On the simplest level, there are only two types of supernovae: those caused when a massive star collapses and those caused when white dwarf stars reach a critical mass. But the better we're able to image supernovae, the more we're finding out that there are distinct subtypes within each of these classes. Or, as the author of a new paper put it, "It turns out that there were about a dozen new classes of ‘exotic’ or ‘peculiar’ transients lurking in the shadows."
Figuring out the physics that creates these peculiarities can be a significant challenge. But the paper provides a possible explanation for something called a "calcium rich supernova." And the explanation involves a binary star system being hurled from its galaxy by a pair of supermassive black holes.
The first instance of these supernovae was recorded in 2005, and while they're rare, they still occur at about five percent the rate of the more common type Ia supernovae. As more of these events have been identified, a strange pattern became clear: they tend to occur far from a galaxy—on the order of 100,000 light years away or more. This led to the suggestion that they were occurring in dwarf galaxies or globular clusters, which would be hard to spot at great distances.
But the author, University of Illinois' Ryan Foley, argues that there's an alternative explanation that fits the data a bit better. He finds a pattern where the further the supernova from the nearest galaxy, the slower it (or more properly, its debris) is moving toward us. The ones that appear closest to a possible source galaxy are moving toward us at velocities of up to 1,700 kilometers a second. That means that either the explosions themselves were asymmetric or the progenitor was moving toward us at some pretty extreme speeds—higher than the escape velocity of a galaxy.
The ones that appear to be furthest from the galaxy, in contrast, appear not to be moving much relative to us.
Foley argues for the explosion of a high-velocity star, ejected from its former host galaxy. If it's ejected perpendicular to our line of sight, it will appear to travel far before exploding, and the explosion won't appear to be shifted in our direction. If it's ejected toward us, it won't appear to travel far from the galaxy since it's moving along the line of sight. But the explosion debris will appear to be moving rapidly.
This implies that there's another class that's ejected away from us, where the explosion debris would appear to be red shifted because it's moving away from us. But these instances would have a galaxy sitting between the explosion and Earth (the one they were ejected from), so we're unlikely to see them.
There's just one problem with this model. The stars that are exploding appear to be white dwarfs, which only explode following mergers or by drawing matter off a companion. Both of these scenarios require a companion star And, if a star has been ejected from its galaxy, it shouldn't have a companion. The physics of ejection requires that a binary system interact with the supermassive black hole at the center of the galaxy. One of the stars gets trapped in orbit around the black hole while the other is shot out into intergalactic space.
While this would seem to be bad news for Foley's model, he has an answer for it. The nearby galaxies generally show signs of having been disrupted by mergers or exist in clusters where mergers should be common. If this is the case, there are two supermassive black holes around. The two of them can now provide the gravitational interactions needed to send both stars of a binary system out of the galaxy.
Foley has put together a pretty persuasive argument, one that fits most of the data available. If it holds up as more of these explosions are observed, then we may have one less peculiarity to try to explain.