Black hole “census”: astronomers manage to measure masses of more than 800 black holes

The most comprehensive research of its kind - in the near or local universe - has been released by the international scientific team of the BASS Survey project, after more than 15 years of extensive research including an outstanding participation of astronomers from the Centro de Astrofísica y Tecnologías Afines (CATA) from Chile. Some of the telescopes used in this study were the Magellan telescopes and the Irénée du Pont Telescope at Las Campanas Observatory.

The research, which was published in the latest edition of the Astrophysical Journal, includes a series of scientific publications that used data from large telescopes in the north of our country, the United States and NASA's Swift Space Observatory, achieving an extensive accumulation of information that has allowed the construction of a map of these active black holes and their intense emissions in the nearby universe.

Hundreds of hours of observation and analysis were required to accomplish this task, revealing 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 the 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, one of the principal investigators of the BASS Survey project.

Cosmic X-ray

According to the research, 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 electromagnetic spectrum, peaking in the optical and ultraviolet range as it falls into the black hole.

Franz Bauer, CATA researcher and academic at the Astrophysics Institute of the Catholic University, 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" nuclei of galaxies (AGN), making it difficult 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.

This barrier could be overcome 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 energetic emissions from supermassive black holes.

"It is similar to the process of taking an X-ray, as this instrument observed at a similar frequency. In this case, it would be like a cosmic X-ray to observe the nuclei of galaxies where these growing black holes are," explains Ezequiel Treister, Deputy Director of CATA and UC astronomer who was also part of the research.

Nearly complete sample

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 have an almost complete sample of accreting black holes in the centers of nearby galaxies," the researcher explains. The rate at which these black holes grow varies greatly," adds the astronomer, "from the equivalent of the mass of Uranus per year to those that "swallow" the equivalent of 30 Jupiter planets in a similar period.

In addition to the Swift Observatory and BAT, more than 10 ground-based optical and infrared telescopes in our country and other parts of the world were used. Ezequiel Treister, emphasizes that "this is a collaborative work, which required the combined work of telescopes in the southern hemisphere and the northern hemisphere, in order to study the active nuclei of galaxies distributed throughout the sky. The mass measurements were possible thanks to the many, many observations from Chile".

Dozens of CATA scientists participated in the extensive accumulation of data using Chilean telescopes over the years, including ESO's Very Large Telescope at Cerro Paranal (in the Antofagasta Region), the Magallanes telescopes and the Irénée du Pont Telescope at Las Campanas Observatory (located in the Atacama Region), along with the SOAR Telescope, located at Cerro Pachón in the Coquimbo Region.

"Measuring black hole masses can be quite complicated, and with this collaborative work we have been able to do it for a very complete sample of objects in the nearby universe," says Claudio Ricci.

High velocity

One of the results published in this study, led from Chile, was obtained using 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 mass of black holes, detecting the motion of clouds rotating at high speed in their surroundings, including completely obscured systems where this was not possible. This shows the importance of combining multiple observatories," says Federica Ricci, who was a FONDECYT postdoctoral researcher at the Catholic University and is currently continuing her research career 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, allowing 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, to analyze the accelerated growth of supermassive black holes, and also to investigate black hole systems that could be considered rare or abnormal, the astronomers conclude.

Source: CATA


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