Sergio Prostak, writing at Sci-News:
Astronomers want to know the black hole’s spin for several reasons. The first is physical – only two numbers define a black hole: mass and spin. By learning those two numbers, you learn everything there is to know about the black hole. Most importantly, the black hole’s spin gives clues to its past and by extension the evolution of its host galaxy.
“The black hole’s spin is a memory, a record, of the past history of the galaxy as a whole.”
According to the team, the supermassive black hole in NGC 1365 spins so fast that its surface is traveling at nearly the speed of light.