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Black holes seem to defy our comprehension and be contrary to conventional understanding. So perhaps it is not entirely surprising to find that supermassive black holes which have a retrograde or backwards spin might be more powerful and produce more ferocious jets of gas. While this new finding goes against what astronomers had thought for decades, it also helps solve a mystery why some black holes have no jets at all.
Powerful jets stream out from the accretion disks that spin around many supermassive black holes. The black holes can spin either in the same direction as the disks, called prograde black holes, or against the flow – the retrograde black holes. For decades, astronomers thought that the faster the spin of the black hole, the more powerful the jet. But there were problems with this “spin paradigm” model. For example, some prograde black holes had been found with no jets.
Theoretical astrophysicist David Garofalo and his colleagues have been studying the motion of black holes for years, and in previous papers, they proposed that the backward, or retrograde, black holes spew the most powerful jets, while the prograde black holes have weaker or no jets.
Their new study links their theory with observations of galaxies across time, or at varying distances from Earth. They looked at both “radio-loud” galaxies with jets, and “radio-quiet” ones with weak or no jets. The term “radio” comes from the fact that these particular jets shoot out beams of light mostly in the form of radio waves.
The results showed that more distant radio-loud galaxies are powered by retrograde black holes, while relatively closer radio-quiet objects have prograde black holes. According to the team, the supermassive black holes evolve over time from a retrograde to a prograde state.
“This new model also solves a paradox in the old spin paradigm,” said David Meier, a theoretical astrophysicist at JPL not involved in the study. “Everything now fits nicely into place.”
The scientists say that the backward black holes shoot more powerful jets because there’s more space between the black hole and the inner edge of the orbiting disk. This gap provides more room for the build-up of magnetic fields, which fuel the jets, an idea known as the Reynold’s conjecture after the theoretical astrophysicist Chris Reynolds of the University of Maryland, College Park.
“If you picture yourself trying to get closer to a fan, you can imagine that moving in the same rotational direction as the fan would make things easier,” said Garofalo. “The same principle applies to these black holes. The material orbiting around them in a disk will get closer to the ones that are spinning in the same direction versus the ones spinning the opposite way.”
Jets and winds play key roles in shaping the fate of galaxies. Some research shows that jets can slow and even prevent the formation of stars not just in a host galaxy itself, but also in other nearby galaxies.
“Jets transport huge amounts of energy to the outskirts of galaxies, displace large volumes of the intergalactic gas, and act as feedback agents between the galaxy’s very center and the large-scale environment,” said team member Rita M. Sambruna, from Goddard Space Flight Center. “Understanding their origin is of paramount interest in modern astrophysics.”
The team’s paper was published in the May 27 Monthly Notices of the Royal Astronomical Society.
Source: JPL
From nonsense to sense, no?
But now the question arise, why would a SMBH be born (develop to be) retrograde? As always, an answer spawns off more questions.
“why would a SMBH be born (develop to be) retrograde?”
Maybe when 2 SMBH collide and merge as one?
If SMBH’s are made by growing MBH’s fed by merging galaxies (maybe each providing a MBH) then it seems natural that the orientations of many black holes are pretty random in terms of the resulting larger galaxy’s orientation.
Thanks Olaf, Excalibur!
I’ll buy that. It means that the sum SMBH grows more by merger than accretion in between (at least as regards angular momentum), which I should have realized in the first place.
Universe – a place that will give you plenty of knocks.
Actually a few microseconds I also did not buy into a reverse accretion dish, but I happen to be viewing you-tube videos predicting how the milky-way would die and happen to see 2 colliding black holes (creating shock-waves of radiation frying Earth when they merge).
You can think of it this way. A rotating gravity field has a Lense-Thirring effect, or frame dragging. This is in some ways similar to a magnetic field with charge, but here it is a gravi-magnetic field acting on charge. A rotating black hole generates this field, and if it is the opposite direction as the rotating matter in the disk the field acts a bit like a repeller.
LC
Oops, I wrote “but here it is a gravi-magnetic field acting on charge.” I meant to write, “but here it is a gravi-magnetic field acting on mass.”
LC
Given (1) retrograde black holes are younger
and (2) the jet of a retrograde black hole is a form of galactic recycling
then a retrograde black hole is, in a few senses, a green black hole. 😉
TerryG, That is very likely the case. The retrograde configuration is not stable, and will evolve into the prograde state. Well of course the final state is that the black hole consumes everything and the rest is ejected away.
LC
I take the retrograde/prograde part as being the BH’s rotation with respect to the accretion disk, wich does not necessarily need to be the same as the overall galaxies rotation (although in many cases it probably is), and that we also need to consider ’tilt’ as a factor.
Every successive merger have the potential to ’tilt’ the accretion disk somewhat due to the infall angle, or in the case of large infalling amount of gas/dust that the new accretion disk ends up retrograde when the old one was prograde.
If the SMBH has an angular momentum J and the rest of the stuff an angular momentum -J’ then after the “rest of the stuff” has been absorbed by the black hole the net angular momentum is J – J’. A tilt on the angle of the accretion disk relative to the angular momentum of the black hole turns this problem into a vector problem. This is an approximate nonrelativistic way to think of it.
A rotating black hole has a region called the ergosphere where any matter entering this region is forced into a rotational motion comoving with the black hole. There is then an interaction via gravity in the process of a particle entering a rotating black hole which can reduce the angular momentum of the BH.
LC