The first map of the “galactic underworld” – a map of the corpses of once-massive suns that have now collapsed into black holes and neutron stars – has revealed a graveyard stretching three times the height of the Milky Way, and which nearly a third of the objects became thrown out of the galaxy altogether.
“These compact remnants of dead stars show a fundamentally different distribution and structure than the visible galaxy,” said David Sweeney, Ph.D. Student at the Sydney Institute for Astronomy at the University of Sydney and lead author of the article in the latest issue of Monthly Bulletins of the Royal Astronomical Society.
“The ‘height’ of the galactic underworld is more than three times that in the Milky Way itself,” he added. “And a staggering 30 percent of the objects were completely ejected from the galaxy.”
Neutron stars and black holes are formed when massive stars – more than eight times larger than our Sun – run out of fuel and suddenly collapse. This triggers a runaway reaction that blasts the outer parts of the star apart in a titanic supernova explosion while the core continues to collapse inward until it becomes either a neutron star or a black hole, depending on its initial mass.
In neutron stars, the core is so dense that electrons and protons are forced to combine into neutrons at the subatomic level, forcing their combined mass into a sphere smaller than a city. When the original star’s mass is more than 25 times that of our Sun, this gravitational collapse continues until the core is so dense that not even light can escape. Both types of stellar bodies curve space, time, and matter around them.
Although billions of humans must have been born since the galaxy was young, these exotic carcasses were flung into the darkness of interstellar space by the supernova that created them, eclipsing the sight and knowledge of astronomers – until now.
By painstakingly recreating the entire life cycle of ancient dead stars, researchers have created the first detailed map showing where their bodies lie.
“One of the problems with finding these ancient objects is that until now we didn’t know where to look,” said Professor Peter Tuthill of the Sydney Institute for Astronomy, co-author of the publication. “The oldest neutron stars and black holes were created when the galaxy was younger and shaped differently, and then underwent complex changes spanning billions of years. It was a big job modeling all of this to find them.”
Newly formed neutron stars and black holes correspond to today’s galaxy, so astronomers know where to look. But the oldest neutron stars and black holes are like ghosts that still haunt a long-torn house, so they’re harder to find.
“It was like trying to find the graveyard of the mythical elephant,” said Professor Tuthill, referring to a place where legend has it that old elephants go alone to die, far away from their group. “The bones of these rare massive stars had to be out there, but they seemed shrouded in mystery.”
Sweeney added that “the hardest problem I had to solve in finding their true prevalence was explaining the ‘kicks’ they get at the violent moments of their creation. Supernova explosions are asymmetric and the remnants are ejected at high speeds – up to millions of kilometers per hour – and, what is worse, it happens in an unknown and random direction for each object.”
But nothing in the universe stands still for long, so knowing the likely magnitudes of the explosive kicks wasn’t enough: researchers had to delve deep into cosmic time and reconstruct how they behaved over billions of years.
“It’s a bit like snooker,” Sweeney said. “If you know which direction the ball is being hit and how hard it is, then you can calculate where it’s going to land. But in space, the objects and velocities are just much larger. Also, the table isn’t flat, so the stellar remnants go on complex orbits that wind through the galaxy.”
“Finally, unlike a snooker table, there’s no friction – so you never slow down. Almost every remnant that ever formed is still out there, gliding through interstellar space like ghosts.”
The intricate models she developed with research fellow Dr. Sanjib Sharma from the University of Sydney and Dr. Ryosuke Hirai of Monash University encoded where the stars were born, where they met their fiery end, and their eventual dissolution as the galaxy evolved.
The end result is a distribution map of the Milky Way’s stellar necropolis.
“It was a bit of a shock,” said Dr. Sharma. “I work every day with images of the visible galaxy as we know it today, and I expected the galactic underworld to be subtly different but broadly similar. I didn’t expect such a radical change in shape.”
In the generated maps, the characteristic spiral arms of the Milky Way disappear in the “galactic underworld” version. These are completely washed out due to the age of most of the remains and the blurry effects of the energetic kicks of the supernovae that created them.
Even more intriguingly, the side view reveals that the galactic underworld is much more “inflated” than the Milky Way — a result of the kinetic energy injected by supernovae lifting them into a halo around the visible Milky Way.
“Perhaps the most surprising result of our study is that the shocks are so strong that the Milky Way will lose some of these remnants entirely,” said Dr. hirai “They get kicked so hard that about 30 percent of neutron stars are ejected into intergalactic space, never to return.”
Tuthill added, “To me, one of the coolest things we’ve found in this work is that even the local stellar neighborhood around our sun is likely to have these spooky visitors. Statistically, our nearest remnant should be only 65 light-years away: more or less in our backyard, in galactic terms.”
“The most exciting part of this research is yet to come,” Sweeney said. “Now that we know where to look, we’re developing technology to hunt them down. I bet the ‘galactic underworld’ won’t remain shrouded in mystery much longer.”
Data from the Gaia space telescope shows the galaxy’s primordial core
David Sweeney et al, The Galactic Underworld: The Spatial Distribution of Compact Remnants, Monthly Bulletins of the Royal Astronomical Society (2022). DOI: 10.1093/mnras/stac2092
Provided by the University of Sydney
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