I’avalanche of confirmations of the black hole theory continues with a statement from theESO which accompanies a publication in the famous newspaper Nature which can be consulted freely at arXiv. In 2015, we celebrated the centenary of Einstein’s final discovery of his theory of general relativity and the same year were detected on Earth for the first time the gravitational waves coming from another spectacular prediction of his relativistic theory of gravitation, black holes. In 2019, it was the disclosure of the first image of a black hole taken with L’Event Horizon Telescopethat of the black hole at the heart of the galaxy M87.
For decades, it has been thought that these relativistic stars in their supermassive form, that is to say containing at least a million masses solar and sometimes several billion, are at the origin of the active nuclei of galaxies (AGN Where active galactic nucleus, in English) highlighted in particular by radio astronomers, but also by astronomers without these AGNs necessarily being sources radio. These AGNs are characterized by particularly energetic phenomena in one form or another, for example jets of relativistic matter over thousands of light yearsand it is a discovery that concerns them that is now coming to the fore thanks to observations made with theinterferometer from Very Large Telescope of the European Southern Observatory (ESO’s VLTI).
Active galactic nuclei (AGNs) are extremely energetic sources fueled by supermassive black holes. This short video provides insight into these particular objects through the recent discovery of an AGN at the center of the Messier 77 galaxy. © European Southern Observatory (ESO)
A zoo of active galactic nuclei
The most spectacular AGNs are quasars which at first glance look like stars seen in a telescope ordinary while being powerful radio sources, but whose precise determination of the distances at which they were observed at the beginning of the 1960s led to the admission that they were objects whose size wasmagnitude of the Solar System, yet capable of continuously releasing as manyenergy than the stars of a great galaxy whole like the Milky Way.
In the end, astronomers understood that AGNs could be described by three major classes, namely radio galaxies, Seyfert galaxies and the quasars with subdivisions. Some are very bright both in the visible and in the radio, others in only one of these spectral bands. Some have jets of matterothers not.
Thus, radio galaxies are ordinary-looking galaxies, rather elliptical galaxies giants or lenticular galaxies, but which emit powerfully in the radio domain. The radio radiation emitted can be hundreds of times more powerful than that of so-called normal galaxies – we know for example the case of the source called Cygnus A, which is a million times brighter than our Milky Way. An important feature of radio galaxies is the presence, sometimes thousands oflight years from their center, from two lobes where is concentrated most of theepisode radio. This is the end of the jets of material expelled at very high speed that we mentioned earlier and of which we can see an artist’s illustration in the video above.
Seyfert galaxies are spiral galaxies first observed in 1943 Karl Seyfert. We can mention the galaxies NGC 1410 in the constellation of the Eridano and Messier 77 in that of the Baleine. They are brighter than the average galaxy not only in radio, but also in the visible with in particular their nucleus which emits as much light as the rest of the stars of these galaxies.
Jean-Pierre Luminet, research director at the CNRS and Françoise Combes, professor at the Collège de France, talk to us about black holes and in particular supermassive black holes in galaxies and which are behind AGNs. © Hugot Foundation of the College de France
We ended up advancing the idea, described by what is called the unified model of AGNs, that behind all these active nuclei of galaxies was hiding the same type of objects, but seen from different angles and at different periods of time. the history of the Universe, namely, as we have said, supermassive black holes emitting an enormous amount of energy following complex, and not always well understood, processes ofaccretion of matter (mainly in the form of cold filaments) and relativistic magnetohydrodynamics.
Thus, in a region hardly larger than the Solar System at most, there must have been a torus of dust and neutral gases surrounding an accretion disk of dust, gas and finally matter ionized by the heat released by viscous friction in this disc and falling on a Kerr black hole in rotation.
The heated plasma entering the ergosphere of the black hole, i.e. the region of thespace-time rotating any body falling radially, then participated in a complex mechanism, elucidated in part by Blandford and Znajeck, where the gravitational energy of the fall of matter and especially the rotational energy of the black hole were converted into intense radiation and jets of matter along the axis of rotation of the compact star.
Astronomers have observed different types of AGN. Some, called blazars, are extremely bright and can exhibit brightness variations on time scales of only hours or days, while another type, called quasar, is also very bright but tends to exhibit less variability. than blazars. Seyfert galaxies, which come in two forms (1 and 2), are another type of AGN, surrounded by easily detectable host galaxies. The Seyfert 1 and Seyfert 2 galaxies are both characterized by a bright core. However, those of the Seyfert 2 type tend to be more discreet. The Unified AGN Model states that, despite their differences, all AGNs have the same basic structure: a supermassive black hole surrounded by a thick ring or torus of dust. According to this model, any difference in appearance between the AGNs results from the angle from which we observe the black hole and its bulky ring from Earth. The type of AGN we observe thus depends on how dark the black hole is along the line of sight, with the ring sometimes eclipsing it completely. © European Southern Observatory (ESO), L. Calçada and M. Kornmesser
A unified AGN model
Today, it is therefore a team ofastrophysicistsled by PhD student Violeta Gámez Rosas from the University of Leiden in the Netherlands, who has just provided new evidence of the relevance of the unified AGN model by making the most precise observations to date of the center of the galaxy. Messier 77, located 47 million light-years from the Milky Way in the whale constellationrevealing the presence of a thick disc of dust and cosmic gases concealing a supermassive black hole.
The ESO press release which exposes this discovery, made possible by the Matisse instrument (Multi AperTure mid-Infrared SpectroScopic Experiment) installed on the VLTI, presents it as a very serious confirmation of the validity of the unified model advanced thirty years ago. Located in the desert from Atacama in Chile, this instrument has combined the light infrared collected by the four 8.2 meter telescopes making up the Very Large Telescope (VLT) from ESO using a technique called interferometry long-base optics and which therefore makes it possible to have virtually a much larger telescope, with a power of resolution superior to observe never-before-seen details.
” Matisse is able to detect a wide range of wavelengths infrared, allowing us to see through dust and measure temperatures accurately. Because the VLTI consists of a very large interferometer, it offers sufficient resolution to study phenomena occurring within galaxies as distant as Messier 77. The images obtained show variations in temperature andabsorption from clouds gas around the black hole “, specifies Walter Jaffe, co-author of the study and professor at the University of Leiden.
” The true nature of the dust clouds, their role in feeding the black hole as well as in the appearance it takes on as seen from Earth have, for three decades, been essential questions for any researcher working on AGNs. Although no single result can answer all the questions that arise, a major step has just been taken in our understanding of how AGNs work.explains Violeta Gámez Rosas who adds, our results should offer a better understanding of the inner workings of AGNs. They could also help us better understand the history of the Milky Way, which harbors a supermassive black hole at its center that may have been active in the past. »
The researchers now wish to extend their observations, using ESO’s VLTI, to a larger sample of galaxies, in order to confirm the validity of the unified AGN model.
Bruno Lopez, one of the team members and principal manager of the Matisse instrument at the Observatoire de la Côte d’Azur, located in Nice, France, adds as for him and always in the press release of the ESO than : “ Messier 77 constitutes a true prototype of AGN. His study pushes us to extend our program of observations and to optimize Matisse in order to study a larger sample of AGN “.
This research program should take on a new dimension when theExtremely Large Telescope (ELT) of ESO will enter service before the end of this decade.
” Supermassive black holes that are too greedy. All galaxies harbor a supermassive black hole at their center, with a mass of between a million and a few billion solar masses. There is a proportional relationship between the mass of these black holes and the mass of the bulge of the galaxies, which suggests that star formation and black hole feeding occur simultaneously. In a way, galaxies and their black holes grow in symbiosis. When gas falls towards the center of the galaxy, the black hole swallows as much of it as possible, but the mass it can absorb is limited. The fall of matter on the black hole releases a considerable amount of energy, in the form of radiation, and also in the form of kinetic energy. The nucleus of the galaxy becomes active, either a Seyfert nucleus or a quasar. The winds and plasma jets emitted by the black hole entrain the surrounding interstellar gas. Flows of molecular gas have recently been detected around active nuclei, carrying away so much mass that they can have a significant impact on the evolution of the host galaxy, regulating or even stopping the supply of gas for star formation. . Gluttonous black holes, by spitting out their food, regulate star formation. We will detail these phenomena, perhaps at the origin of the proportionality between the masses of black holes and bulbs. Françoise Combes is an astronomer at the Paris Observatory in the Laboratory for the Study of Radiation and Matter in Astrophysics (Lerma). His current area of research concerns the formation and evolution of galaxies. © Ecole Normale Supérieure – PSL
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