The supermassive black hole at the center of our Milky Way galaxy, known as Sagittarius A*, has a mass equivalent to that of four million suns. Discovered in the 1970s and recently studied thanks to the Event Horizon Telescopeyour knowledge and research has allowed astronomers to theorize that all galaxies, or at least the most massive ones, have a supermassive black hole at their center.
In some cases those black holes they can eat material from their surroundings and emit large amounts of energy. These objects are known as active galactic nuclei, or AGNs.
The comparison of our Sagittarius A* with the supermassive black hole of the galaxy M87, the first black hole photographed in 2019reveals that despite the colossal size of our black hole, it is a thousand times smaller and less massive than that of the aforementioned galaxy located 55 million light years from our planet and whose mass reaches no less than 6 billion light years. Suns. But how do they get to grow this way? What causes these differences in their sizes?
Although it is known that they are capable of devouring stars – those that have the misfortune to fall under their powerful gravitational attraction -, new research published by CATA astronomers in the Astrophysical Journal, reveals that the dust and gas surrounding these powerful galactic objects play a key role.
“We focus on the relationship between the black holes and the material around them that feeds them”, explains Claudio Ricciastronomer of the Diego Portales University and the TASTING who led the investigation.
For this, data from the largest census of black holes in the nearby or local universeprepared by the international scientific team of the BASS project, which for more than 15 years has been investigating the active nuclei of galaxies, including an outstanding participation of astronomers from the Center for Astrophysics and Related Technologies (CATA) from Chile.
“What we discovered is that the number of black holes in accretion (growth) it decreases when there is less gas and dust around itr, and that this material disappears due to the effect of the radiation from the black hole, which pushes it and carries it away”, says the UDP astronomer.
As the new research details, black holes begin their accretion or growth phase with relatively little gas and dust, so in a first stage they feed very slowly.
Once they receive more material, due to the explosion of nearby stars, for example, they begin to “eat” more quickly, but this means that they begin to emit more radiation, pushing away all the material that feeds them.
Franz Bauer, astronomer of the Institute of Astrophysics of the Catholic University and the CATA that participated in the investigation, explains that “then the black holes are left with very little material in their surroundings, that is, less food. Thus they begin to grow more slowly, until they run out of food again and do not emit energy. As an example we have the supermassive black hole of the Milky Way, which is precisely in a phase like this”, he details.
Precisely, the observations of the last two decades reveal that sagittarius A*the black hole in our galaxy, is in a quiescent phase. “It is thought that a few million years ago the black hole in the Milky Way was in the accretion phase, and it is possible that this phase has stopped for the reasons that we discovered in our study: due to the push of nearby material by the emitted radiation. ”, adds Claudio Ricci.
But various phenomena contribute to black holes returning to their active phase, such as stellar winds, nearby supernova explosions, stars or gas clouds that “venture” too close to the black hole and are engulfed, or even colliding galaxies, such as the collision that will occur between our neighbor Andromeda and the Milky Way, in about 5 billion years.
“This study has a strong participation of the Chilean scientific community, led by our researcher Claudio Ricci. During their development, the telescopes located in Chile have been extremely important, because they allow us to measure the mass of black holes and the properties of the galaxies that host them. We have worked on this project for several years, it started in 2016 and it was possible to conclude it in the last 2 years thanks to the new BASS black hole sample”, he concludes. Ezequiel Treister, deputy director of CATA and astronomer at the UC Institute of Astrophysics.