Astronomers observe supermassive black hole feeding and feedback on sub-parsec scales

A global research team under the direction of Takuma Izumi, an assistant professor at the National Astronomical Observatory of Japan, has made significant progress in using the Atacama Large Millimeter/submillimeter Array (ALMA) to observe the close-by active galactic nucleus of the Circinus Galaxy at an exceptionally high resolution (roughly 1 light-year).

The findings of this discovery are reported in Science under the title "Supermassive black hole feeding and feedback observed on sub-parsec scales."

This is the first quantitative measurement of gas fluxes and their structures in the close proximity of a supermassive black hole in all phase gases, including plasma, atomic, and molecular, down to a few light-years. Consequently, the accretion flow towards the supermassive black hole has been clearly caught by the researchers, and it has been discovered that this accretion flow is produced by a physical phenomenon called "gravitational instability."

Moreover, the group discovered that a large fraction of this accretion flux is not used in the black hole's formation. in an alternative, the majority of the gas is ejected from the black hole's vicinity in atomic or molecule outflows and then recycles back into the gas disk to take part in another accretion flow in the direction of the black hole. This process is similar to a water fountain. These results provide an important step towards a thorough understanding of supermassive black hole formation processes.

"Supermassive black holes" are black holes with masses greater than a million times that of the sun that are located at the core of many massive galaxies. These gigantic black holes: how do they form? A key growth mechanism that has been suggested by earlier studies is "gas accretion" onto the black hole. This is the mechanism via which gas in the host galaxy gravitates in the direction of the black hole in the center.

The supermassive black hole's gravity accelerates gas that congregates extremely near to it at extraordinary speeds. This gas warms up to several million degrees and generates bright light as a result of strong friction between the particles. An active galactic nucleus (AGN) is this phenomena, and it can occasionally be brighter than the galaxy's total starlight. It's interesting to note that outflows are believed to result from the massive energy of this active galactic center blowing away some of the gas that falls towards the black hole (accretion flow).

In-depth understandings of gas accretion mechanisms from the 100,000 light-year scale of the galaxies down to a few hundred light-years at the core have been obtained via both theoretical and observational investigations. But because of its extremely tiny spatial scale, the gas accretion within a considerably smaller region—especially within a few dozen light-years from the galactic center—has remained elusive.

Measurements of the accretion flow rate (the quantity of gas streaming in) and the volumes and types of gases (plasma, atomic gas, molecular gas) that are released as outflows at that tiny scale, for example, are required to quantitatively understand the formation of black holes. Sadly, until lately, observational understanding in this area has not advanced all that much.

In this work, the accretion flow towards the supermassive black hole within the high-density gas disk that spans many light-years from the galactic center was first successfully captured by the research team. Because of the region's tiny size and the intricate movements of gas close to the galactic center, identifying this accretion flow has long been a difficult undertaking.

In this case, however, the researchers identified the precise area where light from the background brightly flashing active galactic center was being absorbed by the foreground molecular cloud. The high-resolution observations with ALMA enabled this identification. This absorbing element is traveling away from us, according to a thorough examination. The fact that there is constantly absorbing material between us and the active galactic nucleus suggests that the team has been effective in capturing the flow of accretion towards the nucleus.

Moreover, the scientific group has also clarified the physical process that causes this gas accumulation. The gravitational pull of the detected gas disk was so great that the pressure derived from the disk's velocity was unable to support it.

This leads to the collapse of the gas disk under its own weight, resulting in the formation of intricate structures and the loss of its ability to sustain steady motion near the galactic center. Consequently, the gas descends quickly in the direction of the center black hole. The physical phenomena at the center of the galaxy known as "gravitational instability" has now been vividly exposed by ALMA.

Furthermore, the quantitative knowledge of gas movements surrounding the active galactic nucleus has been greatly increased by this work. The accretion rate at which gas is fed into the black hole may be computed using the density of the observed gas and the velocity of the accretion flow. This rate was discovered to be surprisingly large, thirty times more than what is really needed to maintain the activity of this active galactic nucleus.

Stated differently, the black hole was not growing because most of the accretion flux surrounding the galactic center, at the 1-light-year scale, was not contributing to its formation. So where did all of this extra gas go? This work has also solved this mystery: ALMA discovered outflows from the active galactic nucleus from high-sensitivity measurements of all phase gases (medium-density molecular, atomic, and plasma; red, blue, and pink areas in the first figure above).

It was discovered by quantitative research that most of the gas that was directed into the black hole was released as discharges of molecules or atoms. They ultimately made their way back to the gas disk since they were unable to escape the black hole's gravitational pull because of their sluggish velocity. There, they completed an intriguing gas recycling process in the galactic center by being recycled back into an accretion flow towards the black hole, much like a fountain (second image).

In terms of this research's accomplishments, Izumi states, "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 indeed monumental achievements in the history of supermassive black hole research."

He goes on, "We need to investigate various types of supermassive black holes that are located farther away from us in order to fully understand the growth of supermassive black holes in cosmic history." We have great hopes for the continued use of ALMA and for planned big radio interferometers in the next generation, as this calls for high-resolution and high-sensitivity observations."