The Spins of Black Holes in Galactic Nuclei
Thursday, February 7, 2013
Science Update - A look at CfA discoveries from recent journals

Supermasssive black holes, having masses of millions or even billions of
suns, are found at the nuclei of galaxies. In dramatic cases like quasars, these black holes are responsible for spectacular phenomena like the ejection of narrow jets of particles at nearly the speed of light. These outflows, discovered at radio wavelengths, are thought to
be driven by matter accreting onto a hot disk around the black hole. A black hole, although thought to be point-like in dimension, is surrounded by an imaginary surface, or "edge," of finite size (its
"event horizon") within which anything that ventures becomes lost forever to the rest of the universe. But black holes can be ringed by disks of matter, and when infalling material interacts with the disk,
radiation and matter can be ejected.

A black hole is so simple that it can be completely described by only three parameters: its mass, its spin, and its electric charge, but determining these values is not simple. The charge is usually considered to be negligibly small and neglected in modeling. The mass can be found when the black hole has orbiting material or companions from the
periodic orbital motions. The spin is derived from modeling the shape of emission lines, for example from iron, a very abundant species. The atoms rotate in the disk around the black hole, some close to the minimum possible orbit before being captured; the rotation of the disk
measured by these atoms in turn reflects the rotation of the black hole itself.

CfA astronomer E. Nardini and four colleagues used the Japanese Suzaku
X-ray satellite to complete a unique, systematic study of twenty-five galaxies with active nuclei whose black holes are vigorously accreting material and heating their surroundings. The scientists chose a sample of galaxies seen face-on with little or no dust along the line-of-sight to the nucleus that could diminish the radiation and thereby complicate the analysis. The astronomers modeled both the X-ray emission from the continuum and from hot atoms in these galaxies, and report two significant conclusions. First, most of the supermassive black holes are indeed rapidly rotating. As material accretes onto a black hole it can either increase the rotation (if it is an ordered and gradual process) or decrease the rotation (if the infall is more random), and therefore this conclusion implies that the feeding of nuclear black holes was a steady and ordered process. Secondly, in a related point, they conclude that the strength of the radio emission of a galactic nucleus is not directly related to the black hole spin.