In the field of cosmology, and in other fields of astrophysics as well, the James Webb Space Telescope (JWST) has not disappointed. It allows us to explore stratastrata of light from the history of the observable Universe better than Hubble and even some that were beyond its reach, less than 500 million years after the Big BangBig Bang.
The JWST observations thus revealed that a large number of galaxiesgalaxies already well evolved and massive existed earlier than generally thought less than a billion years after the Big Bang. In particular, large supermassive black holessupermassive black holes already containing more than massemasse than that of our Milky WayMilky Way today. It is a problem for the standard cosmological modelstandard cosmological model based on dark matter anddark energydark energy. We are not yet at a refutation, but it could eventually arrive and lead us to adopt a new law of the gravitationgravitation and/or mechanics within the framework of Mond theory.
Jean-Pierre Luminet, research director at the CNRS and Françoise Combes, professor at the Collège de France, talk to us about black holes. © Fondation Hugot du Collège de France
We understand quite well how black holes can be born by collapsecollapse gravitational ofstarsstars massive containing a few tens of solar masses. We then obtain what we call stellar black holesstellar black holes. It is not the same with supermassive black holes containing from a few million to a few billion solar masses at the heart of galaxies. They are responsible for the existence of active galactic nucleiactive galactic nuclei and in particular those discovered since 1963 and which we call quasars.
The mysterious quasars
We know that these objects influence the evolution of galaxies and especially that these stars grow together, at least as far as the large ones are concerned. galaxies spiralesgalaxies spirales and elliptical, because there is a remarkable relationship of proportionality between the mass of the black holes at the heart of these galaxies and the mass they contain in the form of stars.
The existence of supermassive black holes is already a problem in itself and several theories have been proposed to explain the formation of these giants. In any case, mechanisms of genesis and rapid growth must be involved. In particular, it was thought that those that were active less than a billion years after the Big Bang must be different from those at work in the cosmoscosmos observable for several billion years, which would also explain why there were more quasarsquasars at the beginning of the history of the Universe than today.
Let us recall that quasars are very bright active galactic nuclei which result from a significant accretionaccretion of mattermatter by supermassive black holes, much of which eventually falls into them.
Did you know ?
About 60 years ago, the technique of occultations made it possible to determine the counterpart in the visible of what was then only an astonishingly powerful radio source, 3C 273. When Maarten Schmidt, a Dutch astronomer, then made a spectral analysis of the light of the star still in the visible, he discovered with amazement hydrogen emission lines strongly shifted towards the red. However, 3C 273 appeared in the visible as a star while this result implied that it was located outside the Milky Way at a cosmological distance. To be observable from so far away, the object must therefore have been of prodigious luminosity. Other quasi-stellar radio sources, quasars according to the name proposed in 1964 by the Chinese astrophysicist Hong-Yee Chiu, were soon to be discovered. Today, more than 200,000 of them are known.
Astrophysicists very early sought to understand the nature of these stars which, although releasing enormous quantities of energy, seemed to be small in size. It was first thought that they could be enormous stars dominated by the effects of general relativity, notably responsible for the spectral shift, before fairly quickly considering that they could be supermassive black holes accreting large quantities of gas. In the bestiary of relativistic stars that we began to explore seriously during the 1960s, some, such as the Russian Igor Novikov and the Israeli Yuval Ne’eman, even proposed that quasars were in fact white holes. That is to say, either regions of the Universe whose expansion at the time of the Big Bang had been delayed (the hypothesis of lagging core), or the other end of wormholes ejecting the matter they had absorbed in the form of black holes into another part of the cosmos, or even into another Universe.
It can be shown that the radiation produced by the gazgaz hot from accretion diskaccretion disk exercises a pressionpressionthe famous radiation pressureradiation pressureon matter falling onto a black hole. There is even a brightnessbrightness limit beyond which radiation stops the flow of matter. This is just one example to make it clear that the theoretical determination of how a black hole grows is not simple. More generally, there are uncertainties about how a black hole grows by swallowing matter.
Theories suggesting a different diet for black holes less than a billion years after the Big Bang could be tested by observing quasars from that time with the JWST. This is what a team ofastrophysicistsastrophysicists had undertaken to do by observing the most distant quasar known so far in the noosphere in January 2023, during the first cycle of JWST observations.
J1120+0641, a laboratory quasar
The data collection lasted about two and a half hours and therefore concerned the spectrespectre of the famous quasar J1120+0641 located in the constellation du Lionconstellation du Lionobserved in theinfraredinfrared average with the instrument MiriMiri of the JWST during the so-called period ofDawnDawn cosmic. In this case for this quasar, barely 770 million years after the Big Bang (red shiftred shift z = 7).
The analysis of the observations was entrusted to Sarah Bosman, postdoctoral researcher at the Max-PlanckPlanck of Astronomy (MPIA) and member of the European consortium Miri, and it is exhibited with the work of its colleagues in an article of Nature Astronomy.
Françoise Combes, professor at the Collège de France, talks to us about supermassive black holes. © École normale supérieure, PSL
In a press release from the MPIA, the researcher bluntly reveals what was discovered. Overall, the new observations only add to the mystery: the first quasars were surprisingly normal. Whatever the wave lengthwave length At which we observe them, quasars are almost identical at all times in the Universe. »
His statement is supplemented by explanations in the same press release: “ The general shape of the mid-infrared spectrum (“continuum”)) encodes the properties of a large dust torus that surrounds the accretion disk in typical quasars. This torus helps guide matter toward the accretion disk, “feeding” the black hole… the torus, and by extension the feeding mechanisms in this very early quasar, appear to be the same as for its more modern counterparts. The only difference is one that no model of rapid growth of early quasars had predicted: a slightly higher dust temperature of about a hundred kelvinskelvins than the 1300 K found for the hottest dust in less distant quasars ».
Thus, J1120+0641, with a luminosity equivalent to 63 × 1012 times that of the SoleilSoleil generated by a supermassive black hole of about 2 billion solar masses appears to be mature in both size and mass early in the history of the cosmos, which is difficult to explain within the framework of current models and perhaps impossible.
In particular, it was thought that its mass had been overestimated due to the presence of a large amount of dust in the galaxy hosting the quasar, which would have modified its spectrum to the point of making it grow. But it now seems that this is not the case, at least for J1120+0641.
Three scenarios for producing supermassive black hole seeds
Let us recall that three hypotheses have been mainly put forward to explain the birth of supermassive black holes.
The first involves primordial cosmological black holes, remnants of the high-density phase of the Big Bang less than a second after its start, where large amounts of matter could gravitationally collapse directly into these black holes.
The second involves stars exoticexotic very massive, a few hundred to several thousand solar masses in particular but perhaps more, forming part of the very first stars in the observable cosmos, those called Population III, born during the first hundreds of millions of years of the history of the observable cosmos. These stars would then have formed in the particular conditions of the Universe at the time of the dark agesdark ageswhereas the baryonic matter from which all stars originated consisted of an almost pure mixture ofhydrogenhydrogend’héliumhélium and their isotopesisotopeswithout any trace of heavy elements such as carbonecarbonethe siliconsilicon and the ferfer.
This difference is important, for billions of years, the existence of silicate and carbonaceous dust has been necessary to allow the collapse of cloudsclouds molecular and dusty where star nurseries are born. Indeed, by collapsing under their own gravitygravitythese clouds heat up and a pressure appears which stops the collapse, unless an agent dissipates part of the heatheat in these clouds, causing them to cool. At the end of the Big Bang, without this dust, the formation of stars could not be the same. In fact, we also have problems in giving birth to supermassive stars which, after exploding in supernovaesupernovaecould leave giant black holes containing much more than a few hundred solar masses, germsgerms supermassive black holes called intermediate-mass black holes.
The last hypothesis assumes, still at the time when the first stars are formed, that immense clouds of matter collapse, directly giving black holes of intermediate masses during the first hundreds of millions of years after the Big Bang. We would therefore obtain very quickly large black holes and this is precisely what is suggested by the observations concerning J1120+0641 now. The researchers also deduce that these first black holes formed by direct gravitational collapse must already have contained at least 100,000 solar masses.
In all cases, the massive black holes generated will then grow by accreting matter, notably in the form of cold filaments which also make galaxies grow according to the paradigm that has been imposed over the past decade. Collisions between galaxies, followed by fusionfusionwill also cause massive and supermassive black holes to coalesce and grow. But there remains the “final parsec problem” in this regard.
