The first-ever chart of the "galactic underworld" reveals that almost a third of the objects in the galaxy have been thrown out of the galaxy altogether, as their bodies have collapsed into black holes or neutron stars.
The Milky Way is our home galaxy and exploring it is of great essence. Despite the fact we are still in the infancy of doing so, we are making great progress in understanding what our cosmic neighborhood is like and how it behaves. And while we search for distant stars and exoplanets that might harbor life or be habitable for life to thrive on them, looking for long-gone stars is also of great importance.
The first-ever chart of the “galactic underworld” reveals that almost a third of the objects in the galaxy have been thrown out of the galaxy altogether, as their bodies have collapsed into black holes or neutron stars. The graveyard stretches three times as high as the Milky Way, and almost a third of the objects are buried three times as deep as the Milky Way. Doctoral student David Sweeney says the structure and distribution of this compact remnant of dead stars differ fundamentally from the visible galaxy. “The galactic underworld has a height three times greater than our Milky Way,” he said. Thirty percent of the galaxy’s objects have been expelled completely.”
Large stars—more than eight times larger than our sun—exhaust their fuel and collapse, forming neutron stars and black holes. The supernova explosion blows apart the outer portions of the star, but the core continues to compress in on itself until it becomes a neutron star or black hole based on its starting mass. The outer parts of the star are blown apart by this runaway reaction.
A neutron star has an extremely dense core that forces electrons and protons to combine into neutrons at the subatomic level, reducing the total mass to a sphere smaller than a city. A star whose mass exceeds 25 times that of our sun will continue to collapse through gravity until its core is so dense that even light cannot escape. Space, time, and matter are distorted by stellar corpses of both types. The supernova that created these exotic carcasses sucked them into the darkness of interstellar space and thereby slipped beyond astronomers’ sight and knowledge until now, despite the fact that billions must have existed since the galaxy was young.
These researchers have created the first detailed map of where the ancient dead stars’ corpses lie by recreating their full lifecycle. Professor Peter Tuthill, the co-author of the paper, noted that one of the challenges in finding ancient objects is that we have no idea where to look until now. When the galaxy was younger and shaped differently, neutron stars and black holes were created, and they were then subjected to complex changes spanning billions of years. The modeling of all of this to find them has been a major undertaking.”
As new neutron stars and black holes form, they conform to today’s galaxy, making them easy to spot. It is harder to find the oldest neutron stars and black holes because they are like ghosts haunting a house that had been demolished long ago. Professor Tuthill compared the search to finding “the mythical elephant’s graveyard,” which is a place where, according to legend, old elephants die alone, away from their friends and family. “The bones of these rare massive stars had to be out there, but they seemed to shroud themselves in mystery.”
The researchers had to delve deep into cosmic time to determine how the explosive kicks behaved over billions of years since nothing in the universe stands still for long. Sweeney refers to it as something akin to snooker. You can predict where a ball will end up if you know its direction and how hard it was hit. But in space, we are dealing with vastly larger objects and speeds. Moreover, the table is not flat. Thus, stellar remnants follow complicated orbits throughout the galaxy.
“Finally, unlike a snooker table, there is no friction—so they never slow down. Almost all the remnants ever formed are still out there, sliding like ghosts through interstellar space.” Their complex models – created jointly with University of Sydney Research Fellow Sanjib Sharma and Monash University’s Dr. Ryosuke Hirai – encode where the stars were born, where they died, and their eventual dispersion as the galaxy evolved.
Using this data, they create a map showing the stellar necropolis distribution across the Milky Way. Dr. Sharma said it was a bit of a shock. “I work every day with images of the visible galaxy we know today, and I expected that the galactic underworld would be subtly different but similar in broad strokes. I was not expecting such a radical change in form.” As a result of the ‘galactic underworld’ mapping, the characteristic spiral arms of the Milky Way are absent. As most of the remnants are old and blurred due to the energetic kicks they received from supernovae, most of these are completely lost.
As a result of kinetic energy being injected by supernovae and elevating them into a halo around the Milky Way, the galactic underworld appears much puffier than the Milky Way. The study found that some of these remnants will disappear entirely due to the kicks, which is perhaps the most surprising finding. “They are kicked so hard that about 30 percent of the neutron stars are flung out into intergalactic space, never to return.”
The study was published in Monthly Notices of the Royal Astronomical Society.