Figure 1. The central region of the Circinus Galaxy observed with ALMA. The distributions of carbon monoxide (CO; reflecting the presence of medium-density molecular gas), atomic carbon (C; reflecting the presence of the atomic gas), hydrogen cyanide (HCN; reflecting the presence of high density molecular gas), and the hydrogen recombination line (H36α; reflecting the presence of ionized gas), are shown in red, blue, green, and pink, respectively. There is an active galactic nucleus at the center. This galaxy is known to have a tilted structure from the outer to the inner regions, with the central region resembling a nearly edge-on disk. The size of the central dense gas disk (green) is approximately 6 light-years: this has been observed clearly thanks to the high resolution of ALMA (see the inset for the zoom-up view). The plasma outflow travels almost perpendicular to the central dense disk. Credit: ALMA (ESO/NAOJ/NRAO), T. Izumi et al.

At the centers of many massive galaxies, there exist “supermassive black holes” with masses exceeding a million times that of the Sun. How are these supermassive black holes formed? One of the crucial growth mechanisms proposed by previous research is “gas accretion” onto the black hole. This refers to the process in which gas in the host galaxy somehow falls toward the central black hole.

The gas that gathers very close to supermassive black holes is accelerated at high speeds due to the gravity of the black hole. As a consequence of intense friction between gas particles, this gas heats up to temperatures reaching several million degrees and emits brilliant light. This phenomenon is known as an active galactic nucleus (AGN), and its 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.

Both theoretical and observational studies have provided detailed insights into gas accretion mechanisms from the 100,000 light-year scale of the galaxies down to a scale of a few hundred light-years at the center. However, the gas accretion within a much smaller region, especially within a few dozen light-years from the galactic center, has remained unclear due to its extremely limited spatial scale. For instance, to quantitatively comprehend 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 in this regard has not progressed significantly until now.

An international research team led by Takuma Izumi, an assistant professor at the National Astronomical Observatory of Japan (affiliated with NAOJ and Tokyo Metropolitan University at the time of this study), has achieved a world-first success by quantitatively measuring gas flows and their structures for all phases (plasma, atomic, and molecular) at an extremely small spatial scale of just a few light-years around a supermassive black hole, by using the Atacama Large Millimeter/submillimeter Array (ALMA). Observations of multiphase gases can provide a more comprehensive and thorough understanding of the distribution and dynamics of matter around a black hole. The observed object was the Circinus Galaxy, a representative active galactic nucleus in the nearby Universe. The achieved resolution was approximately 1 light-year. This marks the highest resolution ever achieved for multiphase gas observations in an active galactic nucleus.

In this study, the research team initially succeeded in capturing, for the first time, the accretion flow heading towards the supermassive black hole within the high-density gas disk extending over several light-years from the galactic center (green part in Figure 1). Identifying this accretion flow had long been a challenging task due to the small scale of the region and the complex motions of gas near the galactic center. However, in this instance, the research team pinpointed the location where the foreground molecular gas was absorbing the light from the background brightly shining active galactic nucleus. This identification was made possible through high-resolution observations with ALMA. Detailed analysis revealed that this absorbing material is moving in the direction away from us. Since the absorbing material always exists between the active galactic nucleus and us, this indicates that the team has successfully captured the accretion flow towards the active galactic nucleus.

Furthermore, the research team has also elucidated the physical mechanism responsible for inducing this gas accretion. The observed gas disk itself exhibited a gravitational force so substantial that it could not be sustained by the pressure calculated from the motion of the gas disk. When this situation arises, the gas disk collapses under its own weight, forming complex structures and becoming incapable of maintaining stable motion at the galactic center. As a result, the gas rapidly falls towards the central black hole. Now ALMA has clearly revealed this physical phenomenon known as “gravitational instability” at the heart of the galaxy.

In addition, this study has significantly advanced the quantitative understanding of gas flows around the active galactic nucleus. From the density of the observed gas and the velocity of the accretion flow, the accretion rate at which gas is supplied to the black hole can be calculated. Surprisingly, this rate was found to be 30 times greater than what is actually required to sustain the activity of this 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? This mystery has also been unraveled in this study—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. Through quantitative analysis, it was revealed that the majority of the gas that flowed towards the black hole was expelled as atomic or molecular outflows. However, due to their slow velocities, they couldn’t escape from the gravitational potential of the black hole and eventually returned to the gas disk. There, they were recycled back into an accretion flow towards the black hole, akin to a fountain, thus completing a fascinating gas recycling process at the galactic center (Figure 2).

Regarding the achievements of this study, Takuma 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 emphasizes the significance of this accomplishment. Looking ahead to the future, he also continues, “To comprehensively understand the growth of supermassive black holes in cosmic history, we need 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 further use of ALMA, as well as for upcoming large radio interferometers in the next generation.”

Figure 2. An illustration depicting the distribution of interstellar medium in the active galactic nucleus based on the results of this observation. High-density molecular gas flows from the galaxy towards the black hole along the plane of the disk. The material accumulated around the black hole generates a huge amount of energy, causing the molecular gas to be destroyed and transformed into atomic and plasma phases. Most of these multiphase gases are expelled away via outflows from the nucleus (including plasma outflows primarily occurring in the direction above the disk, and atomic or molecular outflows mainly occurring diagonally), but most of these outflows will fall back to the disk, acting like a gas fountain. Credit: ALMA (ESO/NAOJ/NRAO), T. Izumi et al.

These observation results were published as Takuma Izumi et al. “Supermassive black hole feeding and feedback observed on sub-parsec scales” in Science on November 3, 2023 (DOI: 10.1126/science.adf0569).

This work is supported by NAOJ ALMA Scientific Research Grant No. 2020-14A, 2022-21A, ALMA Japan Research Grant for the NAOJ ALMA Project Code NAOJ-ALMA-271, a Grant-in-Aid from the Japan Society for the Promotion of Science (JP20K14531, JP21H04496, JP17H06130, JP21K03632, JP19K03937, JP20K14529, JP20H00181, JP22H00158, JP22H01268).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI). ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.