On February 12, the internationally renowned academic journal Science Advances published online a new study by Professor He Zhicheng, special associate researcher of the team of Professor Wang Tinggui and Professor Liu Guilin, Department of Astronomy, School of Physics, University of Science and Technology of China. He Zhicheng has created a new set of methods to measure the physical properties of ionized gas in galaxies. For the first time, the central high-speed outflow of active galaxies (galaxies with intense activity) is accelerated on the scale of 100 light-years, which is more than two orders of magnitude larger than the classical black hole-accretion disk wind scale.

Rapid outflow affects galaxy evolution

The current galaxy formation and evolution theory believes that in general, there is an active supermassive black hole in the center of the galaxy, the mass of this giant black hole can be more than 1 billion times the mass of the sun, and the huge energy released by it will effectively regulate the evolution of the galaxy, prevent the galaxy from growing too large, this mechanism is called "active galactic nucleus feedback mechanism".

An active supermassive black hole acts as the "engine" at the center of an active galaxy, forming a rotating accretion disk as it accrets material around the black hole, ionizing and "blowing" large amounts of gas into interstellar space, creating a high-speed outflow. The speed of this high-speed outflow can reach one-tenth of the speed of light, and the intensity of the resulting intense electromagnetic radiation can be more than a thousand times the total luminosity of the stars in its galaxy (that is, the intensity of electromagnetic radiation), and has an important impact on the overall evolution of the galaxy in which it is located. This outflow "transfers" matter and energy from the galactic nucleus region to the outskirts of the galaxy, and is one of the main forms of feedback mechanism of active galactic nuclei. However, the physical properties, origin, acceleration mechanism and its effect on the evolution of active galaxies are still poorly understood.

The scale of high velocity outflow is one of the core parameters to understand its origin and measure its impact on the galactic environment. This is because the scale of the high-speed outflow is as large as the scale of the galaxy, so that it can interact with the matter in the entire galaxy, and then affect the evolution of the entire galaxy.

But it's not easy for scientists to study the rapid outflow of galaxies. Because the galaxies in the universe are relatively far away from the Earth, the high-speed outflow scale of these galaxies is very small from the Earth, and it is only a "point" on the image, and its specific size cannot be directly inferred.

Previously, the density sensitive ion excited state absorption line was generally used to calculate the high velocity outflow scale, but this method is technically complicated and the measurement of the gas density of the high velocity outflow is not accurate enough, which affects the measurement results of the high velocity outflow scale. Therefore, in the past 30 years, people have used this method to calculate the high-speed outflow scale of only a few dozen active galaxies, and the magnitude of the scale is still controversial.

Acceleration on a scale of hundreds of light-years

The origin, scale and energy of the high velocity outflow are related to the accretion process of supermassive black holes and the evolution of galaxies. Scientists believe that the high-speed outflow is likely suppressing star formation in the galaxy. In order to get the exact answer to this question, He Zhicheng kept trying and exploring. To break through the dilemma, he proposed a new way to measure the physical parameters of the high-speed outflow from the new perspective of the absorption line light change (that is, the change in the depth or speed of the absorption line), and using the Sloan Large sample Survey data, found that most of the high-speed outflow is more than tens of light-years in size, and its energy is enough to affect the overall evolution of the galaxy. His work was published in the journal Nature Astronomy in 2019.

After several years of hard work, He developed a new method using both amplitude and phase information of the response function, and successfully obtained kinematic information about the high-speed outflow of quasars (the most distant and luminous class of active galaxies from Earth). Using this new method, He found for the first time the acceleration of the high velocity outflow from the center of an active galaxy on the scale of 100 light-years, and more than 100 times the scale of the classical black hole-accretion disk wind.

Interstellar dust or speed 'engines'

The next question is, where did these rapid outflows come from? What is their acceleration mechanism? He et al. hypothesized that interstellar dust is likely to play an important role in the high-speed outflow acceleration process, because the ultraviolet radiation interaction between dust and the black hole accretion disk is much larger than the free electron Thompson scattering cross section, so the dusty gas is more easily accelerated by the accretion disk radiation.

Further analysis confirmed this hypothesis: the researchers calculated that the amount of dust needed to accelerate the high-speed outflow of gas in the black hole accretion disk radiation-dust interaction model was identical to the observed dust extinction (the phenomenon of light passing through the dust and being darkened by the dust) within one standard deviation.

This finding suggests that dust plays a key role in the coupling between accretion disk radiation and the interstellar medium, allowing the high-speed outflow to have a significant effect on star formation activity in the host galaxy (the galaxy where the supermassive black hole resides) : If the medium in the galaxy contains dust, the radiation from the black hole's accretion disk will more easily transfer energy to the medium in the galaxy, and it will be easier to accelerate the medium and blow it directly out of the galaxy, so there will be less medium available to form stars, and the galaxy will not continue to increase in mass.

It is worth mentioning that the new method established by He Zhicheng et al. to measure the density of ionized gas in galaxies overcomes the shortcomings of the traditional method which is limited by the velocity dispersion of gas (that is, it can only be applied to narrow absorption lines). He Zhicheng said that in today's era of rapidly changing time domain astronomy and large accumulation of repeated observation data, the research team will fully explore the potential of this new method and develop it into a universal method for measuring the density of ionized gases.



Article source: "Science and Technology Daily" (February 21, 2022) reporter Wu Changfeng

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