Galaxies Use "Seat Belts" Too

Some astronomers used computer simulations to prove that in a galaxy collision, the net momentum carried by the radiation produced by the merger of the two central black holes gives the remnant black hole a large kick in the opposite direction, and that would make it recoil at speed up to ten million miles per hour, which is actually fast enough to traverse an entire galaxy in a cosmically short time of only ten million years.

Now, another team of astronomers think that the fact that no such evidence of "empty nest" galaxies missing their central black holes has ever been found may indicate that most galaxies are equipped with "seat belts" that keep their colossal black holes from flying out into space after violent collisions.

Erin Bonning of the Paris Observatory in France and her colleagues looked at 2600 observations of potential fugitive black holes made by the Sloan Digital Sky Survey, but came up empty-handed. She concluded that there must be a mechanism that prevents the black holes from getting kicked out of their host galaxies.

Tamara Bogdanovic and Christopher Reynolds of the University of Maryland in College Park, US, might have an explanation for the phenomenon, in the form of a galactic "seat belt" that restrains the black holes by minimizing the strength of the birth kick.

It seems that the actual kick that should propel the black hole actually depends on how the spin of the black hole is, so if the spin axes of the two merging central black holes are in the same plane as their orbit about each other, the merger gives them a strong kick. If the spin axes are perpendicular to the orbit, the kick is weak.

"What we believe we've identified is a surprisingly powerful mechanism that can take black holes in [one] configuration and twist them around," says Reynolds.

However, scientists did not completely rule out the possibility that some black holes get evicted from their galaxies, they just said the reason why they don't see this happening is the low probability of such an event escaping the "seat belt" restraint.