Black Holes and their Spin

There are two basic characteristics that describe black holes: Mass and Spin. Mass determines the size of the event horizon, the gravitational mass, and many of the ways the black hole can gravitationally shred people, planets and just about anything else. Spin is related to the magnetic field (which can also shred people because of the magnetic properties of water), and it exerts many relativistic effects on its surrounding, such as frame dragging. Black hole spin also allows the black hole’s associated accretion disk to extend closer in toward the event horizon, creating a (with future higher resolution telescopes) a directly imaginable effect.

In a trio of spin related press releases, scientists described how to measure spin, the consequences spin has on how black holes merge, and results on a test to check if our understanding is wrong.

Measuring Spin: To measure the spin of the black hole, scientists start from the knowledge that Black Hole magnetic fields couple with accretion disks, and the inner accretion disk and the rotation of the black hole are also coupled via other, far more complex physics. By looking at spectra of accretion disks in active galaxies, it is possible to get at the disks’ rotation rates. According to soon-to-be Dr. Laura Brenneman (University of Maryland), they used the iron lines in disk spectra to measure disk rotation rates. To do this, they had to model how doppler effects broaden the line, and how relativistic effects alternatively enhance the blue and spread the red. They had to integrate across the entire width of the accretion disk to develop a model for the aggregate line profile as a function of spin rate. Applying this dissertation making profile to actual data, they found that super massive black holes spin at a variety of rates.

With this info, they looked at how spins effect black hole mergers. If two black holes try to merge with spins that are pointed north pole to north pole with the axis in the plane of the merger, than there is a ~10% or more probability that one of black holes will get ejected during the merger. The thing is, we never see these ejected black holes, so some mechanism must/should/could exist to torque the black holes such that they don’t have this ejection causing alignment. If instead, the black holes merged with their north poles both aligned perpendicular to their shrinking orbits with the N poles pointed in the same direction, then black holes wouldn’t be ejected. In looking for a way to get the black holes to align, Drs. Christopher Reynolds and Tamara Boganovic (University of Maryland), explained that it is possible for the fluid dynamic (drag / friction/ etc) and gravitational torques from the disk formed during the merger to flip the black holes and their disks, such that everything aligns in the same way, with all disks (the two accretion disks and the disk formed during the galaxy merger) all align with the black holes aligned perpendicular to the disk. Using advanced computer models, they demonstrated that the time scales to rotate the disks into a non-flinging alignment are short and should prevent all black holes in gas-rich galaxy-galaxy mergers from escaping. Problems solved.

Problem solved, sorta.

In gas poor mergers, there won’t be a central disk of gas and dust formed during the merger, so there won’t be anything to torque the disks. So, free ranging super massive black holes may be able to happen in 10% of the specifically aligned gas poor galaxy mergers (in other words, it might happen in very rare events).

Looking for ejections: Not one (or two) to trust theory, Drs. Erin Bronning (Observatoire de Paris) and Greg Shields (University of Texas) searched through 2600 Sloan spectra of mergers looking for signatures of a black hole being ejected. No ejected super massive black holes were found. Theory 1, Observations 0

Next on the agenda: The Sun and its Danger Zone (the Chromosphere).


  1. Star Stryder » Blog Archive » Deja Press Release: More on Black Hole Spin- by Pamela L. Gay - June 6, 2007

    […] Last week at the AAS there was a press conference focused on how super massive black holes interact during galaxy mergers and why/how black holes aren’t ejected during mergers. That press conference highlighted the work of University of Maryland researchers lead by Chris Reynolds that took into careful account the spin of the black holes and the angles between the merging black holes’ axes of rotation. Observational data mining by University of Texas researchers lead by Greg Shields was also discussed. […]

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