Outcast supernovae that explode all alone in space present a scientific mystery. Prof Ryan Foley from the University of Illinois has developed a theory for where these doomed stars come from and how they arrived at their current homes.
These Hubble images show elliptical galaxies with dark, wispy dust lanes, the signature of a recent galaxy merger. The dust is the only relic of a smaller galaxy that was consumed by the larger elliptical galaxy. The X in the images marks the location of supernova explosions that are associated with the galaxies. Each supernova may have been gravitationally kicked out of its host galaxy by a pair of central supermassive black holes. When two galaxies merge, so do their supermassive black holes. Prof Foley suggests the supernovae were stars that were once part of double-star systems. These systems wandered too close to the binary black holes, which ejected them from their galaxies. Eventually, the stars in each system moved close enough together to trigger a supernova blast. These outcast supernovae are located at various distances from their home galaxies. SN 2000ds (left) is at least 12,000 light-years from its galaxy, NGC 2768; SN 2005cz (right) is at least 7,000 light-years from its galaxy, NGC 4589. NGC 2768 resides 75 million light-years from Earth, and NGC 4589 is 108 million light-years away. Image credit: NASA / ESA / R. Foley, University of Illinois.
Prof Foley traced 13 high-velocity exploding stars back to the galaxies they came from to find the peculiar combination of events leading to the stars’ lonely explosions.
“Looking around where these supernovae exploded, there’s nothing there – no trace of star formation, no clusters of old stars, there’s nothing nearby. So I knew that these things were starting somewhere else and moving long distances before they explode,” said Prof Foley, who is an author on the study published in the Monthly Notices of the Royal Astronomical Society.
Examining the locations and kinematics of calcium-rich supernovae, he was able to determine that the stars that exploded had been kicked out of their galaxies at very high speeds, millions of years before they exploded. To understand how they got so far from their galaxies – up to half a million light years away – moving at such high speeds, he looked at the galaxies that had produced the stars before ejecting them.
First, Prof Foley noticed that many of the galaxies were composed only of old stars, which meant that the calcium-rich supernovae had to come from a population of older stars such as white dwarfs. Most stars become white dwarfs after they stop producing new energy.
In order to produce the kind of explosions observed, a white dwarf has to drain mass from a companion star. In this case, the two stars are in a binary system where the pair circle one other until tidal forces rip one apart. That material is dumped on the other star, which causes an explosion.
Thousands of such supernovae have been found within galaxies, but how did these odd cases end up on solo hypervelocity flights through space?
Looking more closely, Prof Foley then noticed that all of the galaxies that had produced the runaway supernovae showed signs of merging – two galaxies colliding and rearranging into one big galaxy. That is when all the puzzle pieces fell together for the astronomer.
“The velocities were incredible, on the order of 4.5 million miles per hour. There is only one way to get a binary star system moving that fast: a slingshot from a close flyby of a binary supermassive black hole. How do you get a binary supermassive black hole? Merge two galaxies,” Prof Foley said.
This illustration offers a plausible scenario for how vagabond stars exploded as supernovae outside the cozy confines of galaxies: (i) a pair of black holes comes together during a galaxy merger, dragging with them up to a million stars each; (ii) a double-star system wanders too close to the two black holes; (iii) the black holes then gravitationally catapult the stars out of the galaxy. At the same time, the stars are brought closer together; (iv) after getting booted out of the galaxy, the binary stars move even closer together as orbital energy is carried away from the duo in the form of gravitational waves; (v) eventually, the stars get close enough that one of them is ripped apart by tidal forces; (vi) as material from the dead star is quickly dumped onto the surviving star, a supernova occurs. Image credit: NASA / ESA / P. Jeffries & A. Field, STScI.
According to Prof Foley’s scenario, after two galaxies merge, their black holes migrate to the center of the new galaxy, each with a trailing a cluster of stars.
As the black holes dance around each other, slowly getting closer, one of the binary stars in the black holes’ entourage may wander too close to the other black hole. Many of these stars will be flung far away, and those ejected stars in surviving binary systems will orbit even closer after the encounter, which speeds up the merger.
“With a single black hole, occasionally a star will wander too close to it and have an extreme interaction. With two black holes, there are two reservoirs of stars being dragged close to another black hole. This dramatically increases the likelihood that a star is ejected,” Prof Foley said.
“While the black hole at the center of the Milky Way may eject about one star a century, a binary supermassive black hole may kick out 100 stars a year,” he added.
After getting booted out of the galaxy, the binary stars move closer together as their orbits continue to accelerate which speeds up the binary stars’ aging process. The binary stars are likely both white dwarfs, which are burned out relics of stars. Eventually, the white dwarfs get close enough that one is ripped apart by tidal forces. As material from the dead star is quickly dumped onto the surviving star, an explosion occurs, causing the supernova.
The time it takes for one of these ejected stars to explode is relatively short, about 50 million years. Normally, these kinds of binary stars take a long time to merge, probably much longer than the age of the Universe, which is more than 13 billion years.
Prof Foley hopes that in the future, these types of supernovae can be used to find more binary supermassive black hole systems, which themselves are rare and interesting phenomena that could give insight into gravity, general and special relativity, quasars, dark energy and other mysteries of astronomy and physics.
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Ryan J. Foley. 2015. Kinematics and host-galaxy properties suggest a nuclear origin for calcium-rich supernova progenitors. MNRAS 452 (3): 2463-2478; doi: 10.1093/mnras/stv789
