On March 24, Beijing time, the international scientific journal Nature published a major achievement in the form of a cover article by researchers at the Max Planck Institute for Astronomy in Germany in collaboration with Dr. Maosheng and Professor Hans-Walter Rix. Based onGuo Shoujing Telescope, a major national scientific and technological infrastructure operated by the National Astronomical Observatory of the Chinese Academy of Sciences(LAMOST) andESA Astrometry Satellite Gaia Telescope(Gaia) sky survey data, researchers have obtained the most accurate large sample of stellar age information to date, clearly reconstructing the formation and evolution of the Milky Way during its infancy and adolescence according to the time series, rewriting people’s understanding of the early formation history of the Milky Way.
The age of a star is one of the most difficult physical quantities to accurately measure in astronomy.
The beautiful and vast Milky Way in the night sky has aroused countless imagination and endless exploration since ancient times. Our Milky Way is an ordinary disk galaxy in countless cosmic islands. Like other similar galaxies, it has integrated hundreds of billions of stars over the past ten billion years. These stars are mainly distributed in the Milky Way’s halo and silver disk, which in turn includes a geometrically relatively thick disk and a relatively thin and more extended disk. However, when did the Milky Way’s halo and Milky Way disk form, and how did they assemble and evolve into today’s colorful galaxy?It has always been a scientific mystery that astronomers urgently need to solve.It is also the main scientific goal of a large-scale astronomical survey program for multiple ground and space telescopes around the world.
LAMOST publishes tens of millions of star spectra, becoming the cornerstone of the digital galaxy. The Gaia satellite launched by ESA provides maps of the positions and movements of 1.40 billion stars. This perfect combination provides astronomers with a unique advantage in tracing the integration and evolution history of the Milky Way. To Dr. Maosheng and Professor Rix based on LAMOST and Gaia data,High-quality data samples containing 250,000 subgiant stars were constructed and their precise ages were obtained.The age of a star is the most difficult physical quantity to accurately determine, and it can also be said to be one of the most difficult physical quantities to accurately measure in the field of astronomy.
Thanks to the LAMOST Milky Way Survey and other international sky surveys, obtaining the ages of large samples of stars has become a reality in the past few years. However, the typical age error of large samples of stars obtained by previous studies is 20% or more, and the star samples that achieve 10% age determination accuracy are very small, and the space and parameter range of the samples are also very limited. Using the LAMOST spectroscopic big data, the atmospheric parameters of 7 million stars were accurately determined in the Lush, and combined with the Gaia data, high-precision stellar luminosity and orbital kinematics parameters were obtained. 250,000 subgiant stars were selected from these 7 million stars, and their precise ages were determined.The average age accuracy of the sample is 7%.The metal element abundance coverage ranges from -2.5 (one-300th of the metal content of the sun) to 0.5 (three times the metal content of the sun), with a space coverage of 30,000 light-years. This is the first time that such a large sample of stars has been obtained within such a vast space and stellar metal abundance range of the Milky Way. It has successfully broken through the limitations of data and taken a landmark step for the study of the history of the formation and evolution of the Milky Way.
[The first "clear picture" of the history of the formation of the Milky Way]
Based on their motional characteristics and chemical DNA (elemental abundance), they divided 250,000 stars into two groups: those characterized by the formation of the Milky Way’s extended thin disk during a relatively dynamically quiet process, and those characterized by the formation of the Milky Way’s halo and thick disk during a dynamically violent turbulent process. The research team found that the ages of these two groups of stars, bounded by about 8 billion years, were equally clearly divided into two distinct groups. That is, in terms of time, the integration and evolution history of the Milky Way is divided into two well-defined stages, 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 stage formed the Milky Way’s thick disk and halo, and the late stage formed the Milky Way’s thin disk.
The ultra-high temporal resolution allowed the research team to obtain a clear picture of the early integration and augmentation of the Milky Way: stars in the Milky Way’s thick disk have been forming since 13 billion years ago, just 800 million years after the Big Bang (corresponding to a cosmological redshift of 7). The oldest thick disk stars are even about 1 billion-2 billion years older than the stars in the Milky Way’s inner halo. The inner halo structure of the Milky Way is thought to be mainly caused by the collision of the Hundred-Handed Giant Enceladus Dwarf Galaxy (GSE) into the Milky Way and were accreted and formed in time. That is,The early thick disk formed 1 billion-2 billion years before the main stellar halo structure we see today, refreshing conventional knowledge of the galaxy’s early formation history.
Diagram of the early formation and evolution of the Milky Way:138Billions of years ago, the Big Bang.130Billions of years ago, thick plates began to form,110Million years ago, the halo formed.80Hundreds of millions of years ago, the Milky Way’s thin disk formed. (Image source: Yu Jingchuan)
After further investigation, Xiang Maosheng et al. also found that although the formation of the thick disk lasted for about 5 billion years from 13 billion years ago to 8 billion years ago, the metal content increased by 30 times. However, although the cycle lasted for 5 billion years, most of the thick disk stars formed in a concentrated explosion about 11 billion years ago. At the same time, they found that the merger of the 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 are highly consistent, which the research team believes is no accident, but strongly suggests that the star formation activity of the thick disk was significantly stimulated by the GSE impact event.
The gas that formed the thick disk stars was exhausted about 8 billion years ago, and the formation of the thick disk stopped. Around the same time, new gas began to gather from around the Milky Way into a thinner disk to form the Milky Way’s thin disk stars. The thin disk formation process continues to this day. At this point, a precise image of the formation and evolution of the early Milky Way on a timeline is presented, Nature journalThe reviewers’ evaluation of this result is the first to provide such a clear picture of the history of the galaxy’s formation.The Milky Way, as a representative of ordinary galaxies, is a key laboratory for our study of the formation and evolution of ordinary galaxies in the universe, which can help astronomers trace the fascinating stories that have taken place from the very early universe to today.
