Material cannot fall directly onto a black hole all at once. An active black hole has a disk or torus (doughnut) of gas and dust surrounding it which feeds the hungry black hole. Sometimes, not all of that material goes in and in an energetic process that is not yet fully understood, some of that material is blasted away from the black hole as high-speed, thin jets.
David Garofalo and his collaborators looked at the different types of active galaxies, those that have strong jets, those that have weak jets, and those that have no jets at all. Their model shows that black holes that are spinning in the opposite direction to the spinning disk around them can create powerful jets. These systems have a gap between the black hole and disk where powerful magnetic field can form which help drive the jets.
Some black holes spin in the same direction as their surrounding disks, and these do not produce jets. In addition, backwards spinning black holes will evolve over time and spin the other way, producing a weaker and weaker jet until it shuts off altogether.
This scenario may explain why only 10% of active galaxies have radio jets, and how active galaxies evolve over time. Since these powerful blasts of material significantly affect the surrounding galaxy, and even galaxy cluster in some cases, this work fits into the larger picture.
It also reminds me of how the vast array of different types of active galaxies seen in optical and radio were unified into one model with a supermassive black hole just a couple of decades ago. Maybe we are on the verge of another such shift in our thinking about these cool and powerful objects!
Above artist's concept shows a galaxy with a supermassive black hole at its core. The black hole is shooting out jets that are visible in the radio. Image credit: NASA/JPL-Caltech This research is available on Arxiv.