Based on data from two telescopes, astronomers have drawn images of the early evolution of the Milky Way

The Beijing News reporter learned today (March 24) that based on the sky survey observation data of the Guo Shoujing Telescope (LAMOST) and the Gaia Telescope (Gaia), researchers obtained the most accurate large sample of star age information to date, clearly depicting the formation and evolution of the Milky Way in its infancy and adolescence - 13 billion years ago to 8 billion years ago the Formation of the Milky Way Thick Disk, 11 billion years ago the formation of the Silver Halo, 8 billion years ago to the formation of the Milky Way Disk. This study refreshes the understanding of the early formation history of the Milky Way.

On March 24, Beijing time, the international scientific journal Nature published this important research result in the form of a cover article.

Schematic diagram of the early integration and evolution of the Milky Way: the Big Bang 13.8 billion years ago, the thick disk began to form 13 billion years ago, the silver halo formed 11 billion years ago, and the thin disk of the Milky Way formed 8 billion years ago to the present. Photo/ Yu Jingchuan

The two telescopes are "beaded together", and the astronomical observation big data opens the dusty history of the Milky Way

The beautiful and vast Milky Way in the night sky has triggered endless imagination and exploration since ancient times. The Milky Way is an ordinary disk galaxy among countless cosmic islands, and like other similar galaxies, it has integrated hundreds of billions of stars over the past ten billion years. Depending on their position, these stars are mainly distributed on several characteristic structures, including nucleospheres, halos, and silver disks, of which the silver disks include a geometrically relatively thick disk and a relatively thin and more extended disk, on which the Sun is located.

However, when and how did these structures of the Milky Way galaxy form? How did it assemble and evolve into today's brilliant Milky Way? This series of origins questions has been a scientific mystery that astronomers have worked hard to solve, and it is also the main scientific goal of the large-scale astronomical survey observation program of multiple ground and space telescopes around the world.

Past research has generally suggested that the Milky Way underwent a violent process of formation during infancy (very early), with large amounts of metal-poor gas collapsing (elements other than hydrogen and helium are astronomically called metals) or gas-rich galaxies colliding with each other and merging to form the Galactic Halo. The gas then gradually cooled to form 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 continued from about 8 billion to 10 billion years ago to the present.

"That is, previous studies, to be precise, prior to 2018, generally agreed that silver halos predate the formation of silver discs." The first author of the article, Dr. Xiang Maosheng, a researcher at the Max Planck Institute of Astronomy in Germany, told the Beijing News reporter.

However, these images are mainly derived from numerical simulations and speculation about fragmented observational evidence. "A 2018 study showed that the actual image is more complex than this pattern, and the silver halo may have been the result of the early Milky Way accretion dwarf galaxy." But little is known about this early milky way, what did it look like? What effect did the accretion merger event that led to the birth of the silver halo have on it?

Fortunately, the emergence of astronomical observation big data is rewriting the evolution of the Milky Way, and the era of opening the dusty history of the Milky Way has arrived.

The Guo Shoujing Telescope, a major national scientific and technological infrastructure operated by the National Astronomical Observatory of the Chinese Academy of Sciences, released tens of millions of stellar spectral data, which has become the cornerstone of the digital Milky Way. The Gaia Telescope, an astrometric satellite launched by the European Space Agency, provides high-precision position and movement maps of 1.4 billion stars. The combination of the two offers astronomers a unique advantage in tracing the history of the galaxy's integration and evolution.

Get the exact age of 250,000 subgiants

Based on data from the Guo Shoujing Telescope and the Gaia Telescope, researchers at the Max Planck Institute for Astronomy in Germany, Dr. Mao Sheng and Professor Hans-Walter Rix, constructed a high-quality data sample containing 250,000 subgiant stars and obtained their precise ages.

The age of a star is the most difficult to accurately determine the physical quantity of the star, and it is arguably one of the most difficult physical quantities in the field of astronomy to accurately measure. "Human lifespan is only 100 years, and it is impossible to time a star with a lifespan of up to 10 billion years, so scientists judge the age of the star by its temperature, luminosity, and element abundance (content)." Xiang Maosheng said.

Stars largely retain the chemical composition of the environment in which they were born, and their origins can be traced back by analyzing the chemical composition of stars. The spectrum of stars is like the DNA of stars, through which astronomers can determine the temperature of stars, the abundance (content) of metal elements, etc., and solve the "mystery of life" of stars.

The Guo Shoujing telescope can observe 4,000 celestial bodies at the same time, released tens of millions of stellar spectral data, thanks to the Guo Shoujing telescope Milky Way Sky Survey and other projects, the age of obtaining large samples of stars has gradually become a reality in the past few years. Although the problem of "large samples" was solved, the previous study obtained a stellar average age error of 20% or greater, a stellar sample with an error of 10% was small, and the spatial and parameter range of the samples was also very limited.

"So this time, we chose subgiant stars in the stars as the object of study." Xiang Maosheng explained that subgiant stars are stars that are in the transition from the evolutionary stage of the main sequence of stars to the evolutionary stage of red giants, and when the abundance of elements is known, their luminosity and age have a very good one-to-one correspondence, and the luminosity of stars of different ages varies greatly. Therefore, once their elemental abundance and luminosity are known, their age is easier to accurately determine.

But on the other hand, stars evolve very rapidly during the subgiant stage, and subgiant stars in the "window" of certain ages are relatively rare. Using the spectral big data of Guo Shoujing's telescope, xiang Maosheng accurately determined the atmospheric parameters of 7 million stars, and combined with the data of Gaia telescope, the high-precision stellar luminosity and orbital kinematics parameters were obtained. From these 7 million stars, 250,000 subgiant stars were screened and their precise ages were determined, with an average age error of 7%, metal abundance coverage from -2.5 (1/300th of the solar metal content) to 0.5 (3 times the solar metal content), and a spatial coverage of 30,000 light-years.

This is the first time that the high-precision age of such a large sample of stars has been obtained in such a wide space range and the metal abundance of stars in the Milky Way, 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 evolutionary history of the Milky Way is divided into two distinct phases

Based on motion characteristics and chemical DNA (elemental abundance) identification, the researchers divided the 250,000 stars into two groups: stars formed in the Milky Way's extended thin disks in a relatively quiet process of dynamics; and stars in a silver halo and thick disk formed in a dynamic process of violent turbulence.

The team found that the two groups of stars, bounded by the age of about 8 billion years, were also clearly divided into two distinct groups. That is, in terms of time, the history of integration and evolution of the Milky Way is 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.

The ultra-high temporal resolution gave the team a clear picture of the early integration and enrichment of the Milky Way :the Milky Way's thick disk stars began to form 13 billion years ago, just 800 million years after the Big Bang. The oldest pachyderm stars are even about 2 billion years older than halo stars in the Milky Way (silver halos mainly refer to internal halos). The galactic halo structure is thought to have been formed primarily when the Gaia-Sausage-Enceladus dwarf galaxy (GSE) collided with the Milky Way and was accretized and merged.

"That is, early thick disks formed 2 billion years before the halo structure of major stars." This also means that when answering the question of when the halo and the silver disc formed, the answer is that the silver disc predates the formation of the halo, which provides a clear picture of the timeline of the early formation of the Milky Way. Xiang Maosheng said.

Dwarf galaxy impact events accelerate the formation of thick disks

Solving the question of "when to form", then how did the halo and silver disc "form"?

After further research, Xiang Maosheng et al. found that the formation of thick plates lasted for about 5 billion years from 13 billion years ago to 8 billion years ago, during which the metal element content increased by 30 times. However , most pachyllic stars formed in a concentrated eruption about 11 billion years ago. At the same time, through age data studies, they found that the merger of dwarf galaxy GSE and the early Milky Way occurred about 11 billion years ago, which is 1 billion years earlier than previously thought. The two ages coincide highly, and the team believes that this is no accident, but rather a strong hint that the star-forming activity of the thick disk was significantly stimulated by the GSE impact event.

The researchers reasoned that the gas that formed the thick disk star ran out about 8 billion years ago, and the thick disk formation stopped. Around the same time, new gas began to accumulate from around the Milky Way into a thinner disk to form a thinner disk star in the Milky Way. The thin disc formation process continues to this day.

Eventually, a precisely documented 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 understanding and understanding the history of the integration and evolution of our galaxy, which is the key significance of the current work." Emphasis on Mao sheng.

As a representative of ordinary galaxies, the Milky Way is a "key laboratory" for human research on the formation and evolution of general galaxies in the universe, which can help astronomers trace the wonderful stories that have occurred from the very early universe to today.

Reviewer Timothy C. Beers commented that this is the clearest image of the evolution of the Milky Way. Invited by the journal Nature, Timothy C. Beers wrote an opinion piece for the study, saying that the study successfully helped us better understand how the Milky Way formed by estimating the date of birth (age) of stars through an innovative approach. And the approach is scalable, and the picture will become clearer as data from larger samples of stars in the Milky Way become available.

Beijing News reporter Zhang Lu

Edited by Liu Mengjie Proofreader Yang Xuli