Tsinghua University is the first detailed imaging of the galaxy’s circulating influx

In the night sky, the most striking thing is the Milky Way across the sky. Our Earth and solar system are also a drop in the ocean in the vast galaxy. However, how did massive galaxies such as the Milky Way form and evolve, and how did massive stars emerge? The current scientific community does not explain it well.

Recently, the team of the Department of Astronomy of Tsinghua University directly detected the detailed process of gas around galaxies in the early universe into galaxies through full-band data, confirming that the “cyclic inflow” with high abundance of heavy elements is the key to driving galaxy star formation, which is an important step in understanding the galaxy “ecosystem” and galaxy evolution.

On May 5, the relevant research results were published online in Science in the form of a long article.

Traditional theories contradict observations

When it comes to galaxies, the first thing that comes to mind is often a “big disk” that keeps rotating in the vacuum of the universe, but with the rapid progress in the scientific community’s understanding of galaxies in recent years, it has been found that many galaxies grow in a “gas environment” – there is a huge dark matter halo around the galaxy, which is rich in various gases. Galaxies are constantly receiving gas from their surroundings and forming an “inflow” that flows to the galaxy. At the same time, the giant black hole at the center of the galaxy converts huge amounts of gravitational potential energy into light energy, pushing the gas inside the galaxy into large-scale space, thus forming a “cosmic ecosystem”.

In the astrophysical plan for the next ten years announced by the United States in 2021, the “cosmic ecosystem” was listed as one of the important problems to be solved. A key problem in this is to explain the mechanism by which massive galaxies form and evolve.

“The traditional theory holds that massive galaxies have huge gravitational potential energy, causing the material to be heated by shock waves during the collapse process, so that the gas flowing into the galaxy produces a high temperature and cannot be effectively cooled, so that it cannot smoothly gather together to form a star.” In an interview, Cai Zheng, a professor in the Department of Astronomy at Tsinghua University, said.

However, this theory contradicts recent observations, because massive galaxies have been found in the very early universe that are forming stars violently. “This means that the detailed process of gas flowing into galaxies is not fully understood, and the process by which incoming gas drives star formation has not been revealed.” Cai Zheng said.

Details of gas entering the galaxy Courtesy of Tsinghua University

U-turn “back” airflow

Cai Zheng’s team has been exploring related issues for many years. In the process, they discovered a galaxy 11 billion light-years away in 2017, which is less than 100,000 light-years away, but Cai’s team detected a large amount of gas within 1.5 million light-years around the galaxy.

“These gases are so dense that they emit light of a certain brightness, which makes our observation very convenient.” Cai Zheng’s team used advanced imaging spectrometers to successfully detect hydrogen elements and a variety of heavy element radiation in the gas around the galaxy, and further estimated the large-scale spatial distribution of heavy elements.

However, the resulting images puzzled the team members—they found that at least 100 kiloparsecs (astronomical measure, 1,000 parsecs is about 3260 light-years), the abundance of heavy element distribution in this region is roughly equivalent to that of heavy element distribution around the sun.

“For example, if the periphery of the solar system is compared to a bustling city, the area we are exploring is the desolate no-man’s land in the west. But the detection results show that the distribution density of stores (heavy elements) in these two areas is similar, which is obviously difficult to understand. Cai Zheng said.

Faced with such a map of the distribution of heavy elements, the research team first explained the possible cause as a large number of eruptions of heavy elements in the galaxy’s interior, but this explanation could not be consistent with the actual observed dynamics of galactic matter. In this regard, the team members have thought and verified a lot, and in this process, the words of Xu Dandan, professor of astronomy at Tsinghua University, gave Cai Zheng a great inspiration.

“If you assume that the gas rich in heavy elements in the galaxy is not pushed outward after being pushed out of the galaxy, but turns around and flows inward with a certain angular momentum, it should explain the dynamic phenomenon you see.” Xu Dandan said.

Through further spectroscopic and numerical simulation analysis, the research team found that these ionized gases rich in heavy elements were most likely ejected around the galaxy by the active galactic nucleus at the center of the galaxy, cooled down through composite radiation, forbidden transition radiation and other processes, and returned to the galaxy under the combined action of gravity and environmental angular momentum, forming a “circulating cold gas flow”. This model fits perfectly with the dynamic phenomena actually observed by the team.

A clear picture for the first time

By modeling the observed gas dynamics, a majestic picture of the universe was presented to the team members – galaxies constantly pushing gas towards the surrounding dark matter halo through light pressure. At the same time, three giant “cantilever” structures like the galactic cantilever but much larger than the galactic cantilever and distributed in the dark matter halo flow into the galaxy with a certain angle and angular momentum, and a large amount of gas flows to the galaxy, thus forming a huge cycle spanning two million light years.

The inflow of gas is closely related to the circulation inflow and galaxy formation Courtesy of Tsinghua University

“This discovery provides a clear picture of how galaxies exchange matter with large-scale environments, suggesting that ‘recycling inflow’ is an important mechanism that drove the formation of massive galaxies in the early universe.” Cai said the discovery also further explains the formation of massive stars in early galaxies.

He explained that it is precisely because the gas pushed out of the galaxy and back to the galaxy contains a large amount of heavy elemental material, which is easier to cool than light elements, which greatly improves the efficiency of star formation in the galaxy.

The black hole at the center of the galaxy pushes the gas enriched by heavy elements to the dark matter halo, which is like dirt, and the “fertilized” gas is fed back to the galaxy, “nourishing” new stars. “This is a picture staged on a cosmic scale, ‘falling red is not a heartless thing, turning into spring mud is more protective of flowers’.” Cai Zheng said the process had previously only appeared in digital simulations of cosmology, but it had not attracted attention. This is the first time they have clearly presented this picture through observation.

Cai Zheng said that the research results are only for a massive galaxy. In the future, they can image more massive galaxies of different masses and environments, and understand in detail how galaxies have evolved to this day. At the same time, the current study of galaxies and cosmological research is interrelated, and this research will also play a certain role in people’s in-depth study of cosmological problems such as “dark matter” and “dark energy”. (Source: Chen Bin, China Science News)

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