Supermassive black holes in the universe are swallowing gas round them. The infalling gas is called black hole accretion flow. Recently, the research group lead by Prof., Feng Yuan at Shanghai Astronomical Observatory, together with the group lead by Prof. Zhiyuan Li at Nanjing University, find direct evidence for the existence of strong wind launched from accretion flow around black holes. This result indicates that we have made an important step toward understanding accretion processes around black hole. This result was recently published in Nature Astronomy.

There exists a supermassive black hole in almost every galaxy in the universe. The gas round the black hole will be accreted and form an accretion disk. Strong radiation is emitted from the accretion disk, which is the origin of the radiation in the first image of black holes people have obtained in 2019.

Depending on the gas temperature, black hole accretion flows are divided into two types, namely cold and hot ones. Theoretical studies performed by the group in SHAO over ten years ago predicted that strong wind must exist in hot accretion flows. Most recently, these winds are believed to play a crucial role in galaxy evolution, according to the famous cosmological simulation Illustris-TNG.

However, the direct observational evidence for the wind has always been difficult to obtain. By analyzing the high-resolution observational data from the Chandra telescope, we find redshifted and blueshifted iron emission lines. The data tells us that these emission lines come from gas with outflowing velocity of 3000 km per second, and the temperature of the gas should be about 12 KeV.

To interpret these observational data, we have performed magnetohydrodynamical numerical simulations of accretion flows. We then calculated the emission lines emitted by the wind launched from accretion flows. We find that the predicted emission lines are in excellent agreement with the observational data. This study provides direct evidence for the existence of wind, confirming the previous theoretical predictions. It is an important step for advancing our understanding of black hole accretion and galaxy evolution.

The observed blueshifted and redshifted iron emission lines and their comparison with the theoretical predictions based on numerical simulations. Taken from Shi et al. 2021, Nature Astronomy (in press).

Paper link：https://www.nature.com/articles/s41550-021-01394-0