CATA Astronomers discover key details about the growth of Supermassive Black Holes

Research using data from the largest survey of black holes, published in the latest edition of the Astrophysical Journal, indicates that materials such as dust and gas in their vicinity play a critical role in their evolution.

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 Telescope, 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 such black holes can eat material from their surroundings and emit large amounts of energy. These objects are known as active galaxy nuclei or AGN.

The comparison of our Sagittarius A* with the supermassive black hole of the galaxy M87, the first black hole photographed in 2019, reveals 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 nothing less than that of 6 billion suns. But how do they grow in this way? What causes these differences in their sizes?

While it is known that they are capable of devouring stars – those unfortunate enough to fall under their powerful gravitational pull – new research published by CATA astronomers in the Astrophysical Journal reveals that the dust and gas surrounding these powerful galactic objects plays a key role. “We focused on the relationship between black holes and the material around them that feeds them,” explains Claudio Ricci, an astronomer at Diego Portales University and CATA who led the research.

To achieve this, the scientists used data from the largest survey of black holes in the near or local universe, created 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 have discovered is that the amount of accreting (growing) black holes decreases when there is less gas and dust around them, and that this material disappears due to the effect of the black hole’s radiation, which pushes it away,” says the astronomer.

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According to this new research, black holes begin their accretion or growth phase with relatively little gas and dust, so in the first stage they grow very slowly. Once they receive more material, due for example to the explosion of nearby stars, 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 at the Astrophysics Institute of the Catholic University of Chile and CATA researcher, who participated in the study, explains that “black holes are left with very little material in their surroundings, i.e., less food. In this way 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 explains.

Precisely, the observations of the last two decades reveal that Sagittarius A*, the black hole of our galaxy, is in an inactive phase. “It is thought that some millions of years ago the black hole of the Milky Way was in an accretion phase, and it is possible that this phase has stopped for the reasons we discovered in our study: due to the push of the nearby material by the emitted radiation,” adds Claudio Ricci.

But several phenomena contribute to black holes going back 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 galaxy collisions, such as the collision that will occur between our neighbor Andromeda and the Milky Way in about 5 billion years.

“This study has an important participation of the Chilean scientific community, led by our researcher Claudio Ricci. 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 been working 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”, concludes Ezequiel Treister, CATA subdirector and astronomer at the Instituto de Astrofísica UC.


About the main image: Active galactic nucleus (credit ESO): Artist’s impression of the active nucleus of a galaxy, surrounded by an accretion disk composed of dust and gas.