Looking back, it became clear that the Light from the galaxy called J1120+0641 took almost as long to reach Earth how long it took the universe to develop until today. It is inexplicable how the black hole at its center could then have weighed more than a billion solar masses, as shown by independent measurements.
Recent observations of the material in the vicinity of the black hole should have revealed a particularly efficient feeding mechanism, but they found nothing special. This result is even more extraordinary: it could mean that Astrophysicists understand less about the development of galaxies than they thought. And yet, they do not disappoint at all.
These findings are published in the journal Nature Astronomy.
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 get so massive so fast? The new observations described here provide strong evidence against some proposed explanations, in particular against an “ultra-efficient feeding mode” for early black holes.
The limits of growth of a supermassive black hole
The stars and galaxies have changed enormously over the last few 13.8 billion years, the life of the universe. Galaxies have grown and acquired more mass, either by consuming the surrounding gas or (occasionally) merging with each other. For a long time, Astronomers hypothesized that supermassive black holes at the centers of galaxies would have gradually grown along with the galaxies themselves..
But the growth of black holes cannot be arbitrarily fast. Matter falling onto a black hole forms a “accretion disk“bright, hot and spinning. When this happens around a supermassive black hole, the result is an active galactic nucleus. The brightest objects, known as quasarsare among the brightest astronomical objects in the entire cosmos. But that brightness limits the amount of matter that can fall onto the black hole: light exerts a pressure that can prevent additional matter from falling in.
How did black holes become so massive and so fast?
That’s why astronomers were surprised when, over the last twenty years, observations of distant quasars revealed very young black holes which, however, had reached masses up to 10 billion solar masses . Light needs time to travel from a distant object to us, so looking at distant objects means looking into the distant past. 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 formed.
Explaining those first massive black holes is a challenge considerable for 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 are numerous explanations proposed at this time, but none are widely accepted.
A closer look at the early growth of black holes
Deciding which explanation (if any) is correct requires a more complete picture of quasars than had previously been available. With the arrival of JWST space telescope, specifically the telescope’s MIRI mid-infrared instrument, astronomers’ ability to study distant quasars took a giant leap. To measure spectra of distant quasars, MIRI is 4,000 times more sensitive than any previous instrument.
Instruments like MIRI are built by international consortia, where scientists, engineers and technicians work closely together. Naturally, a consortium is very interested in checking whether its instrument works as well as intended.
In exchange for building the instrument, consortia typically receive a certain amount of observation time. In 2019, years before the launch of the JWST, the European MIRI Consortium decided to use part of this time to observe what was then the most distant quasar known.an object bearing the designation J1120+0641.
Observing one of the first black holes
The analysis of the observations fell to Dr. Sarah Bosman, postdoctoral researcher at the Max Planck Institute for Astronomy (MPIA) and member of the European MIRI consortium. MPIA’s contributions to the MIRI instrument include the construction of a number of key internal parts. Bosman was asked to join the MIRI collaboration specifically to provide expertise on how to best use the instrument to study the early universe, particularly the first supermassive black holes.
Observations were carried out in January 2023during 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 comes not from an image, but from a spectrum: the rainbow-shaped decomposition of light from the object into components of different wavelengths.
Tracking fast-moving dust and gas
The general shape of the mid-infrared (“continuum”) spectrum encodes the properties of a large torus of dust surrounding 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 somewhat higher dust 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 simply be overestimating the masses of the first black holes due to the extra dust are also not a solution.
The first quasars are “surprisingly normal”
The quasar’s broad line region, where clumps of gas orbit the black hole at near-light speeds, allowing deductions about the black hole’s mass and the density and ionization of the surrounding matter, also appears normal. In almost all properties that can be deduced from the spectrum, J1120+0641 is not different from quasars of later epochs.
“Overall, the new observations only add to the mystery: the first quasars were surprisingly normal. No matter what wavelengths we observe them in, quasars are almost identical at all times in the universe.“Bosman says. Not only the supermassive black holes themselves, but also their feeding mechanisms were apparently already fully “mature” when the universe was just 5% of its current age.
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: they are “primordial” or “big seeded”. 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.
Reference
Sarah EI Bosman et al, A mature quasar at cosmic dawn revealed by JWST rest-frame infrared spectroscopy, Nature Astronomy (2024). DOI: 10.1038/s41550-024-02273-0
This entry was posted in News on 29 Jun 2024 by Francisco Martín León
