Some supernovae seem to be lost in space, and now astronomers may have figured out why.
Once in a while, astronomers will stumble across a supernova that seems to have gone off in the wrong place, thousands of light-years away from the nearest galaxy where it might belong. First discovered in 2005, the type of supernovae in question occur at about five percent the rate of normal supernovae, John Timmer writes for Ars Technica. Known as “calcium-rich supernovae,” these supernovae are caused by white dwarf stars that blast out high levels of their namesake element as they explode. In fact, these calcium spewing stars could even be the source of all calcium on Earth.
While astronomers and physicists at first believed the wandering supernovae might belong to dwarf galaxies that would be hard to detect at great distances, a new study suggest that these rogues may have been fired away from their home systems by binary black holes like a rock from a slingshot.
“Looking around where the supernovas exploded, there’s nothing there – no trace of star formation, no clusters of old stars, there’s nothing nearby,” Ryan Foley, the study’s author, tells Liz Ahlberg for the News Bureau of the University of Illinois, where Foley is a professor of astronomy and physics. “So I knew that these things were starting somewhere else and moving long distances before they die.”
By compiling data from several telescopes, including NASA’s Hubble telescope, Foley traced the origins of 13 rogue supernovae to their home galaxies. According to the study published in the Monthly Notices of the Royal Astronomical Society, these galaxies were mostly made up of older stars. According to Foley, the data suggests that each of these supernovae were originally a pair of dwarf stars ejected from their home galaxy.
There’s one problem with this theory: in order for this type of calcium-rich supernova to occur, a white dwarf has to have siphoned off enough mass from a companion star to fuel the chemical reaction. Foley’s theory says that the competing black holes could have fired both stars in the same trajectory. “You have two dancing partners, they do-si-do, and one pair gets flung away,” Foley tells Ahlberg. “The white dwarf and its partner are ejected out like from a slingshot, and after traveling at a high speed for about 50 million years, explode out in the middle of nowhere.” But Timmer writes that it’s highly unlikely that the companion star they were leaching from would have followed a white dwarf in that kind of flinging. "The physics of ejection requires that a binary system interact with the supermassive black hole at the center of the galaxy," Timmer explains. "One of the stars gets trapped in orbit around the black hole while the other is shot out into intergalactic space."
Foley has an explanation for that though. He suggests that rather than a single galaxy's black hole flinging both the dwarf star and its companion, the slingshot was powered by two black holes, when one galaxy galaxy collided with another one. It’s possible that these dwarf stars were caught up in the push and pull of the supermassive black holes at the centers of these galaxies before being flung off into deep space.
Astronomers need to find more of these lone supernovae to test against Foley’s theory. But in the meantime, they can bask in the glow of the brightest supernova ever discovered.