by Scientists led by Takuma Izumi from the National Astronomical Observatory of Japan have successfully observed the active galactic nucleus of the Circinus Galaxy, a nearby galaxy to the Milky Way, using the Atacama Large Millimeter/Submillimeter Array (ALMA) in Chile. This breakthrough observation provided a high-resolution view of gas flows and their structures around a supermassive black hole, shedding light on the feeding mechanism and gas recycling process.
The Circinus Galaxy has a supermassive black hole at its center, which is a million times more massive than the Sun. The formation mechanisms of such supermassive black holes have been a mystery to astronomers. One proposed mechanism suggests that gas accretes onto the black hole as it moves towards the center of the galaxy.
As gas approaches the supermassive black hole, its intense gravitational pull causes the gas to accelerate. This acceleration leads to an increase in friction between gas particles, resulting in the heating of the gas to temperatures as high as several million degrees. This process emits brilliant light, known as an active galactic nucleus (AGN), which can be brighter than all the stars in the galaxy combined. Additionally, some of the gas that falls towards the black hole, known as the accretion flow, is thought to be blown away by the immense energy of the AGN, resulting in outflows.
Previous studies have provided insights into gas accretion mechanisms on larger scales but understanding the process on a smaller scale, a few dozen light-years from the galactic center, has been a challenge. To comprehend the growth of black holes in more detail, scientists need to measure the accretion flow rate and determine the type of gases expelled as outflows.
Using ALMA, Izumi and his team were able to capture the accretion flow heading towards the supermassive black hole in the Circinus Galaxy. The high-density gas disk extending over several light-years from the galactic center made the identification of this accretion flow challenging due to the complexity of gas motions in this region. However, the researchers were able to pinpoint the location where foreground molecular gas absorbed light from the bright AGN in the background. Analysis confirmed that this absorbing material is moving away from Earth, thus indicating the successful capture of the accretion flow towards the AGN.
The study also revealed the physical mechanism responsible for inducing the gas accretion. The observed gas disk exhibited a significant gravitational force that could not be sustained by the pressure calculated from the gas disk’s motion. This led to the collapse of the gas disk under its own weight, forming complex structures and losing its ability to maintain stable motion at the galactic center. As a result, the gas rapidly falls towards the central black hole, a phenomenon known as gravitational instability.
Quantitative analysis of gas flows around the AGN provided by ALMA allowed the researchers to calculate the rate at which gas is supplied to the black hole. Surprisingly, the rate was found to be 30 times greater than what is needed to sustain the AGN, suggesting that the majority of the accretion flow was not contributing to the black hole’s growth.
Further observations with ALMA detected outflows from the AGN, indicating that the majority of the gas flowing towards the black hole was expelled as atomic or molecular outflows. However, these outflows were not able to escape the gravitational pull of the black hole and eventually returned to the gas disk. This created a fascinating gas recycling process at the galactic center.
The findings of this study represent a significant advancement in our understanding of the mechanisms behind the growth of supermassive black holes. Izumi emphasizes the importance of investigating various types of supermassive black holes located farther away in order to gain a comprehensive understanding of their growth throughout cosmic history. High-resolution and high-sensitivity observations, such as those conducted with ALMA and upcoming large radio interferometers, will play a crucial role in furthering our knowledge of these cosmic phenomena.
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- Source: Coherent Market Insights, Public sources, Desk research
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