◎ Science and Technology Daily reporter Lu Chengkuan
For thousands of years, people have never stopped exploring in order to uncover the mysteries of the vast galaxy and the vast universe.
On March 24, Nature published a major advance in the study of the Milky Way in the form of a cover article. Based on sky survey observation data from the Guo Shoujing Telescope (LAMOST) and the European Space Agency's Gaia Space Telescope (Gaia) operated by the National Astronomical Observatory of the Chinese Academy of Sciences, the researchers obtained the most accurate large sample of star age information to date, and "mapped" the formation and evolution of the Milky Way in chronological order, refreshing people's understanding of the early formation history of the Milky Way.
March 24 Cover of Nature magazine
A series of origin questions are mysteries to be solved
The beautiful and vast Milky Way in the night sky has triggered endless imaginations since ancient times.
The Milky Way has integrated hundreds of billions of stars over the past ten billion years, and its basic structure includes the nuclear sphere, halo, and silver disk at the center of the Milky Way, which in turn includes thick and thin disks.
However, when did the Galactic Halo and Disk form? How is it formed? How did it assemble and evolve into today's colorful galaxy?
"This series of origins questions has long been a scientific mystery that needs to be solved by astronomers, and it is also a major scientific goal of the large-scale sky survey observation program of multiple ground and space telescopes around the world." The corresponding author of the paper, a researcher at the Max Planck Institute of Astronomy in Germany, told Dr. Mao Sheng.
Past studies have generally suggested that our galaxy underwent a dramatic process of formation during infancy. First, large amounts of metal-poor gas collapsed or gas-rich galaxies collided and merged to form the Galactic Halo. These gases then gradually cooled, forming the early silver disk, the galactic disk. Finally, over time, the gas cools further and begins to form a thin disk of the Milky Way.
"The formation of thin plates is a long and orderly process that has lasted from about 8-10 billion years ago to the present." Introduction to Maosheng.
However, "the images of these processes are mainly derived from numerical simulations and speculations about fragmented observational evidence, which need to be further confirmed and refined." Xiang Maosheng said.
The formation and evolution of the Milky Way can be divided into two stages: morning and evening
Fortunately, the emergence of astronomical observation big data has made the evolutionary image of the Milky Way being rewritten, and the era of opening the dusty history of the Milky Way has arrived.
LAMOST released tens of millions of stellar spectra data, and the Gaia satellite provided high-precision position and movement maps of 1.4 billion stars. "This combination of pearls provides a unique condition for astronomers to trace the history of the formation and evolution of the Milky Way." Xiang Maosheng sighed.
Based on data from LAMOST and Gaia, the researchers constructed a high-quality data sample containing 250,000 stars and obtained their precise ages and metal abundances.
Xiang Maosheng said that this is the first time that such a large number of large samples of stars with the highest age accuracy have been obtained in the vast space range of the Milky Way and the abundance of stellar metals, successfully breaking through the limitations of data and taking a landmark step for the study of the formation and evolution history of the Milky Way.
The study found that, chronologically, the history of the formation and evolution of the Milky Way can be divided into two distinct phases: the early stage from 13 billion years ago to 8 billion years ago and the late stage from 8 billion years ago to the present. The early stages formed the thick disks and silver halos of the Milky Way, and the late stages formed the thin disks of the Milky Way.
Schematic diagram of early integration and evolution of the Milky Way. Source: Yu Jingchuan
"The Milky Way's thick disk stars have been forming since 13 billion years ago, just 800 million years after the Big Bang. This is comparable to the time when the hubble space telescope can currently detect the emergence of the most distant galaxies. Xiang Maosheng said that the early thick disk formed about 2 billion years earlier than the major stellar halo structures we see today, which refreshed the traditional understanding of the early formation history of the Milky Way.
For the first time, the history of the formation of the Milky Way is clearly depicted
After further research, Xiang Maosheng et al. found that the formation of thick plates lasted from 13 billion years ago to 8 billion years ago, a total of about 5 billion years, during which the metal element content increased by 30 times. However, "although this cycle lasted 5 billion years, we also found that most of the pachyderm stars formed in a concentrated eruption about 11 billion years ago." Xiang Maosheng said.
Meanwhile, through age data, the researchers found that about 11 billion years ago, the hundred-handed giant Enserados dwarf galaxy (GSE) collided with the milky way as a teenager. "That's 1 billion years earlier than previously thought."
Xiang Maosheng said, "The two times coincide highly, and we think it is not accidental, but a strong hint: the dwarf galaxy GSE impact event has a significant impact on the star formation activity of the thick disk." ”
The gas that formed the thick disk star ran out about 8 billion years ago, and the thick disk formation stopped. At about the same time, new gases began to accumulate from around the Milky Way into a thinner disk, and the Milky Way disk gradually grew into the way it grows today like a sandwich cake to become larger and fatter at both ends, and the process of thin disk formation continues to this day.
At this point, an accurately depicted image of the formation and evolution of the early Milky Way on a timeline is presented. "Placing a series of important events in the formation of the Milky Way on a precise timeline to form a detailed history of the Milky Way is very important for us to recognize and understand the integration and evolutionary history of the galaxy in which we are located, which is the key significance of the current work." Emphasis on Mao sheng.
Invited by the journal Nature, reviewer Timothy Beers wrote a review article for the study. He said the study successfully helped us better understand how our milky way formed through an innovative way to estimate the date (age) of stars; and that the approach is scalable, and the picture will become clearer as data from larger samples of stars in the Milky Way become available.
Source: Science and Technology Daily Photo courtesy of the National Astronomical Observatory of the Chinese Academy of Sciences
Editor: Zhang Shuang
Review: Wang Xiaolong
Final Judgement: Wang Yu