Free Astronomy Magazine January-February 2024

40 JANUARY-FEBRUARY 2024 ASTRO PUBLISHING absorbing material al- ways exists between the active galactic nu- cleus and us, the team has successfully cap- tured the accretion flow towards the ac- tive galactic nucleus. Furthermore, the re- search team has also elucidated the physi- cal mechanism re- sponsible for inducing this gas accretion. The observed gas disk ex- hibited a gravitational force so substantial that it could not be sustained by the pres- sure calculated from the motion of the gas disk. When this situa- tion arises, the gas disk collapses under its weight, forming complex structures and becoming inca- pable of maintaining stable motion at the galactic center. As a result, the gas rapidly falls towards the cen- tral black hole. ALMA has revealed this physical phenom- enon known as “grav- itational instability” at the galaxy’s heart. In addition, this study has significantly ad- vanced the quantitative understand- ing of gas flows around the active galactic nucleus. The accretion rate at which gas is supplied to the black hole can be calculated from the den- sity of the observed gas and the ve- locity of the accretion flow. Surprisingly, this rate was found to be 30 times greater than what is re- quired to sustain the activity of this active galactic nucleus. In other words, most of the accretion flow at atomic or molecular outflows. However, due to their slow ve- locities, they couldn’t escape from the grav- itational 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 com- pleting a fascinating gas recycling process at the galactic cen- ter. Regarding the achievements of this study, Takuma Izumi states, “Detecting ac- cretion flows and outflows in a region just a few light-years around the actively growing supermas- sive black hole, partic- ularly in a multi- phase gas, and even deciphering the ac- cretion mechanism it- self, are indeed mon- umental achieve- ments in the history of supermassive black hole research.” He emphasizes the sig- nificance of this ac- complishment. Look- ing ahead to the fu- ture, he also continues, “To compre- hensively understand the growth of supermassive black holes in cosmic history, we need to investigate vari- ous types of supermassive black holes located farther away. This re- quires high-resolution and high-sen- sitivity observations, and we have high expectations for the further use of ALMA and for upcoming large radio interferometers in the next generation.” T he 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 recombina- tion line (H36 α ; reflecting the presence of ionized gas), are shown in red, blue, green, and pink, respectively. There is an active galactic nu- cleus 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 ap- proximately six light-years: this has been observed 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. [ALMA (ESO/NAOJ/NRAO), T. Izumi et al.] 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 study also unravels this mys- tery—high-sensitivity observations of all phase gases with ALMA-de- tected outflows from the active galactic nucleus. Quantitative analysis revealed that most of the gas that flowed toward the black hole was expelled as !