Scientists observe the Milky Way in a new way – 06/30/2023 – Science

For millennia, humans have gazed in awe at the vast torrent of stars—bright and dim—in Earth’s night sky that includes the Milky Way. Our home galaxy, however, is now being observed for the first time in an entirely new way.

Scientists said on Thursday they had produced an image of the Milky Way based not on electromagnetic radiation — light — but on ghostly subatomic particles called neutrinos. They detected high-energy neutrinos in pure ice deep beneath Antarctica’s soil and then traced their source to locations in the Milky Way — the first time such particles have been observed to emerge from our galaxy.

This view differs fundamentally from what we can see with our own eyes or with instruments that measure other electromagnetic sources, such as radio waves, microwaves, infrared, ultraviolet, X-rays, and gamma rays. These are not stars, planets and other things observable thanks to their light, but the mysterious sources of neutrinos originating in the galaxy, perhaps leftovers from the explosive deaths of stars called supernovae.

Neutrinos were detected over a decade at the IceCube Neutrino Observatory at a US scientific research station at the South Pole, using more than 5,000 sensors in an area the size of a small mountain.

“This observation is groundbreaking. It defined the galaxy as a source of neutrinos. All future work will refer to this observation,” said physicist Ignacio Taboada of Georgia Tech, spokesman for the IceCube research.

“When we discovered neutrinos of cosmic origin in 2013, it was a surprise that we did not find a flux originating from the nearby sources of our own galaxy. Galactic sources should dominate the sky, as they do in all wavelengths of light. It took us a decade to discover our own galaxy,” said physicist Francis Halzen of the University of Wisconsin, IceCube’s principal scientist.

Neutrinos are electrically neutral, undisturbed by even the strongest magnetic field, and rarely interact with matter, earning them the nickname “ghost particles”. As neutrinos travel through space, they pass unimpeded through matter—stars, planets, and, in this case, people.

“Just as light passes through glass without stopping, neutrinos can pass through everything, including the entire planet Earth”, said Taboada.

“The neutrino is an elementary particle, which means that it is not made of anything smaller. They are not building blocks of ‘things’ such as electrons and quarks, but are created in nuclear processes. They are also created when protons (particles (subatomic) 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 explained 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 allows for more robust examination, as does confirmation of ripples in the fabric of spacetime called gravitational waves, announced in 2016, opening up another new frontier. This field is called “multimessenger astrophysics”.

Neutrinos are produced by the same sources as cosmic rays, the highest-energy particles ever observed, but they differ in one fundamental respect. Cosmic rays, being electrically charged particles, cannot be traced directly to their source because strong magnetic fields in space alter their trajectory. But the direction of arrival of neutrinos points directly to their original source.

The researchers leveraged machine learning to help distinguish neutrinos originating in our galaxy from those coming from elsewhere. They released an illustration of their findings with Milky Way neutrinos represented by light, with strong concentration in the galaxy’s core.

How 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 sources as yet unknown.

“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,” said Halzen.

“The most likely source of neutrinos and cosmic rays in our galaxy,” added Taboada, “are the remains of ancient supernova explosions. But this has not been proven so far.”

^{Translated by Luiz Roberto M. Gonçalves}