We usually think of black holes as anchors—gravitational sources so extreme that they wind up functioning as focal points for vast galaxies. Isolated, liberated black holes hurtling through the gaseous wreckage of a supernova at nearly 100 times the speed of sound? Not so much. But, according to astrophysicists writing in the Astrophysical Journal Letters, this is a likely explanation behind an astrophysical anomaly known just as the "bullet."
The Keio University-based astrophysicists behind the discovery observed the bullet in 2014 and 2015 using the Atacama Submillimeter Telescope, a 10 meter radio telescope located in northern Chile. The bullet lives at the edge of a massive expanding gas cloud serving as a shell surrounding the supernova remnant (SNR) W44. It's the energy-transfer processes of this SNR that the researchers were studying when they happened upon a feature with an unusually high density and temperature beyond what could be explained by the SNR alone.
Looking closer, the Keio researchers found that this mysterious something was both hauling ass and trailing a long, thin tendril of gas picked up from W44's shell. The bullet's extreme kinetic energy is a mystery. One possible explanation is just that a second supernova explanation occurred elsewhere within the W44 gas shell, which is indeed large enough to accommodate an additional such event.
Massive stars tend to form in groups, so it's not impossible. They calculated that the chance probability of a second supernova occurring is around 10^-3.2, or .00063. The Keio team apparently wasn't satisfied with those weak odds and sought other explanations. They came up with black hole two scenarios that could explain the phenomenon. Both involve anomalous cosmic violence.
In the first scenario, the "explosion model," the gas cloud surrounding W44 passes a static black hole (a "normal" but isolated black hole), which has the effect of abruptly pulling a bunch of gas into a relatively tiny space (owing to the black hole's extreme gravity). The violently condensed gas then explodes as W44 moves away from the then-activated black hole, spewing out the observed tendril of gas in the process. Here, the bullet is gas alone.
In the second scenario, the "shooting model" illustrated above, a high-velocity black hole that just happened to have been bouncing around the general vicinity of W44 just happened to have collided with the gas shell, resulting in the same trail of gas, only this time following in the wake of the interloping black hole.
It's weird to think about, but it's thought that stray black holes may be common in the Milky Way—a group of astronomers estimated in 2002 that the Milky Way may house nearly billion strays. Still, the Keio group thinks the explosion model is more plausible because we're actually observed phenomena best explained by rapidly contracting and then exploding gas: the Tornado nebula, and a more vague, recently discovered feature located near the center of our galaxy.