A simulated map of the Milky Way shows the location of our galaxy’s stellar corpses — and they’re not where you might think they’d be.

Astronomers have drawn the first map of the distribution of ancient dead stars in and around our Milky Way, in what they dub the “galactic underworld.” In a surprising twist, the shape of this stellar cemetery is radically different from that of the galaxy itself.

Map of stellar corpses throughout the galaxy
This simulated map of the Milky Way maps its stellar corpses — neutron stars and black holes — in face-on (left) and side (right) views. Their distribution is markedly different than that of currently burning stars, below.
University of Sydney

The most massive stars die in cataclysmic supernova explosions, leaving behind compact corpses such as neutron stars and black holes. Those that formed recently are easy to find, but astronomers have found it far more difficult to track down ancient ones. “The oldest neutron stars and black holes were created when the galaxy was younger and shaped differently, and then subjected to complex changes spanning billions of years,” says study team member Peter Tuthill (University of Sydney, Australia). The study appears in the November issue of the Monthly Notices of the Royal Astronomical Society.

Simulated Map of Milky Way
This simulated map of the Milky Way's star, for comparison, reproduces the spiral arms and thin disk in the face-on (left) and side (right) views, respectively.
University of Sydney

“The hardest problem . . . in hunting down their true distribution was to account for the ‘kicks’ they receive in the violent moments of their creation,” says team lead David Sweeney (also University of Sydney). Supernovae are asymmetrical and the uneven explosion can boot the remnant out of the main disk of the Milky Way. Young remnants won’t have moved very far in the short time since their demise; older ones are long gone. Based on calculations, the team found that 40% of neutron stars and 2% of black holes are kicked so hard that they’re ejected from the galaxy entirely to wander intergalactic space alone.

The remainder are still caught in the Milky Way's gravitational clutches, and Sweeney and his team set out to find out where they are. “It was like trying to find the mythical elephant's graveyard,” says Tuthill. “The bones of these rare massive stars had to be out there, but they seemed to shroud themselves in mystery.”

Sweeney and his team constructed a simulated map of their current locations using a highly detailed statistical model that factored in the birth, death, and ejection of these ancient stars. They found that, rather than being located in one plane like the disk-shaped Milky Way, they likely form a spherical cloud that stretches to three times the disk’s height. This reflects the completely random nature of the original kicks.

“It was a bit of a shock,” says team member Sanjib Sharma (University of Sydney). “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.” The spiral structure that a galaxy like ours is so famous for was also completely absent.

Karen Masters (Haverford College), who was not involved in the research, is impressed. “I think this work is a really clever use of well-established models to investigate the Milky Way,” she says.

If this result bears out, the stars that are ejected completely could have implications for our understanding of dark matter – the missing mass required to keep a galaxy like the Milky Way together. “We assume these stars remain around, but we can't see them,” Masters says. If they are in fact lost to intergalactic space then the visible mass of the Milky Way is lower, which means dark matter makes up an even larger proportion of the galaxy's total mass.

When it comes to the stars that aren't ejected entirely, Masters notes that observing such objects in the real galaxy is “challenging.”

And that is the natural next step. “The most exciting part of this research is still ahead of us,” says Sweeney. “Now that we know where to look, we’re developing technologies to go hunting for them. I'm betting that the ‘galactic underworld’ won't stay shrouded in mystery for very much longer.”



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