An international team of astrophysicists have used the James Webb Space Telescope (JWST) to study one of the most massive and distant black holesat a distance of 13 billion light-years, when the universe was about 800 million years old. Surprisingly, the black hole feeds in the same way as current black holes in our nearby cosmic environment.
Astrophysicists have been trying to explain how these black holes early in the universe gain their extraordinary mass. The new results, published in the journal Nature Astronomyrule out the existence of exotic mechanisms proposed as a possible solution.
Study rules out exotic mechanisms to explain how supermassive black holes early in the universe gained their extraordinary mass
The first billion years of cosmic history pose a challenge: the first known black holes at the centers of galaxies have surprisingly large masses. How did they become so massive and so fast? These new observations provide strong evidence against some proposed explanations, in particular against an extremely effective feeding mode for increasing the mass of the first massive black holes.
Stars and galaxies have changed enormously over the last 13.8 billion years, the life of the universe. Galaxies have grown and gained more mass, either by consuming surrounding gas or (occasionally) merging with each other. Astronomers have long assumed that supermassive black holes at galactic centers would have gradually grown along with the galaxies themselves.
Black hole, AGN and quasar
But the growth of black holes cannot be arbitrarily fast. The matter that falls on them forms a bright, hot, rotating “accretion disk.” When this happens around a supermassive black holethe result is a active galactic nucleus (AGNfor its acronym in English), from which large amounts of energy are released from the accretion of that gas and dust on the central hole.
The most luminous AGN, known as quasars (powerful sources of radiation), are among the brightest astronomical objects in the entire cosmos. But that brightness limits the amount of matter that can fall onto the black hole: the light exerts pressure that can prevent additional matter from falling.
That is why astronomers were surprised when, over the past twenty years, observations of distant quasars revealed very young black holes that had nevertheless reached masses of up to 10 billion solar masses. Light takes time to travel from a distant object to us, so looking at distant objects means looking into the distant past.
Cosmic Dawn Quasars
We see the most distant known quasars as they were in an era known as “cosmic dawn,” less than a billion years after the Big Bang, when the first stars and galaxies were formed. Explaining these early massive black holes is a considerable challenge to current models of galaxy evolution.
Could it be that early black holes were much more efficient at accreting gas than their modern counterparts? Or could the presence of dust affect quasar mass estimates in a way that caused researchers to overestimate the masses of early black holes? There is numerous proposed explanations at this time, but none are widely accepted.
Deciding which explanation is correct requires a more complete study of quasars than has been available until now. With the arrival of the JWST space telescope, specifically the MIRI mid-infrared instrument, astronomers’ ability to study distant quasars took a giant leap.
He MIRI instrument It was built by an international consortium with the participation of scientists and engineers from the Higher Council for Scientific Research (CSIC) and the National Institute of Aerospace Technology (INTA). In exchange for building the instrument, the consortium received a certain amount of observation time. In 2019, years before Webb’s launch, the European MIRI consortium decided to use some of this time to observe what was then the most distant quasar known, an object bearing the designation J1120+0641.
The study was carried out with Webb’s MIRI mid-infrared instrument and focused on the quasar J1120+0641, just 770 million years after the Big Bang.
The redshift (z) of a light source helps astronomers deduce its distance and age. «To date there are nine confirmed quasars at redshifts of more than 7, and J1120 was the first to be detected above (z=7.08), but there are currently three that are further away, at shifts between 7. .51 and 7.62 (about 700 million years from the beginning of the Big Bang)”, explains one of the authors, Luis Colinafrom the Astrobiology Center (CAB, CSIC-INTA).
Hill and Alvarez Marquez, also from the CAB, were in charge of designing the quasar data collection and its subsequent calibration, correcting the instrumental effects. The analysis of the observations fell on Sarah Bosmanpostdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) in close collaboration with Spanish scientists.
Spectrum at different wavelengths
The observations were carried out in January 2023, during the first cycle of JWST observations, and lasted approximately two and a half hours. They constitute the first mid-infrared study of a quasar in the cosmic dawn period, just 770 million years after the Big Bang (redshift z=7). The information does not come from an image, but from a spectrum: the rainbow-shaped decomposition of light from the object into components of different wavelengths.
A ‘bull’ that does not change
The general shape of the mid-infrared (continuous) spectrum encodes the properties of a big dust bull that surrounds the accretion disk in typical quasars. This torus helps guide matter toward the accretion disk, “feeding” the black hole. The bad news for those whose preferred solution to the first massive black holes lies in alternative modes of rapid growth: the torus, and by extension the feeding mechanism in this very early quasar, appears to be the same as that of its more modern counterparts.
The only difference is one that no model of rapid early growth of quasars predicted: a slightly higher powder temperature, about a hundred Kelvin warmer than the 1,300 K found for the hottest dust in less distant quasars. The shorter wavelength part of the spectrum, dominated by emissions from the accretion disk itself, shows that for us as distant observers, the quasar’s light is not dimmed by more dust than usual. Arguments that we may be overestimating the masses of the first black holes due to additional dust are also not the solution.
The quasar broad line region, where clumps of gas orbit the black hole at speeds close to the speed of light, allowing deductions about the mass of the black hole and the density and ionization of the surrounding matter, also appears normal. In almost all the properties that can be deduced from the spectrum, J1120+0641 does not differ from quasars of later epochs.
In almost all the properties that can be deduced from the spectrum, J1120+0641 does not differ from quasars of later epochs
“Overall, the new observations only add to the mystery: the early quasars are surprisingly normal. No matter what wavelengths we observe them at, quasars are almost identical at all times in the universe,” says Bosman. Not only the supermassive black holes themselves, but also their feeding mechanisms were apparently already fully “mature” when the cosmos was just 5% of its current age.
Primordial supermassive black holes
By ruling out a number of alternative solutions, the results strongly support the idea that supermassive black holes started out with considerable masses from the beginning, in astronomy jargon: that they are «primordial» or «large sown».
Supermassive black holes did not form from the remains of the first stars and then became massive very quickly. They must have formed early with initial masses of at least one hundred thousand solar masses, presumably through the collapse of huge early gas clouds, studies like this one reveal.
