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“Down in a hole and they’ve put all the stones in their place. I’ve eaten the sun so my tongue has been burned of the taste…” For the first time the evolution of the spin of the supermassive black holes has finally been examined. New research hints that supermassive black holes enlarged by swallowing matter will barely show spin, while those that merge with other black holes take on a rapid spin rate. Outta’ control? Let’s check the evidence.
Dr Alejo Martinez-Sansigre of the University of Portsmouth and Prof. Steve Rawlings of the University of Oxford made the new discovery by using radio, optical and X-ray data. Their findings were that giant black holes are – on the average – spinning faster than ever. With masses anywhere between a million and billion times that of the Sun, the net they weave isn’t visible to the eye – but the accretion disk is. The material becomes superheated, emitting X-rays detectable by space-telescopes. And, like great rock music, they emit some powerful radio waves able to be picked up by terrestrially based equipment.
But that’s not all these powerful babies kick up. We also know that twin jets are often associated with black holes and their accretion disks. The evolution of the jets can be caused by many factors, but now we’re beginning to associate spin rate with their formation as well. Through sampling radio observations Dr Martinez-Sansigre and Professor Rawlings were able to deduce the power of the jets and how they acquire material. From there, they could hypothesize how quickly these objects are spinning. These same observations provided data on black hole evolution. According to their research, the early Universe black holes had a much slower spin rate compared to the fraction of those found rapidly spinning in the present.
“The spin of black holes can tell you a lot about how they formed. Our results suggest that in recent times a large fraction of the most massive black holes have somehow spun up.” said Dr Martinez-Sansigre. “A likely explanation is that they have merged with other black holes of similar mass, which is a truly spectacular event, and the end product of this merger is a faster spinning black hole.”
Professor Rawlings adds: “Later this decade we hope to test our idea that these supermassive black holes have been set spinning relatively recently. Black hole mergers cause predictable distortions in space and time – so-called gravitational waves. With so many collisions, we expect there to be a cosmic background of gravitational waves, something that will change the timing of the pulses of radio waves that we detect from the remnants of massive stars known as pulsars.
Radio waves? You bet. “Down in a hole. Outta’ control…”
Alice In Chains lyrics quotation FTW! (Bonus points for avoiding “Supermassive Black Hole” by Muse, given the recent article saying that the galactic core should not have “superstars sucked into the supermassive”.)
(BTW, please fix the last line from “Down in hole”.)
done. “i’d like to fly… but my wings have been so denied…”
“Black Hole Sun” is by Soundgarden 🙂
Now just use that style for a formal abstract… or hell, just a preprint.
Also acceptable: Black Hole Sun.
It is funny that black holes would tend to spin up this way. The area of a black hole event horizon is A = 4?r^2 and r = m + sqrt(m^2 – J^2), where m is the gravitational mass m — > GM/c^2 and J^2 is the mass equivalent of the angular momentum. The entropy of a black hole is S = k A/L_p^2. L_p is the Planck mass. The crucial things is that the entropy of the black hole is larger for zero spin. As a rule the second law of thermodynamics has dS/dt >= 0, so the entropy increases. So these SMBHs are being spun up by the input of external matter with a significant angular momentum.
It is not clear to me how this would be accomplished by black hole mergers. Most often the gravity waves produced carry off angular momentum, where the gravity wave has some twisting structure.
LC
I agree. I do not see how this could work without great angular momentum (high speed relative motion) between the two black holes at just the right angle, speed and distance upon approach which seemingly would be the exception ration than the rule. Simply two black holes merging together would seemingly spin down both resulting in a more massive black hole with seemingly a slower spin rate for the resulting singular black hole relative to the background stars in the new galaxy, in more cases than it would spin up the rate of the combined black hole.
The Kerr solution describes a rotating black hole. This is something students work through in a graduate level course on general relativity. It is fairly involved and complicated. The mass of a black hole for the Schwarzschild solution defines the event horizon by the mass as r = 2GM/c^2. A rotating black hole has two event horizons. The outer horizon is at r_+ = m + sqrt{m^2 – J^2}. Beneath this is a spacelike region similar to the interior of a Schwarzschild black hole. There is then an inner event horizon defined at r_- = m – sqrt{m^2 – J^2}, beyond which is another timelike region which contains the singularity with a ring shaped geometry. For the angular momentum J = m in gravitational units r_+ = r_- and this is the extremal limit of a black hole.
The entropy of a black hole is determined by the area of the outer limit. This is
S = k A/4L_p.
For a rotating black hole of a given mass the horizon has its smallest area at the extremal limit, where r_+ = m < 2m = horizon radius at Schwarzschild limit for J = 0. The law of black hole thermodynamics is then
dm = 1/8? dA + ?dJ + ? dQ
for ? the rotational velocity of BH, ? the electrostatic potential and Q the charge of the black hole. This has some deep physics connected with string theory and the Regge trajectory
J = 0,
where the maximal spinning up is where this is zero, and a black hole entirely formed this way is at the extremal limit. Maximum entropy growth is where ?dA = 8??dm and J = 0. So if material is falling into a black hole with random directions, spins and so forth I would tend to expect the net angular momentum to be zero. If there is a rotating accretion disk from a system with some angular momentum you can then have a spin up.
LC
Doesn’t the speed of an in-falling black hole need to be greater than the spin rate of lets say the larger black hole and moving on close approach and on the same plane to increase spin rate? How often would this happen? It seems to me that spin down would be a much more common result. If the relative speed between the two were too great they most often would not ever merge and the galaxies would pass through each other both leaving remnants of each in the other galaxy, right?
I think it is unnecessary to think of a black holes as anything different from very massive highly condensed bodies somewhat like huge neutron stars without internal radiation. If there observational conclusion is that spin-up of the central black increases as a spiral galaxy ages, then I would expect that there is another reason for it other than black hole mergers or maybe not gravity in general. Any reason other than gravity would be theory shaking I think :).
I would like to contemplate other hypothetical scenarios that could spin up black holes.
regards, forrest
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LC,
Black hole entropy is S = k Ac^3/4G?.
I forgot to square the Planck length L_p = sqrt{G?/c^3}. This is the same as S = kA/4L_p^2
LC
I believe I see your point of this being unintuitive.
But wouldn’t your model equations and analysis be used to support the idea here? AFAIU accretion from the disk would be a small or insignificant source of anisotropic angular momentum going into your model equations here and below. Mergers OTOH would supply that plenty unless there was an unlikely head on merger.
I suppose I am not enough of an astrophysicist to think this through completely. An accretion disk that is stable will confer angular momentum bit by bit over a long period of time. SMBHs have enormous accretion disks as I understand and over many hundreds of millions of years it could grow not just the mass of the BH, but its angular momentum.
Maybe the limiting factor is that if the SMBH grows from such an accretion disk the ratio of angular momentum to mass = J/m of the supermassive black hole will not surpass that of the accretion disk. Maybe on the other hand black holes with an orbital angular momentum in orbit around the SMBH can transfer a much larger ?(J/m) to the SMBH than an accretion disk can.
LC
Surely the angular momentum comes from that of one BH around the other rather than that of the indvidual BHs. Two BH orbiting light years apart would slowly approach until they merge so the speed would rise as the moment reduced with radius.
I would have assumed that frame dragging would spin up the accretion disc at the expense of the BH’s angular momentum thus slowly reducing the spin rate. Some of that would be recovered as the material fell of the inside of the disc but that carried of in the jets would be lost from the system.
A rotating black hole with a Kerr metric can interact with material to
transfer angular momentum to the exterior. Material which is falling
inwards can bifurcate in the ergosphere in a manner so the escaping
material has absorbed angular momentum that has kinetic energy greater
than the mass-energy content which enters the black hole. On the other
hand if there is no escaping of some material back outwards then the
angular momentum of the accretion disk is absorbed by the black hole.
I am not enough of a specialist in this area of astrophysics to discuss
the conditions for either of these two situations. The “spinning up” of
a black hole requires that the black hole absorbs angular momentum from
the exterior world.
LC
Angular momentum is additive. So anything with a rotational angular
momentum or an orbital angular momentum will contribute their angular
momentum to the black hole. So everything which builds up the black hole
needs to have on average angular momenta which point in the same direction.
An accretion disk will do this. The same holds for any matter orbiting the
black hole in the same direction. So it is maybe not that crazy to think
the angular momentum of a black hole can increase. It is just that I find
it odd that it does so more through collisions with other black holes, for
in that case I would think their rotational angular momenta would be
randomly pointing. Further, black hole coalescense generates lots of
gravity waves which carry off angular momentum.
Of course the black hole can’t have angular momenta greater than its mass,
here in gravitational units. In that case the inner and outer horizons
cancel each other and the black hole is a naked singularity. For various
reasons this is not allowed. In effect a “time machine” has been generated
in the universe. So as a black hole absorbs angular momentum up to the
extremal limit J –> m more of this is radiated away in gravity waves.
LC
Again, I generally agree with you. Accordingly it seems to me that just as often or maybe more often merging black holes would cause the spin of the merged black holes to decrease or remain about the same rather than just increase as the article suggests. The question is not whether the angular momentum (rotation speed) can increase upon merger, we both agree that this is possible, the question concerning this article : is it reasonable to expect that the rotation speed after merging will more often be faster than either black hole was individually. I don’t believe it would happen that way the majority of the time.
I think the very nature of a massive black hole is to have some ridiculously high spin rate. Though not close to the speed of light, but in the vicinity. We know black holes force any surrounding material to revolve around it until it gets close enough to the event horizon and eventually “falls in”. But what if the surrounding material doesn’t just “fall in”, but instead, it pulled in close enough to the surface/event horizon to meet/match the spin rate and thus be affected by the tidal forces of the huge amount of gravity being exerted by the black hole. The material is then broken down where the resulting solid mass & light are contained, but x-ray, gamma burst energy is carried or “spun up” to the poles and emitted via the jet streams.
Since we know black holes emit these super-heated jets of gases & energy (x-ray, gamma, etc), should we look at the magnetic field surrounding the black hole has “carrier system” for the emitted jet materials. Essentially, scientists should be able to detect that these jet streams of materials have some sort of “spin rate” and would confirm that the black hole does rotate at some enormous rate of speed less than that of light.