A pair of supermassive black holes are said to have carved out giant cavities in a cluster of galaxies

Chandra’s x-ray observations in space and radio observations from Karl G. Jansky Very Large Array (VLA) and EVN on Earth show huge cavities carved into the plasma at the center of a distant cluster of galaxies. They suggest that they were dug by the perpendicular jets of two supermassive black holes whose orbit size is the smallest known to date for this type of giant stars.

As Futura has explained on several occasions, black holes are objects so compact, not necessarily dense, that one would have to overcome the speed of light to escape their gravitational pull once inside a spherical region, the boundary of which is defined by what is called a event horizon, surrounding the star. It is by no means necessary that there be a singularity of thespace-time in the center of this region so that there is a black hole. Although we have more and more reason to believe that black holes do exist in the observable cosmos, it remains to demonstrate that the objects that we observe and which from the astrophysical point of view behave like black holes do indeed have an event horizon.

Indeed, the existence of these objects is not obvious and it has hardly been more than 50 years since the scientific community began to take the existence of black holes seriously, thanks to the work of the pioneers that were John Wheeler, the Nobel Prizes of physique Roger Penrose and Kip Thorne, Stephen Hawking, Without forgetting Yakov Zeldovich and Igor Novikov. You would actually have to add a good dozen more names at least, like those of Roy Kerr, Brandon Carter, Vitaly Ginzburg, Jacob Bekenstein, etc.

Jean-Pierre Luminet, research director at the CNRS and Françoise Combes, professor at the Collège de France, talk to us about black holes. © Hugot Foundation of the Collège de France

Supermassive black holes that influence galaxies and clusters of galaxies

We understand rather well how black holes can be born by collapse gravitationalstars massive containing a few dozen masses solar. We then obtain what we call stellar 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 and we speculate on their birth, especially from primordial black holes of the Big Bang. What is certain is that these giants are responsible for the existence of the active nuclei of galaxies (AGN) and in particular those discovered since 1963 and which are called quasars. We know that these objects influence the evolution of galaxies and especially that these stars grow together, at least with regard to large spiral and elliptical galaxies, because there is a remarkable proportionality relationship between the mass of black holes at the heart of these galaxies and the mass they contain in the form of stars.

We are trying to elucidate these growth processes. There must of course be some giant black hole coalescing process when two galaxies merge and the black holes begin to sediment under the influence of the gravitational frictional force of the matter in the resulting galaxy to reach its center. But it is certain that there are also processes of accretion of vast filaments of cold matter, that this accretion releasesenergy in the form of radiation and is also at the origin of powerful jets of matter rushing through the intergalactic medium and influencing both the host galaxy and the galactic cluster where the AGNs at the origin of these jets are found.

The study of these processes is important for understanding the joint evolution of galaxies and supermassive black holes. Another laboratory allowing this study to be carried out was found by astrophysicists using observations in the field of X-rays provided by satellite Chandra from Nasa. This resulted in an article published in The Astrophysical Journal Letters and which can be found in free access on arXiv.

Two supermassive black holes 250 light years apart?

Chandra’s gaze had been turned in the direction of themistresses of galaxies RBS 797, located at approximately 3.9 billionlight years from Milky Way, in addition to that in the field radio of the famous network of radio telescopes called the Karl G. Jansky Very Large Array (VLA), and even telescope Hubble.

As a result, four enormous cavities, or bubbles, appeared in the images in the center of the cluster, in the millions of degrees hot plasma bathing the galaxies.

These bubbles seem to go in opposite pairs along perpendicular axes. The simplest way to interpret these associated bubbles is to involve the jets of two supermassive black holes in orbit close.

Cavities of this kind have already been detected, but this time a double pair seems to indicate a rare event where two supermassive black holes are almost simultaneously erupting with jets. However, the researchers cannot exclude that we see the effect of two eruptions close together in time, if we can say on a cosmic scale (the analysis of Chandra’s data shows that the age difference for east-west and north-south cavities are less than 10 million years old), and a single supermassive black hole which for one reason or another may be under the influence of gravitational disturbances from another hole black, would have flipped, which would have changed the direction ofepisode of these jets (remember that the jets do not come from inside the black hole but are the product of a complex relativistic magnetohydrodynamic process in the disc of matter outside the rotating black hole).

A summary of the discovery concerning RBS 797. To obtain a fairly accurate French translation, click on the white rectangle at the bottom right. The English subtitles should then appear. Then click on the nut to the right of the rectangle, then on « Subtitles » and finally on « Translate automatically ». Choose « French ». © Chandra X-ray Observatory

However, it should not be forgotten that the European radio telescope network VLBI (European VLBI Network or EVN) has indeed identified at the heart of RBS 797 two powerful radio sources separated by only about 250 light years. These would be precisely two supermassive black holes and if so, this is the closest pair discovered to date.

In this hypothesis, the two compact objects will continue to approach each other by emitting increasingly large quantities ofgravitational waves, up to one fusion cataclysmic.

Françoise Combes, professor at the Collège de France, tells us about supermassive black holes. © École normale supérieure, PSL

What you must remember

  • Four hot gas cavities in a single cluster of galaxies have been discovered.
  • Astronomers found this quartet of x-ray cavities in the cluster about 3.9 billion light years from Earth using Chandra.
  • This suggests that there is a pair of supermassive black holes, both of which erupted and spouted out around the same time.
  • If confirmed, the two supermassive black holes would be the closest pair ever discovered, with a separation of only about 250 light years.

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