A team of international researchers, led by the University of Cambridge, has discovered what is, to date, the oldest black hole ever observed. The object, with a mass 1.6 million times greater than the Sun, emitted the light that allowed its detection 13.4 billion light years ago, which implies that it was active, devouring matter, only 400 million years after the Big Bang.
The fact that a black hole could have reached so much mass at such an early time in the universe, and the realization that the rate at which it feeds on the surrounding material is much higher than that predicted by theoretical models, may mean, in the words of Roberto Maiolino, principal investigator of the team of scientists that carried out the study, a “giant leap forward” in the knowledge we have about the formation and development of the first black holes that existed in the cosmos.
The discovery was made possible thanks to the unique observational capabilities of the James Webb Space Telescope and has been published in the journal Nature.
The discovery was made from the study of light coming from an extraordinarily distant (and distant in the past) galaxy called GN-z11. The detailed analysis of the spectrum of this light (the decomposition of radiation obtained with a prism) shows the typical structure of the so-called active galaxies, objects that contain, in their center, black holes towards which enormous amounts of matter fall.
Although, by definition, light cannot escape from a black hole once it is inside, the material that is attracted by the object’s gravity swirls around the outside, forming disks that fall with a spiral movement similar to that observed. in a drain (these disks are called accretion disks). The friction generated during the process causes the matter in the disk to heat up to millions of degrees and emit radiation that allows our instruments to infer the presence of black holes in the centers of galaxies.
The spectral analysis also provides information about the age and size of the discovered black hole. On the one hand, the expansion of the universe affects the light of distant objects, stretching its wavelength, an effect that in cosmology is known as redshift and that allows us to estimate the time during which the light has been touring space since it aired.
The light from the galaxy GN-z11 has a redshift that places it in the remote past, about 13.4 billion years ago. This means that the radiation was emitted (by the activity of the black hole inside) when the cosmos was just 400 million years old.
On the other hand, the light spectrum contains information that astronomers can relate to the mass of the black hole. In this specific case, researchers have estimated a mass of 1.6 million suns, which is surprising considering the age of the object.
Science knows two main types of black holes. The so-called stars are the result of the gravitational collapse suffered by stars much more massive than the Sun in the last moment of their life, when they have exhausted the nuclear fuel that supports their weight. These black holes usually have masses on the order of tens that of our star.
So-called supermassive black holes, on the other hand, have masses of millions of suns (and, in some cases, billions). But unlike stellar black holes, the origin of supermassive black holes is not clear, and there are two major theoretical models that could explain their enormous mass.
One of these models is based on the gradual growth of stellar-mass black holes as they devour matter. This process could require long periods of time for the black hole to reach the large masses typical of these objects.
The second model predicts much faster formations, based on the direct collapse of gigantic concentrations of gas in the early universe. According to this hypothesis, these black holes would already be born with a great mass.
In principle, the discovery of the black hole inside the galaxy GN-z11 seems to favor this second theoretical scheme, since it is a massive object that existed in very early times of the universe, apparently without enough time for its mass to be easily explained through a process of gradual growth.
Analysis of the light received by the Webb telescope also allows us to calculate the rate at which matter falls into the black hole. And to the researchers’ surprise, the results suggest that this rate is up to five times higher than what theoretical patterns allow.
The radiation emitted by the material that forms the accretion disk around a black hole exerts an outward pressure that opposes the fall of new matter. That is, the activity of the black hole itself ends up regulating the speed with which the object is able to grow. According to this concept, current astrophysical models have established a limit, called Eddington, that predicts the maximum rate at which a black hole can feed.
The new discovery challenges this model, with its cadence five times greater than that dictated by the Eddington limit, and opens the door to the possibility that supermassive black holes could grow much faster than expected. And this would return us to square one, making it possible that, even with a short lifespan of the universe, these objects can develop progressively, rescuing, in this way, the first formation model for supermassive black holes.
In fact, the authors of the study have estimated that, maintaining the observed growth rate, the black hole of the galaxy GN-z11 could have needed only 100 million years to reach the detected mass.
This discovery, which could revolutionize the field of study of supermassive black holes, has been possible thanks to the extraordinary power of the James Webb space telescope, which is capable of capturing light in the infrared range. This allows you to observe very old objects, the radiation from which has undergone a large red shift (to the point that its light has completely shifted towards infrared wavelengths).
“It is a new era: a giant leap in sensitivity, especially in the infrared” declared Roberto Maiolino. For the researcher, the revolution that the Webb telescope is making possible with its findings is equivalent to “converting, overnight, Galileo’s original telescope into a modern instrument.”
