galaxy.”
But Adams says she’s excited about both: neutrinos that originated in outer space or within the Milky Way. “The extra-galactic ones have the highest energy. They are like beautiful castles you like to visit, but we want to understand our own home, too.”
