Astrophysics (index)about

retrograde accretion

(accretion disk spinning in opposite direction to the body)

Retrograde accretion is that of an accretion disk rotating in the opposite direction from the accreting body's rotation. Naturally, the rotation axes might not be parallel.

A theory is that central galaxy SMBH's are sometimes like this, (retrograde accreting black holes, i.e., with retrograde accretion disks), due to the black hole rotating in the opposite direction as the galaxy. Further, that such accretion can be a factor in generating extreme emission from some galaxies, a reason why active galactic nucleus's appear to fall into two classes: normal versus very bright. Such a situation is presumed to come about from galaxy mergers placing a SMBH in a galaxy rotating in the opposite direction.

The accretion will slow the SMBH's rotation, and sufficient time may have passed to reverse it. If the SMBH has doubled in mass over its lifetime, this is a definite possibility. This is one possible mechanism for an observed reduction in the number of quasars over time: that mergers were common once, and sufficient time has passed to reverse the rotation of many of the SMBH's.

(black holes,accretion)