In June, scientists published the most unusual portrait of the Milky Way to date. Astronomers have mapped our Galaxy using something other than light beams for the first time, a finding that heralds a new way of studying what’s going on in the busy center of the Milky Way.
The data comes from the IceCube Observatory at the South Pole, which detects neutrinos, the lightest and most penetrating elementary particles in the universe. The observatory is a huge array of 5,160 sensors buried more than 1.5 kilometers below the surface of the ice sheet. Over the course of ten years, the field detected neutrinos one by one and measured the direction they came from, allowing researchers to map their origin to the center of the Milky Way.
The first neutrino map of our Galaxy is a milestone for the nascent field of neutrino astronomy, researchers say. And it’s just the beginning. Several neutrino detection fields are currently under construction in locations from the Mediterranean Sea to Siberia.
Neutrinos are known to arise through a myriad of subatomic processes: from nuclear fusion in the Sun’s core and radioactivity in Earth’s rocks to high-energy collisions of interstellar particles with the atmosphere. Over the past decade, the IceCube Neutrino Observatory has peered deeper into space, detecting record-high energy neutrinos and determining their origin in distant cosmic sources.
In fact, it was easier for astronomers to detect neutrinos from the far reaches of space than neutrinos from the Milky Way. “We now have more information about extragalactic sources than galactic ones,” says IceCube spokesman Francis Halzen, a physicist at the University of Wisconsin-Madison. “Which is wonderful.
IceCube and other observatories are turning neutrinos into a tool for peering into otherwise inaccessible places, such as the dense vortices of matter swirling around supermassive black holes at the centers of galaxies, where extreme energies could reveal new physical phenomena. And the approach could eventually help reveal the source of cosmic rays—protons, or heavier atomic nuclei, that travel at nearly the speed of light.
The second challenge is geography. IceCube gets cleanest neutrino detection from below. This is because neutrinos are the only known particles that can travel undisturbed across the planet and reach the South Pole ice from below. This means that signals from points below the horizon can be more easily distinguished from the cacophony of particles hitting the detector from above, which mostly come from electrons’ massive cousins ​​called muons. However, in order to see the Milky Way, IceCube did not have the luxury of using Earth’s mass as a shield. Most of the Galaxy’s mass is concentrated in a single band of the southern sky around the constellation Sagittarius permanently above the horizon as seen from the South Pole.
So the team had a huge sorting job to find high-energy galactic neutrinos. The researchers analyzed 59,592 detections between May 2011 and May 2021 in the energy range of 500 GeV to a few petaelectronvolts. They estimated that only 7% of these were neutrinos originating from deep space. They then used machine learning techniques to show that while most of the events had points of origin scattered across the sky, a small number were concentrated in regions of the Milky Way that also have high γ-ray emission, suggesting that they must have been neutrinos from the Galaxy (see ‘Different Views of the Galactic Center’). The team reported the results in the June 30 issue of Science.
However, the long-term effects of a supernova could be different. Over the centuries, the expanding shock waves from such an explosion could act like slot machines, accelerating protons to higher energies. “Each time a proton crosses a shock wave, it gains a bit of energy,” says Montaruli.
IceCube has been much more successful in locating point sources outside the Galaxy, specifically those in active galactic nuclei (AGN) supermassive black holes that emit bright radiation as they absorb large amounts of matter from their surroundings.
In 2018, this collaboration combined a single, extremely energetic neutrino with a blazar event a flare from an AGN that temporarily brightens it2. And last year, the team reported collecting around 80 neutrinos coming from the Squid Galaxy AGN, also known as NGC 10683.
However, the octopus heart is surprisingly not bright in γ radiation. “NGC 1068 appears to be a source of neutrinos, but the photons that should be produced in the same interactions are not getting through,” says Elisa Resconi, an astroparticle physicist at the Technical University of Munich in Germany. This means that its central black hole must be surrounded by a layer of dust thick enough to block γ-rays. This makes neutrinos all the more valuable because they are the only available medium for understanding the physics around a giant black hole, Resconi says. At just 14.4 megaparsecs from Earth, Squid is one of the closest and best-studied AGNs.
Resconi is a spokesman for the Pacific Ocean Neutrino Experiment, a proposed deep-ocean neutrino observatory off Vancouver Island, Canada, that would have an even larger volume than IceCube more than 2 cubic kilometers. Another large neutrino observatory, called the Cubic Kilometer Neutrino Telescope, is being built at three separate sites in the Mediterranean Sea. And Russia is building cube neutrino observatories in Lake Baikal in Siberia. Together, these observatories would form a network that could collect unprecedented amounts of neutrinos from across the sky.
The Squid Galaxy findings are the kind of discovery that has astronomers particularly excited about neutrino astronomy. The extreme environments surrounding black holes offer natural laboratories for testing the limits of known laws of physics and open the possibility of finding new ones, Resconi says. “It doesn’t get any better than this for scientists.
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Reference: https://www.nature.com/articles/d41586-023-02427-6
