A comprehensive study of the supermassive black holes in the nearby local Universe —giants that grow up devouring everything, found in the so-called active nuclei of galaxies and reaching billions of times the mass of our Sun— has been released by the BASS international scientific team. This extensive research was carried out over more than 15 years, with the critical participation of CATA astronomers in Chile.

This research, published in the latest edition of The Astrophysical Journal, includes a series of nine scientific publications based on data from large telescopes in the North of Chile, continental United States, Hawaii, and NASA‘s Swift Space Observatory. The team achieved an extensive accumulation of information that has allowed for the construction of a map of the most active black holes in the nearby Universe and their intense emission.

Hundreds of hours of observations and analysis were required to accomplish this task, allowing the scientists to measure the masses of supermassive black holes in galactic centers with a level of detail that had been impossible to achieve until now. "The map is representative of active black holes in the local universe. It has more than 800 supermassive black holes in a distance range of more than 5 billion light-years. The main novelty is that we were able to estimate the most important physical properties for a large sample of supermassive black holes, such as their masses and growth rates," explains Claudio Ricci, CATA astronomer at the Diego Portales University and one of the principal investigators of the BASS project.

According to our understanding, when a substantial amount of dust and gas surrounds a supermassive black hole, it can form an accretion disk that emits large amounts of light across the entire electromagnetic spectrum. This radiation peaks in the optical and ultraviolet range as it falls into the black hole. 

Franz Bauer, CATA researcher and professor at Universidad Catolica de Chile, who also participated in the research, explains that this same dust and gas, however, can also block our view of the so-called central engine, or active galactic nucleus (AGN), making it challenging to observe these giants with traditional instruments and techniques. "This implies that although many supermassive black holes are actively accreting material and growing, we do not see them easily at visual wavelengths and do not take them into account," he says.

It was possible to overcome this limit thanks to the instrument aboard the Swift Observatory known as BAT (Burst Alert Telescope), capable of detecting high-energy X-rays, also known as "hard X-rays," associated with high-energy emissions from supermassive black holes. "It is similar to the process of taking an X-ray radiography, as this instrument is observed at a similar frequency. In this case, it would be like taking a cosmic radiography to observe the nuclei of galaxies where these growing black holes are," explains Ezequiel Treister, Sub-director of CATA, an astronomer at Universidad Católica de Chile, and part of the scientific team.

Claudio Ricci points out that at these energy levels, the radiation interacts very little with the material in its path, allowing "to detect also some of the most obscured black holes. This has made it possible for us to construct an almost complete sample of accreting black holes in the centers of nearby galaxies," says the researcher. The rate at which these black holes grow varies greatly," adds the astronomer, "from the equivalent of the mass of the planet Uranus per year to those that "swallow" the equivalent of 30 Jupiter planets in a similar period”.

In addition to the Swift Observatory, the team used more than ten ground-based optical and infrared telescopes in our country and other parts of the world. Ezequiel Treister emphasizes that "this is a collaborative work, which required the combined work of telescopes in both hemispheres, to study the active nuclei of galaxies distributed throughout the sky. Mass measurements were possible thanks to numerous observations from Chile".

Dozens of CATA scientists participated in the considerable accumulation of data using Chilean telescopes over the years, including ESO‘s Very Large Telescope at Cerro Paranal (in the Antofagasta Region), the Magellan Telescopes, and the Irénée du Pont Telescope (located in the Atacama Region, managed by the Carnegie Institution for Science), along with the SOAR Telescope (AURA), located at Cerro Pachón in the Coquimbo Region. "Measuring black hole masses can be quite complicated. With this collaborative work, we have been able to do so for a nearly complete sample of objects in the nearby universe," explains Claudio Ricci. 

One of the results published in this study, led in Chile, used infrared spectroscopy to measure the mass of more than 300 highly obscured supermassive black holes. "Thanks to these data, we have been able to measure the masses of black holes, detecting the motion of clouds rotating at high speed in their surroundings, including completely obscured systems where this was formerly impossible. This demonstrates the importance of combining multiple observatories," says Dr. Federica Ricci, a former FONDECYT postdoctoral researcher at the Universidad Católica de Chile. She is currently continuing her research career, investigating the growth of supermassive black holes, in Italy.

The astronomers conclude that the large sample of objects and the enormous amount of data accumulated in recent years will make it possible to improve the understanding of black holes and allow a better understanding of their relationship with their host galaxies. The new data will make it possible to study phenomena such as the accumulation of gas in galaxies and its influence on the formation of certain stars, analyze the accelerated growth of supermassive black holes, and investigate black hole systems that could be considered rare or unique.