Radiative Feedback Shaping the Close Environments of Accreting Massive Black Holes

Kyoto, Japan -- How are black holes formed? We may now have a clearer picture.

Yoshihiro Ueda and his international research group have completed a large systematic study of black holes in unprecedented detail. The results shed light on a longtime mystery regarding the location and evolution of the obscuring material surrounding supermassive black holes.

The study, published in Nature, was conducted in collaboration with multiple institutions including Pontificia Universidad Católica in Chile, and ETH Zurich in Switzerland.

In the universe, the majority of black holes are obscured by surrounding gas and dust. The location and evolution of these materials have been the subject of intense research and debate.

As the surrounding matter is accreted by a supermassive black hole, it emits intense radiation. And then, this object is observed as an active galactic nucleus, AGN. It has been known the covering fraction of the surrounding mass decreases with the luminosity. Many scientists have suggested that this phenomenon is driven by an increase in the inner radius of the obscuring material due to sublimation of dust heated by the radiation.

However, the mechanism regulating obscuration remained unclear due to a lack of a large and complete database on the dynamics of black holes.

Fortunately, NASA launched the Swift X-ray observatory carrying Burst Alert Telescope in 2004 and discovered more than several hundred black holes. Since then, researchers have been measuring their characteristics using X-ray and optical telescopes. In 2013, they began to assemble their data and began this project named BAT AGN Spectroscopic Survey (BASS).

In this collaborative study, they found that antagonism between the strong gravity of a black hole -- the inward force -- and the radiation pressure coming from it -- the outward force -- is the primary mechanism regulating the distribution of the circumnuclear material. The results imply that the bulk of the obscuring dust and gas in these objects is located within the black hole's sphere of influence.

The research group states that this analysis has brought insight into the evolutionary growth of black holes. For example, when a small black hole with low gravity swallows the surrounding gas rapidly -- so that the radiation output becomes higher than its mass -- the radiation pressure expels most of the obscuring material.

As a result, the black hole stops growing as it can only feed on things that fall into it. They also comment that observed outflows of dusty gas could ultimately interact with the host galaxy potentially affecting future star formation.

Ueda states that with further technological developments of high resolution spectrometers, measurement of dusty gas can be done with higher accuracy. He hopes to continue using the orbiting X-ray astronomy telescope, to further elucidate the physical state of the gas.