2023-11-07 21:45:12
At the centers of many massive galaxies there are supermassive black holes, with masses that can exceed 1 million times the mass of the Sun. How do these black holes accumulate so much mass? One of the most important mechanisms detected in previous research is gas accretion, a phenomenon in which gas from the host galaxy flows towards the black hole hosted at its center.
The gas that accumulates very close to a supermassive black hole reaches high speeds when attracted by the star’s gravity. The intense friction between the gas particles causes the gas to heat up to several million degrees and emit a bright light. This phenomenon is known as an active galactic nucleus, and its brightness can exceed that of all the stars in the galaxy combined. The curious thing is that part of the gas that flows towards the black hole (the accretion flow) seems to be expelled in an intense jet due to the enormous energy generated by this active galactic nucleus.
Both theoretical studies and observations carried out to date have provided detailed information on the mechanisms of gas accretion in the center from galactic scales of 100,000 light-years to scales of a few hundred light-years. However, the phenomenon of gas accretion in much smaller regions, a few dozen light-years from the center of a galaxy, was unknown until now due to the very small spatial scale it covers. In order to understand the growth of black holes in quantitative terms, it is necessary to measure the accretion rate and determine the amount and types of gas (plasma, atomic gas and molecular gas) that are ejected in these jets at such small scales. Unfortunately, observational studies have not made much progress in that area, until now.
An international research team led by Takuma Izumi, a professor at the National Astronomical Observatory of Japan (who was affiliated with the observatory and Tokyo Metropolitan University at the time of the study), has reached a globally unprecedented milestone by measuring quantitatively the gas flows and their structures in all phases (plasma, atomic and molecular) at a tiny scale—a few light-years—around a supermassive black hole, using the Atacama Large Millimeter/submillimeter Array (ALMA) observatory. ). Observations of multiphase gases can help to reasonably understand the distribution and dynamics of matter surrounding a black hole. In this study, the Circinus galaxy, which constitutes a typical active galactic nucleus in the nearby universe, was observed at a resolution of approximately one light-year, the highest achieved to date in observations of multiphase gas in an active galactic nucleus.
The research team managed to observe for the first time the accretion flow towards the supermassive black hole within the high-density gas disk that extends several light years from the center of the galaxy. Identifying this accretion flow had always been a difficult task due to the tiny scale of the observed region and the complex motions of gas near the galactic center. However, this time the research team determined precisely where the molecular gas observed in the foreground was absorbing light from the bright active galactic nucleus behind it. This was possible thanks to the high-resolution observations achieved with ALMA, which revealed, after detailed analysis, that this absorbing material is moving away from us. And since this material is always between the active galactic nucleus and us, the team was able to observe the accretion flow moving in the direction of the active galactic nucleus.
The team also managed to elucidate the physical mechanism of gas accretion. The gas disk generates a gravitational force so intense that it cannot be sustained by the pressure exerted by the movement of the disk. In this type of situation, the gas disk usually collapses under its own weight and gives way to complex structures that are unable to maintain stable movement in the galactic center, after which the gas flows rapidly towards the black hole in the center. ALMA made it possible to reveal this physical phenomenon, known as gravitational instability, in the heart of the galaxy.
Illustration depicting the distribution of the interstellar medium in the active galactic nucleus according to the results of the new study. High-density molecular gas flows from the galaxy toward the black hole following the plane of the disk. The material accumulated around the black hole generates a large amount of energy that destroys the molecular gas and returns it to atomic and plasma states. Most of these multiphase gases are ejected in core-projected jets (including plasma jets up the disk and mainly diagonal molecular or atomic jets). Most of these jets will return to the disk, similar to the jets of an ornamental water fountain. (Image: ALMA (ESO/NAOJ/NRAO), T. Izumi et al.)
The study also contributes to considerably improving quantitative knowledge of gas flows around active galactic nuclei. The accretion rate of the gas flowing toward the black hole can be calculated from the density of the observed gas and the speed of the accretion flow. To the surprise of the scientific team, this rate turned out to be 30 times higher than what is necessary to sustain activity in the active galactic nucleus. In other words, most of the 1 light-year-scale accretion flow around the galactic center does not contribute to the growth of the black hole. Hence the question: where did all the excess gas go? The study also made it possible to elucidate this mystery through highly sensitive observations of gases in all their phases in the jets of the active galactic nucleus detected by ALMA. Quantitative analyzes revealed that most of the gas flowing into the black hole is ejected in atomic or molecular jets. However, since they are not fast enough, these jets cannot escape the gravitational force of the black hole and end up returning to the gas disk. There, they again form an accretion flow towards the black hole, in a fascinating phenomenon of gas recycling in the galactic center similar to the cycle of an ornamental water fountain.
Takuma Izumi celebrates: “Detecting accretion flows and outgoing jets in a region of a few light-years around a growing supermassive black hole, and in particular in a multiphase gas, and even deciphering the accretion mechanism itself, are achievements monumental in the history of supermassive black hole research.” And about future prospects, he adds: “To fully understand the growth of supermassive black holes in cosmic history, we have to study several types of distant supermassive black holes. For that, we need to make high-resolution, high-sensitivity observations, and we have high hopes for future observations with ALMA and next-generation large radio interferometers.”
The study is titled “Supermassive black hole feeding and feedback observed on sub-parsec scales.” And it has been published in the academic journal Science. (Source: Atacama Large Millimeter/submillimeter Array (ALMA))
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