The astronomer then turned his attention to the aging galaxies in the area of the speeding supernovae. Studying Hubble archival images, he confirmed that many are massive elliptical galaxies that were merging or had recently merged with other galaxies. The lanes are the shredded remnants of a cannibalized galaxy. Other observations provided circumstantial evidence for such encounters, showing that the cores of many of these galaxies had active supermassive black holes fueled by the collision. Many of the galaxies also reside in dense environments at the heart of galaxy clusters, a prime area for mergers. The telltale clue was strong dust lanes piercing through the centers of several of them.
The location of the supernovae in relation to ancient galaxies indicates that the original stars must have been old, too, Foley reasoned. And if the stars were old, then they must have had companions with them that provided enough material to trigger a supernova blast.
How does a double star system escape the boundaries of a galaxy?
Foley hypothesizes that a pair of supermassive black holes in the merging galaxies can provide the gravitational slingshot to rocket the binary stars into intergalactic space. Hubble observations reveal that nearly every galaxy has a massive black hole at its center. According to Foley's scenario, after two galaxies merge, their black holes migrate to the center of the new galaxy, each with a trailing 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," Foley said. "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." While the black hole at the center of the Milky Way may eject about one star per century, a binary supermassive black hole may kick out 100 stars each year.
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 the 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.