Observing the Milky Way with Neutrinos

The arc of the Milky Way above our heads at night has been seen and recorded for as long as humans have existed.

We see the Milky Way in visible light, but much else can be revealed beyond this. Astronomers in recent decades have begun to observe the Milky Way in a wide range of other wavelengths - radio waves, infrared, ultraviolet even gamma rays.

Announced this month was a map of the Milky Way not by light, but by subatomic particles called Neutrinos.

What's a Neutrino?

A Neutrino is a subatomic particle that's found in vast numbers throughout the Universe and created by a variety of processes. They don't have any electrical charge, have miniscule masses close to zero, and hardly interact with matter. This means that while they're constantly zipping around and through us and everything that surrounds us, they're extremely hard to detect.

Most of the Neutrinos streaming through us come from either the Sun or our atmosphere. However, a tiny number moving at a higher speed arrive from outside the Solar System.

Detecting Neutrinos

As Neutrinos are so hard to detect, a unique observatory was built in the Antarctic. The IceCube Neutrino Observatory is a huge facility with some 5,160 sensors in a cubic array one kilometer on each side, buried deep in the ice at the South Pole.

IceCube neutrino detector's aboveground lab. Image via YUYA MAKINO, ICECUBE/NSF

The detectors located at IceCube are shielded as much as possible from the Sun during winter months and other sources of radiation.  Occasionally Neutrinos will strike the nucleus of atoms in the water molecules in the dense polar ice and break them down into a series of sort lived, subatomic particles, some of which will emit a form of light called Cherenkov radiation.  (This is the same process that gives pools of water surrounding nuclear reactors their blueish glow.)

Since starting operations in 2011, about a million Neutrino observations have been recorded by IceCube.  Some of these detections leave tracks that can be analysed and used to point back to where they originated. Others, with much higher velocities sometimes caused "Cascade Events" where they kicked off other processes. These are harder to establish where they originate from. Their usefulness for Astronomy was limited

Now Astronomers are using AI networks to scour through these events to establish where these Neutrinos have arrived from.

Physicist Naoko Kurahashi Neilson of Drexel University in Philadelphia and her team used Neutrinos to map the Milky Way for the first time in something other than light.

“When I first joined IceCube,” Professor Kurahashi Neilson commented, “I used to do air quotes” when using the phrase neutrino astronomy. “I don’t do that anymore.… I don’t have to because we’re starting to resolve things” in neutrino images that resemble the astronomical images from other telescopes."

The Milky Way as seen in Neutrinos

We have good views of the Milky Way in visible light. Why do we need this?

The very nature of Neutrinos is the reason they offer new and unique views of our local galaxy. Much of the Milky Way is hidden from us by interstellar gas and dust. While telescopes sensitive to different wavelengths of light such as Infrared help get around this, Neutrinos can pass through hundreds of thousands of light years of space unimpeded - and bringing with them information about where they were formed.

What's next?

As always in Astronomy, new telescopes, tools or techniques bring new knowledge and discoveries.  IceCube itself is undergoing a massive in what is simply called IceCube GEN2 (read about it here)

Neutrino Astronomy is a very new field, but one that's opening up new ways to look at the Universe.

Cheers,

Earl White

BINTEL

7th July 2023