Close up of supermassive black holes

If on a clear summer night, far from any source of light pollution, you turn your eyes to the south, you should know that the scattered glow that you will see just above the horizon, in the constellation Sagittarius, is n other than the core of our galaxy ( note for amateurs: this nucleus can be seen much better from the southern hemisphere). Why a diffuse glow? Because the light emitted by the enormous concentration of stars that form this nucleus reaches us weakened by the dust that accumulates in the central part of the Milky Way. But by looking into this core using telescopes capable of seeing through this dust, astronomers have made an astonishing discovery: the stars in the core orbit the galactic center at enormous speeds, up to several thousand kilometers per hour. . second, thus betraying the invisible presence of a cosmic monster that exerts a significant gravitational influence on everything around it. This monster is Sagittarius A * (or Sgr A *), a supermassive black hole located in the heart of the Milky Way, for which it is in a way the focal point. The same, if about 300 scientists from the Event Horizon Telescope (EHT) network, which today includes 11 radio telescopes spread across the entire surface of the globe, have just given us the picture for the first time.

This historical photograph, from an observation campaign conducted in 2017 but published last month (after five years of reprocessing and data analysis) in “The Astrophysical Journal Letters”, looks like two drops of water to the one that made headlines in 2019. This image from 2019, which is also captured by EHT’s combined radio telescopes during their 2017 campaign, also shows us a supermassive black hole enthroned in the center of a galaxy, but it is not ours: it is the distant and gigantic M87, a supergiant elliptical galaxy in relation to which the Milky Way pales in comparison. Opposite each other, the two images of the M87 *, the central supermassive black hole in the M87, and Sgr A *, its counterpart to the Milky Way, show almost two images of the same object. However, there is a difference between them … in size (in both senses of the term).

Accretion discs

Located 55 million light-years from us, that is, 2,000 times longer than Sgr A * (which is only 27,000 light-years away), the M87 *, on the other hand, is 1,500 times larger than him. Its mass is 6 billion times that of the Sun, all concentrated in a virtually infinitely small volume. Like any black hole, M87 * is defined by its “Schwarzchild radius” (named after the German astrophysicist Karl Schwarzchild, who only in 1915 deduced from the general equations of relativity the possible existence of what was not yet called “black holes” ” This Schwarzchild ray draws around the black hole a purely theoretical sphere (it has no physical reality), the surface of which is called the “event horizon” (hence the name of the Event Horizon Telescope network): past this sphere is the journey without return – even for light.The Schwarzchild radius of M87 * is 19 billion kilometers.

Compared to this celestial Gargantua, Sgr A *, although even hundreds of thousands of times larger than the stars’ black holes (those born from gravitational collapse of very large stars), is only a dwarf: its mass is “only” 4 million solar cells mass, and its Schwarzchild radius 12 million km.

Comparative sizes of the two supermassive black holes: if Sgr A * took the place of the sun, its growth disk would not extend beyond Mercury’s orbit (right); by comparison, the M87 * (left) is a monster: its central dark zone extends far beyond Pluto’s orbit; its growth disk begins only at a distance from the Voyager 1 probe, the most distant spacecraft from Earth …EHT collaboration / recognition Lia Medeiros

Above all, it is what separates the two supermassive black holes beyond their size, their growth rate, that is, the amount of matter that by attracting everything in their vicinity to them – dust, gas, pieces of stars shredded by their gravitational field – they engulf continuously. Astrophysicists have now become convinced that most, if not all, large galaxies, whether spiral-shaped like the Milky Way or elliptical like the M87, have a supermassive black hole at their core. They even stated that the more massive the galaxy, the more its central black hole. A context that, as Vincent Piétu of the Institute of Millimetric Radio Astronomy (Iram) points out, suggests that “supermassive black holes and their host galaxy are evolving together” (read below). But among these galactic nuclei, all of which house a supermassive black hole, some are relatively calm and others hyperactive. A hyperactive core hides a super greedy black hole!

M87 * ogre is the voracious type. Every year it swallows the equivalent of 0.002 solar masses, a growth rate tens of thousands, even millions of times higher than Sgr A *. And it’s fortunate for us that the latter is not only much smaller, but much more frugal than its M87 counterpart: if it were not, the dangerous radiation emitted by the core of the Milky Way would probably make the latter not conducive to emergence within that of any kind of life.

This ionized substance (consisting of ions and free electrons), accelerated by the gravitational field of the black hole to speeds close to light and brought to immeasurable temperatures – several billion degrees, ie. a medium warmer than the furnace that reigns in the heart of the stars – is exactly what the two images of EHT show us. At this temperature, matter no longer exists in any of the three states we know of on Earth (solid, liquid, or gaseous), but in a fourth, called plasma. And it is the general theory of relativity that gives this glowing plasma that makes up the black hole growth disk, this remarkable ring shape. It is, moreover, astonishing to think that the theory formulated in 1915 by Einstein applies just as well, and with such precision, to two objects belonging to such different size scales as M87 * and Sgr A *: the images of EHT constitute more than one century later, a new triumph for Einstein. Like gravitational waves, the first of which was discovered in 2015, and which provided the first evidence of the existence of much smaller star-black holes.

One mystery remains: if it is clear that the stars’ black holes were born from the collapse of themselves by the largest stars (those weighing at least thirty solar masses), once they have reached the end of their lives, how do these galactic form hubs, there are supermassive black holes? No one has yet given a definitive answer to this question, “especially due to the lack of observational data on medium-sized black holes, from a few thousand to a few tens of thousands of solar masses,” explains Vincent Piétu. These middleweights, if they exist, continue to dodge our investigations. But the hunt continues.

Cosmic recycling

When we talk about supermassive black holes that would put themselves at the center of almost all large galaxies, physicist Guido Tonelli writes in his fascinating book “Genesis – The great story of origins” (Dunod, May 2022): “It seems that that the presence of these beautiful monsters is necessary to build these wonderful toys, which are the galaxies: dynamic configurations of matter stable over billions of years. One mechanism in particular illustrates this phenomenon of probable co-evolution. even with a huge growth disk, some of the substance that is forced to spin around it at a furious speed is literally “broken up” in the form of rays of particles (and of radiation) coming out of the poles of the black These very energetic rays extend perpendicular to the plane of the galaxy over considerable distances, up to several thousand light-years, but some of the matter thus expelled into intergalactic space returns ge to the galaxy from which it originated. And, just like fuel that feeds an engine, it allows for the creation of new stars …

4 types of black holes

Stellar Born from the gravitational collapse of a very large star. Mass: a few solar masses.

Supermassive (or Galactic) – The house at the center of almost all major galaxies. Mass: One million solar masses or more.

Intermediaries – Lack of link between star and supermassive. Mass: a few thousand solar masses.

Primordialer – With much lower mass, they would be formed just after the Big Bang. Their existence is not confirmed.

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