Human beings for millennia have gazed with awe at the vast torrent of stars - bright and dim - shining in Earth's night sky that comprise the Milky Way. Our home galaxy, however, is now being observed for the first time in a brand new way.
Scientists said on Thursday they have produced an image of the Milky Way not based on electromagnetic radiation - light - but on ghostly subatomic particles called neutrinos. They detected high-energy neutrinos in pristine ice deep below Antarctica's surface, then traced their source back to locations in the Milky Way - the first time these particles have been observed arising from our galaxy.
This view differs fundamentally from what we can see with our own eyes or with instruments that measure other electromagnetic sources like radio waves, microwaves, infrared, ultraviolet, X-rays and gamma-rays. It is not stars and planets and other stuff observable thanks to their light, but rather the mysterious sources of neutrinos originating in the galaxy, perhaps remnants of explosive star deaths called supernovas.
The neutrinos were detected over a span of a decade at the IceCube Neutrino Observatory at a US scientific research station at the South Pole, using more than 5,000 sensors covering an area the size of a small mountain.
"This observation is ground-breaking. It established the galaxy as a neutrino source. Every future work will refer to this observation," said Georgia Tech physicist Ignacio Taboada, spokesperson for the IceCube research.
"When we discovered neutrinos of cosmic origin in 2013, it was somewhat of a surprise to us that we did not find a flux that originated in the nearby sources of our own galaxy. Galactic sources were supposed to dominate the sky, as they do in all wavelengths of light. It took us a decade to discover our own galaxy," said University of Wisconsin physicist and IceCube lead scientist Francis Halzen.
Neutrinos are electrically neutral, undisturbed by even the strongest magnetic field, and rarely interact with matter, earning the nickname "ghost particle." As neutrinos travel through space, they pass unimpeded through matter - stars, planets and, for that matter, people.
"Just as light goes without stopping through glass, neutrinos can go through everything, including the whole planet Earth," Taboada said.
"The neutrino is an elementary particle, meaning they are not made up of anything smaller. They are not the building blocks of 'stuff,' like electrons and quarks are, but they are created in nuclear processes. They are also created when protons (subatomic particles) and (atomic) nuclei interact at very high energies," said physicist Naoko Kurahashi Neilson of Drexel University in Philadelphia, a member of the research team that detailed the findings in the journal Science.
Many aspects of the universe are indecipherable using light alone. The ability to use particles like neutrinos in astronomy enables a more robust examination, much as the confirmation of ripples in the fabric of space-time called gravitational waves, announced in 2016, opened another new frontier. This field is called "multi-messenger astrophysics."
Neutrinos are produced by the same sources as cosmic rays, the highest-energy particles ever observed, but differ in a key respect. Cosmic rays, as electrically charged particles, cannot be traced straight back to their source because strong magnetic fields in space alter their trajectory. The direction from which neutrinos arrive points directly back to their original source.
The researchers harnessed machine learning to help distinguish neutrinos originating in our galaxy from those originating elsewhere. They released an illustration of their findings with neutrinos from the Milky Way represented by light, with a heavy concentration at the galaxy's core.
How the neutrinos originated is a matter of debate. The observations were consistent with the idea of a diffuse emission of neutrinos in the Milky Way, but these particles could arise from specific yet-unknown sources.
"This is now the key question. Neutrinos only originate in sources where cosmic rays are produced. They are tracers of cosmic ray sources. The key question is where these cosmic rays originate," Halzen said.
"The most likely source of neutrinos and cosmic rays in our galaxy," Taboada added, "are the remains of past supernova explosions. But this is unproven so far."