Is there a horizontal event horizon on a spinning black hole?

Question

It seems that most of the information one reads about black holes theorises that they are not spinning. The theories then describe all matter approaching a black hole as being absorbed by it. Wouldn't most black holes be more likely spinning at various rates? Then if they are spinning and the energy (dark matter/light etc?) that surrounds it is accelerated with the gravitational pull of the black hole, is there a point on the outer circumference that has a horizontal event horizon as well as a vertical one?

If then the black hole is centrally spinning fast enough, could the horizontal event horizon be further out than the vertical one and subsequently reject most matter before it could be absorbed by the vertical one? If this is so, could then our milky way black hole be just one of these with the matter in the legs of the galaxy being matter that has been rejected at the horizontal event horizon?

Answer 1

A stellar mass black hole usually forms during a core collapse supernova of a very massive star. Our understanding of star formation is that most stars will have some angular momentum, and some of this angular momentum will be passed on to the black hole which is produced when the core collapses. This means that, as you mention, most black holes will be spinning at various rates.

However, there is no "horizontal" horizon. The space that surrounds a rotating black hole is dragged in the direction of the rotation, an effect known as frame-dragging. There is even a region close to the black hole where it is impossible to stand still with respect to a distant object, because the space around you is being dragged faster than the speed of light (so that locally, you would be moving backwards faster than the speed of light). This region is known as the ergosphere, and is the closest analogy to the "horizontal horizon" you mention in your question.

The size of this region grows as the spin of the black hole increases. Matter spirals into the black hole in a process known as accretion, and the black hole can gain angular momentum from the infalling matter. There is a limit to how fast a black hole can spin, however; once the spin becomes fast enough, any infalling matter is moving backwards relative to the space around the black hole, which is being dragged along by the spin of the black hole. No matter how close to this limit you get, though, the black hole does not have any trouble with rejecting matter. Indeed, the closer a black hole is to its spin limit, the closer matter can get before it falls into the black hole (in technical terms, the radius of the "innermost stable circular orbit" decreases as the black hole's spin increases).