Why Are Some Exploding Stars Fleeing Their Galaxies?

Elliptical galaxy with wispy lanes – the sign of a recent galaxy merger. A supernova was found outside this galaxy. NASA, ESA, and Ryan Foley.

Out in the empty void of space is a lonely traveler. An exploding star zips through the blackness of the universe at breakneck speed, thousands of light-years away from the nearest galaxy. Even more strangely, this star has exploded billions of years prematurely. Thirteen of these rogue supernovas have been spotted outside of their galaxies – a sight that has perplexed scientists.

One scientist decided to tackle this deep space mystery to figure out where these stars came from, why they were moving at such high speeds, and what made them blow up billions of years before their due date. To do this, he traced the trajectory of the runaway stars back to the galaxies they had fled from. His findings are published in the Monthly Notices of the Royal Astronomical Society

A supernova is a bright explosion that happens at the end of a star's life. Usually, it is one of the brightest objects in a galaxy; however, these supernovas were unusually dim, weaker than a dying star. This indicated that the supernovas were from a pair of white dwarfs.

These dim dwarf stars, the remnants of dead stars, were likely locked in orbit around each other until powerful tidal forces tore one of them apart – its matter and dust then dumped onto the other white dwarf. This matter poured in so suddenly that it ignited a supernova explosion, albeit a less bright one since there was less matter to burn through. 

These supernovas were also rich in the element calcium, another clue that indicated they had come from galaxies, specifically ones with mostly old stars that had time to fuse a heavy element such as calcium in their cores. A calcium-rich supernova is associated with a supernova involving two white dwarf stars.

"Looking around where the supernovas exploded, there's nothing there – no trace of star formation, no clusters of old stars, there's nothing nearby," explained Ryan Foley from the University of Illinois, the scientist who is responsible for this galactic detective work. "So I knew that these things were starting somewhere else and moving long distances before they die."

Host galaxies of the rogue calcium-rich supernovae. Hubble Space Telescope/ACS/WFC.

So, what happened to these calcium-rich exploding stars to propel them into outer space? 

According to Foley, a likely possibility – which explains both the stars' velocity and isolation – involves what is found at the center of two colliding galaxies: a pair of supermassive black holes. These are among the most massive objects in the entire universe, with a gravitational attraction so powerful that even light can't escape their clutches. "How do you get a binary supermassive black hole? Merge two galaxies," Foley said.

So the mystery starts to become clear: A galaxy merger caused two central supermassive black holes to collide, creating a strong gravitational force. It was this force that affected a binary white dwarf pair orbiting one another. As they got sucked towards the black holes at an accelerating speed, the two white dwarfs were also pushed closer together. Fortunately for the white dwarf binary system, their trajectory happened to be just far enough away from the black holes that they weren't sucked in. Instead, they followed a gravitational trajectory and were flung out of the galaxy, zooming through deep space at an enormous speed. Because the black holes pushed these two white dwarfs closer together, it also shortened the time it took for them to go supernova. 

"You have two dancing partners, they do-si-do, and one pair gets flung away," Foley summed up. "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. It's a complicated chain of events, but it turns out that it's actually a very logical path to this weird phenomenon of hypervelocity supernovas."

This complicated process is explained wonderfully in the infographic below. 

1. Two galaxies merge. 2. Two white dwarfs are in orbit around each other. 3. The gravitational strength of the merged galaxy's black holes simulatneously pushes the binary white dwarf system together and out of the galaxy. 4. The white dwarfs spiral closer to each other. 5. They are now close enough together for one white dwarf to suck the matter off the other. 6. There is a supernova, isolated in deep space, like the one that Foley saw. NASA/ESA/P. Jeffries and A. Feild/STScI.

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