Artistic rendering of an extremely ancient quasar.
At the core of every galaxy is a supermassive black hole (SMBH). These behemoths are many times the size of regular black holes. And unlike your massive stellar black hole, supermassive black holes did not form from a collapsing star; they formed before… well, we don’t actually know how they formed. But we do know how big they are. An ordinary black hole can have about five to several tens of times the mass of the sun (solar masses), where the SMBH of our own galaxy has about four million solar masses.
Although we do not yet know the mechanism by which SMBHs are formed, the prevailing thought is that they formed (relatively) small, with “only” 100 to 100,000 solar masses. They would then have gained mass over time as they gobbled up matter, eventually growing into the giants we see today.
A new SMBH has now been discovered with about twelve billion solar masses. That in itself is not unprecedented; others have been discovered with about the same mass. The amazing thing about the new discovery is the extreme distance of the SMBH – about 12.8 billion light-years from Earth – and thus how quickly it formed after the Big Bang.
A long time ago…
Because it’s so far away, the light generated by the SMBH takes a long time to get here — so long that the light we see now was emitted when the universe was only 875 million years old, six percent of the current age of 13.8 billion years. years. We see the black hole as it was then. The new SMBH is a quasar, an extremely luminous type of SMBH. The idea that black holes emit light may seem counterintuitive, but the light does not come from the black hole itself; rather, it is produced by friction in the black hole’s accretion disk, a disk of infalling matter. The more matter in the disc, the more friction and therefore more light. Rays of energetic particles emanating from the poles of the black hole also generate light.
It is the most massive SMBH yet discovered in the past, which is one of the reasons why the discovery is so surprising. Black holes take time to get that big, usually quite a bit of time. And since it had already grown to its full twelve billion solar mass in the first 875 million years of the universe’s history, it must have consumed matter at an astonishing rate.
A graph of the brightness of distant quasars versus their masses. The newly discovered quasar is the large red dot at the upper right. The graph is transposed over a beautiful view of the Milky Way, with the Yunnan observatories in the foreground.
The researchers calculate that it must have absorbed the matter at its maximum speed, Eddington’s limit. The Eddington limit exists because the faster a black hole gobbles up matter, the more friction — and therefore more light — is produced in the accretion disk. The extra light beams outward, exerting pressure on the falling matter and slowing it down. (As counterintuitive as it may seem, light actually exerts pressure on objects; given enough light, this can produce significant force). As the consumption rate of the black hole increases, the amount of radiation increases, which in turn slows the rate of consumption. And when it reaches the Eddington limit, the radiation pressure prevents it from accelerating further.
But if the SMBH consumed at the Eddington limit throughout its existence, the accretion should have stopped. It was thought that the intense radiation produced by such a massive object radiating at its Eddington limit — releasing the maximum amount of radiation it can produce that way — should be enough to expel any nearby matter. That clearly did not happen.
…In a (massive) galaxy far, far away
By studying nearby galaxies, researchers have learned that there is a relationship between the mass of an SMBH and the mass of its host galaxy. Typically, an SMBH makes up a small percentage of its host galaxy’s mass, ranging from about 0.14 percent to 0.5 percent. If this relationship holds true in the early universe, the host galaxy of the new SMBH should have as many as four to nine trillion solar masses in stars. (Not to mention the dark matter component, which is by far the most massive part of any galaxy). Galaxies of this mass are not unheard of, as they exist in the current Universe. But if the galaxy does indeed exist in the predicted mass range, it will be the first to be discovered in that era.
Studying such a huge galaxy would provide clues about how galaxies grew in the early universe. One of the important unknowns in galaxy formation is the role played by the galaxy’s SMBH. Why is there a correlation between the mass of the black hole and that of the galaxy? What kind of relationship exists between the accretion of the SMBH and the formation of stars?
Such questions about the role of an SMBH in its host system have puzzled researchers for some time – while there are some compelling models, there is no consensus on which one is correct. By studying the new SMBH and its host system, we could begin to find those answers. “This quasar is a unique laboratory to study the way a quasar’s black hole and its galaxy evolve together,” says Yuri Beletsky, one of the paper’s authors. “Our findings indicate that quasar black holes probably grew faster than their host galaxies in the early Universe, although more research is needed to confirm this idea.”
To go boldly
The light from the quasar could also be used in other ways to learn more about the early universe. On the one hand, its extreme brightness allows researchers to explore the intergalactic medium like never before.
The intergalactic medium is a thin distribution of gas and dust between galaxies, containing hydrogen, helium, and various metals (in astrophysical terms, all elements above helium are known as “metals”). The light from the quasar has to travel through a lot of space to get here, including the intergalactic medium in its vicinity.
When light passes through a gas, some wavelengths of light penetrate the gas better than others, while some elements block specific wavelengths. So by studying an object’s spectrum and seeing which wavelengths are missing from the spectrum, researchers can learn more about the contents of the gas. However, the process is complicated, especially over such great distances. With less light, it is more difficult to distinguish these gaps or lines in the spectrum. The brightness of the quasar will therefore provide a much clearer measurement of the intergalactic medium.
And since the metals in the intergalactic medium were produced by fusion in the cores of stars, better measurements of these elements could help researchers learn more about the star-forming processes going on in the early Universe.
The next generation of telescopes will reveal more. Not only will they be able to study this SMBH further, they will likely be able to discover more massive objects like this in the very distant early Universe.
Nature2015. DOI: DOI: 10.1038/nature14241 (About DOIs)