Mystery Resolved: Black Hole Feeding and Feedback at the Center of an Active Galaxy

 An international research team led by Takuma Izumi, an assistant professor at the National Astronomical Observatory of Japan, has observed in high resolution (approximately 1 light year) the active galactic nucleus of the Circinus Galaxy - one of the closest major galaxies to the Milky Way. The observation was made possible by the Atacama Large Millimeter/Submillimeter Array (ALMA) astronomical observatory in Chile.

This breakthrough marks the world's first quantitative measurement at this scale of gas flows and their structures of a nearby supermassive black hole in all phase gases, including plasma, atomic, and molecular. Such high resolution allowed the team to team to capture the accretion flow heading towards the supermassive black hole, revealing that this accretion flow is generated by a physical mechanism known as 'gravitational instability.' Furthermore, the team also found that a significant portion of this accretion flow does not contribute to the growth of the black hole. Instead, most of the gas is expelled from the vicinity of the black hole as atomic or molecular outflows, and returns to the gas disk to participate again into an accretion flow towards the black hole, much like how water gets recycled in a water fountain. These findings represent a crucial advancement towards a greater understanding of the growth mechanisms of supermassive black holes.

These observation results were published in Science on November 2, 2023.

'Supermassive black holes,' with masses exceeding a million times that of the Sun, exist at the centers of many galaxies. But astronomers have long pondered the mechanisms responsible their formation. One proposed mechanism, as outlined in previous research, suggests that gas accretes onto the black hole as it gravitates towards the center of the host galaxy.

As gas approaches the supermassive black holes, the intense gravitational pull of the black hole causes the gas to accelerate. The resulting increase in friction between gas particles leads to the gas heating up to temperatures as high as several million degrees and results in the emission of brilliant light. Known as an active galactic nucleus (AGN), the brightness can at times surpass the combined light of all the stars in the galaxy. Interestingly, a portion of the gas that falls towards the black hole (accretion flow) is thought to be blown away by the immense energy of this active galactic nucleus, leading to outflows.

Previous theoretical and observational studies have provided detailed insights into gas accretion mechanisms from the 100,000 light-years scale down to a scale of a few hundred light-years at the center. However, gas accretion occurs a few dozen light-years from the galactic center. This limited spacial scale has hindered further understanding of the accretion process. For instance, to comprehend quantitatively the growth of black holes, it is necessary to measure the accretion flow rate (how much gas is flowing in) and to determine the amounts and types of gases (plasma, atomic gas, molecular gas) that are expelled as outflows at that small scale. Unfortunately, observational understanding has not progressed significantly until now.

"Observations of multiphase gases can provide a more comprehensive and thorough understanding of the distribution and dynamics of matter around a black hole and our observation marks the highest resolution ever achieved for multiphase gas observations in an active galactic nucleus," points out Izumi.

Izumi and his colleagues initially captured, for the first time, the accretion flow heading towards the supermassive black hole within the high-density gas disk that extends over several light-years from the galactic center (as shown in the green part in Figure 1). Identifying this accretion flow had long been a challenge due to the small scale of the region and the complex motions of gas near the galactic center. However, the research team pinpointed the location where the foreground molecular gas was absorbing the light from the active galactic nucleus shining brightly in the background. Detailed analysis revealed that this absorbing material is moving away from Earth. As the absorbing material consistently resides between the active galactic nucleus and Earth, this indicates that the team has successfully captured the accretion flow heading toward the active galactic nucleus.

The study also elucidated the physical mechanism responsible for inducing this gas accretion. The observed gas disk exhibited a gravitational force so substantial that it could not be sustained by the pressure calculated from the gas disk's motion. When this situation occurs, the gas disk collapses under its own weight, forming complex structures and losing its ability to maintain stable motion at the galactic center. Consequently, the gas rapidly falls towards the central black hole, A phenomenon known as "gravitational instability" at the heart of the galaxy.

Furthermore, the study advanced quantitative understanding of gas flows around the active galactic nucleus. By considering the density of the observed gas and the velocity of the accretion flow, the researchers were able to calculate the rate at which gas is supplied to the black hole. Surprisingly, this rate was found to be 30 times greater than what is needed to sustain the active galactic nucleus. In other words, the majority of the accretion flow at the 1-light-year scale around the galactic center was not contributing to the growth of the black hole.

So, where did this surplus gas go? High-sensitivity observations of all phase gases (medium-density molecular, atomic, and plasma; corresponding to red, blue, and pink regions in Figure 1) with ALMA detected outflows from the active galactic nucleus. Quantitative analysis revealed that the majority of the gas flowing towards the black hole was expelled as atomic or molecular outflows. However, due to their slow velocities, they couldn't escape the gravitational pull of the black hole and eventually returned to the gas disk. There, they were recycled into an accretion flow toward the black hole, completing a fascinating gas recycling process at the galactic center (as shown in Figure 2).

Reflecting on the achievements Takuma Izumi iterates, "Detecting accretion flows and outflows in a region just a few light-years around the actively growing supermassive black hole, particularly in a multiphase gas, and even deciphering the accretion mechanism itself, are monumental achievements in the quest to reveal more about supermassive black holes." Looking ahead to the future, he continues, "To gain a comprehensive understanding of the growth of supermassive black holes in cosmic history, it is essential to investigate various types of supermassive black holes that are located farther away from us. This requires high-resolution and high-sensitivity observations, and we have high expectations for the continued use of ALMA, as well as for upcoming large radio interferometers in the next generation."

https://www.tohoku.ac.jp/en/press/black_hole_feeding_and_feedback_at_the_center_active_galaxy.html