Guide:
Kant once said: "There are two things in the world that can deeply shake people's hearts, one is the brilliant starry sky above our heads, and the other is the lofty moral code in our hearts." At night, the quiet and deep Galaxy overhead has touched the heartstrings of many people, there are countless stars, there are also historical memories that have been sealed for tens of billions of years, and there are truths and truths that astronomers are struggling to pursue. How did the ten-billion-year-old Galaxy expand its territory and gradually grow into what it is today? What grand stories have happened in the course of its life? The Milky Way is still hanging high, and the mystery of how to solve it is unsolved. With the advancement of astronomical instruments and science and technology, we are constantly approaching this vast and brilliant Milky Way, peeking into its posture, recalling its growth, opening its past, and revealing its truth.
On March 24, Nature published a study in the form of a cover article: Researchers at the Max Planck Institute for Astronomy in Germany, Dr. Mao Sheng and Professor Hans-Walter Rix, used survey observation data from the National Astronomical Observatory of the Chinese Academy of Sciences (LAMOST) and the Gaia Telescope (Gaia), an astrometry satellite of the European Space Agency, to obtain the most accurate information on the age of large samples of stars to date. According to the time series, the formation and evolution of the Milky Way in infancy and adolescence are clearly restored, and people's understanding of the early formation history of the Milky Way is refreshed. In this issue, Mr. Astronomy, let's see the Milky Way from the beginning of time!
Written by | Shuang Li and Dan Wang (LAMOST Operation and Development Center)
Editor-in-charge | Han Yueyang, Lü Haoran
Figure 1: The Milky Way taken in cold lake in Qinghai, Photo: Deng Licai
Our Milky Way galaxy is representative of an ordinary disk galaxy in countless cosmic islands, and like other similar galaxies, it gathers hundreds of billions of stars. These stars are roughly distributed among several characteristic structures, the most important of which are the nuclear sphere at the center of the Milky Way, the flattened silver disk and the silver halo wrapped around the silver disk, which in turn includes a geometrically relatively thick disk and a geometrically thinner and more extended disk. The thick disk is mainly located in the interior of the Milky Way, and from the tangential appearance of a rugby ball, which is the center of activity of old stars. The thin discs, like sandwiches, stretch from the inside to the ends and gradually thicken the edges, where gas, dust and young stars gather. The periphery of the silver disc is surrounded by a silver halo of spherical structures, which is sparsely starred and rare (Figure 2).
Figure 2: Schematic diagram of the structure of the Milky Way, the red is the nuclear sphere, the bright green and blue are the thin disk and the thick disk of the Milky Way, respectively, image source: Xiang Maosheng
From its inception to today, the Milky Way has passed the age of 10 billion. So, in the past ten billion years, what wonderful stories have the milky way gone through, and gradually formed the series of characteristic structures such as silver halos, thick disks, and thin disks that we see today? These questions about the origins of the formation and evolution of the Milky Way are one of the most important frontier topics in astronomy and the main scientific objective of the Massive Astronomical Survey Observation Program of multiple ground and space telescopes around the world. Tracing the true story of the Milky Way's experience and restoring its growth process has also become the first task for astronomers to complete the galaxy's voyage.
Conventional wisdom often holds that our galaxy underwent a more intense process of formation during infancy (very early). First, large amounts of metal-poor gas collapse (astronomically, elements other than hydrogen and helium are called metals) or gas-rich galaxies collide with each other to form the Galactic Halo. The gas then gradually cooled to form the early silver disk, the galactic thick disk. Finally, as the gas cools further over time, the Formation of the Milky Way Thin Disk begins; the formation of the thin disk is a long and orderly process that has continued from about 80-10 billion years ago to the present. To a large extent, however, these evolutionary images are largely derived from numerical simulations and speculations about fragmented observational evidence. Fortunately, the emergence of astronomical observation big data makes the evolution of the Milky Way image is being restored, and the era of opening the dusty history of the Milky Way has arrived.
The latest giant chart of the age of the stars
The Milky Way is the key laboratory of galactic archaeology research, and stars are natural fossils of milky way archaeology: the chemical element content on the surface of stars completely records the chemical composition of the interstellar environment in the Milky Way at the time of its birth; the similarity of stellar motion also provides important clues for astronomers to solve the problem of stellar origin. Therefore, the key to achieving a comprehensive understanding of the integration and evolution of the Milky Way requires obtaining enough information about the position, motion, age, and chemical composition of stars in the Milky Way galaxy that are widely distributed and representative enough.
The Milky Way Spectral Survey using LAMOST is the world's largest stellar spectral survey project currently in operation, and has acquired tens of millions of stellar spectra over the past 10 years, providing an unprecedented spectral data building block for revealing the structure and evolution of the Milky Way. The Gaia satellite launched by ESA provides astronomers with high-precision positions and movement maps of 1.4 billion stars. This became the key to astronomers opening up the dusty history of the Milky Way.
Figure 3: LAMOST vs Gaia, Image source: LAMOST Operations and Development Center
As a prerequisite for the study of galactic archaeology, the age of stars is the most difficult to obtain accurately, and it can also be said that it is one of the most difficult physical quantities in the field of astronomy to accurately determine. The age of the star cannot be measured directly, but can only be estimated by comparing the basic parameters of the measured star with the theoretical model of stellar evolution. Therefore, the accuracy of age determination depends on the accuracy of the measured stellar parameters and the stage of evolution of the star. Estimating the age of a star based on the high-precision atmospheric parameters (effective temperature, surface gravity, and metal abundance) provided by spectral analysis is currently the most efficient way to determine the age of stars. In recent years, large samples of star age provided by large-scale spectral survey observations have led to rapid development in the archaeology of the Milky Way, and the LAMOST spectral survey is the main pathfinder and navigator in this direction.
However, the typical age error of large samples of stars obtained by previous studies is more than 20%, while the age error of less than 10% of star samples is very small, and the spatial and parameter range of samples is also very limited. A subgiant star is a star in the evolutionary stage of the transition from the main sequence evolution stage to the red giant evolution stage. The observable parameters of stars at this stage of evolution, especially the luminosity, are sensitive to their initial mass and age, so the age of these stars is relatively easy to determine precisely. However, the evolution of stars in the subgiant stage is very rapid, resulting in the scarcity of subgiants, and screening and constructing large samples of subgiants requires a large-scale survey of observations.
Dr. Mao Sheng Xiang and Professor Hans-Walter Rix relied on LAMOST survey spectroscopy data to obtain high-precision atmospheric parameters of 7 million stars, and used Gaia survey data to obtain high-precision stellar luminosity and orbital kinematics parameters, on the basis of which an accurate star age sample containing 250,000 subgiants was screened and constructed, with an average age accuracy of 7%, and the metal abundance coverage ranged from -2.5 (1/ of the sun's metal content). 300) to 0.5 (3 times the metal content of the sun), with a spatial coverage of up to 30,000 light-years. This sample has the highest accuracy in measuring the age of stars at present, and also covers a wide range of spatial and metallic abundances, providing a good data basis for finely characterizing the structure of the Milky Way.
Depending on their motion characteristics and metal content, the 250,000 stars were divided into two groups by the authors: a group of thin-disk stars formed in a relatively quiet process of dynamics; and another group of halos and thick-disk stars formed during a dynamic process of violent turbulence. Using them as research objects, astronomers have carved a clear picture of the formation and evolution of the early Milky Way, and the growth of the Milky Way from the old thick disk to the silver halo to the sandwich disk has been restored one by one in the passage of time.
Figure 4: Panorama of the Milky Way, Image source: ESA-Gaia-DPAC
How long does the old star of the thick plate live a little?
13.8 billion years ago, after the Big Bang, the universe was born, and a few years later, the prototype of the Milky Way appeared, and ancient stars began to gather to form a thick section of the Milky Way, and the inhabitants here were basically all indigenous old stars. How long does the old star of the thick plate live a little? The authors of the Nature article analyzed the ultra-high temporal resolution evolution of the Milky Way and found that the Milky Way's thick disk stars began to form 13 billion years ago, just 800 million years after the Big Bang (corresponding to a cosmological redshift of 7). That's comparable to the time it took for the Hubble space telescope to detect the most distant galaxies it can currently detect.
However, it's unclear whether galaxies similar to the early Milky Way were ubiquitous in the depths of the extremely distant universe, namely the long-lost childhood companions of the Milky Way. Perhaps in the near future, the James Webb telescope, which is undergoing intensive testing on the L2 orbit of the Lagrangian point on earth, will give the answer. The oldest pachyderms are even about 2 billion years older than halo stars in the Milky Way. That is, the early thick disks formed 2 billion years ahead of the major halo structures we see today, refreshing the traditional understanding of the early formation history of the Milky Way.
Upon further study, the authors also found that the formation of thick plates lasted about 5 billion years (from 13 billion years ago to 8 billion years ago). In this process, the metallic content of the interstellar medium and stars increased by a factor of 30. Not only that, but due to the high turbulence of the star-forming gas, the metallic elements produced and ejected by the death of the star are rapidly diffused to all corners of the thick disk, so that all thick disk stars formed at any particular moment in the process, regardless of where they are formed, have almost the same metallic abundance (which is very different from the thin disk formed in the quiet process of dynamics).
Surprisingly, however, while the formation of the thick disk lasted 5 billion years, the data shows that most of the thick disk stars formed in a concentrated explosion 11 billion years ago. What is the reason for this? What happened at this point in time?
A turbulent year for the Galactic Halo
In 2018, a study based on data from the Gaia satellite survey and the ground-based spectral survey found that the motion of most halo stars was very inconsistent with earlier predictions, and that the chemical DNA (metal content) of these stars in the halo was similar to that of dwarf galaxies, but very different from the local stars in the Milky Way. This suggests that most of the halo stars come from a dwarf galaxy, and leads to an unknown turbulent past in the history of the Milky Way: the Gaia-Sausage-Enceladus dwarf galaxy (GSE), a hundred-handed giant, collided with the teenage Milky Way, and was subsequently eaten and absorbed by the Milky Way to form the main body of the galaxy.
Later studies of stellar chemical kinematics further showed that the vast majority of stars in the halo are mainly derived from a large number of accretion dwarf galaxies, of which the GSE collision with the Milky Way is the most important accretion and merger event, contributing about 2/3 of the halo stars. So the once home of these migratory stars in the halo should be this ancient dwarf galaxy, and this collision event has become an important clue to unravel the mystery of the Milky Way.
Figure 5: Schematic of the collision between the Milky Way and dwarf galaxies, Image source: Instituto de Astrofísica de Canarias
Astronomers have struggled to determine the exact timing of the GSE impact, having previously speculated that it collided with the Milky Way about 8 to 10 billion years ago. A new study from the Canary Institute for Astrophysics in Spain in 2019 speculated that the GSE collided with the Milky Way 10 billion years ago. However, the latest study yields an image of the early evolution of the Milky Way that gives a more accurate time point for collision events to be about 11 billion years ago, which is 1 billion years earlier than previously thought.
What's more, GSE hit the early Milky Way 11 billion years ago, and star-forming activity in the thick disk peaked 11 billion years ago. The two ages coincide highly, and the authors of the paper believe that this is no coincidence, but rather a hint that the star-forming activity of the thick disk was significantly stimulated by the GSE impact event, and the scene seems to be vividly remembered.
GSE impacts on the Milky Way have been discovered before, but what did the Milky Way look like before the impact? What impact did the impact have on it? Due to the lack of solid evidence, the answers to these questions are unknown. The latest study provides a clear picture of the early Milky Way and GSE impact events.
The sandwich thin plate is flourishing
These 250,000 stars are not only characterized by attribution in two groups, one belonging to the halo and the thick disk, and the other group from the thin disk. Coincidentally, their ages, bounded by about 8 billion years, are also clearly divided into two distinct groups. This means that from a timeline perspective, 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 disk and silver halo of the Milky Way, and the late stages formed the thin disk of the Milky Way.
The younger plates began 8 billion years ago and gradually grew into the way they grow larger and fatter at both ends like sandwiches today. The process as a whole is relatively smooth and boring, but there is no shortage of wonderful stories, the most exciting of which may be the stellar migration. The growth of the disk has witnessed massive stellar migration events, with stars formed in the inner disk region closer to the center of the Milky Way making a long migration to today's solar neighborhood space (about 25,000 light-years from the center of the Milky Way).
Because of the more active star formation and evolutionary activity, these stars that form in the inner silver disk have unusually high metal content, so they are relatively easy for astronomers to identify through observational data. Similarly, a significant portion of the Solar's neighborhood has stellar colonizations from the outer disk region, which are less metallic and easier to identify than native stars. Isn't it wonderful enough that the so-called sandwich thin plate is flourishing, and thousands of stars are flowing endlessly, opening up the territory for the great cause of the Galaxy?
Moreover, galactic merger events have actually been playing out over the past 8 billion years, with the most widely known remnant of mergers probably being the Sagittarius stream, and the Large Magellanic Cloud will also be swallowed up and digested by the Milky Way in the near future. But overall, these events are estimated to be small merger events, and their impact may not be comparable to earlier GSE impact events.
At this point, a relatively clear image of the early formation and evolution of the Milky Way has been established, which is also the most accurate and complete picture of the history of the formation of the Milky Way by astronomers from the timeline.
For thousands of years, people have never stopped exploring the mysteries of this vast galaxy and the vast universe. Today, the giant eye of the universe is helping scientists to gradually see the history of the formation and evolution of the early Milky Way, opening an important chapter in the annals of the Galaxy. As a general representative of disk galaxies, the annals of the Milky Way will become a classic for astronomers to understand the formation and evolution of galaxies, tracing the wonderful stories that have taken place from the very early universe to today. The excitement continues, let's look forward to it together.
Figure 6: Schematic diagram of the formation history of the Milky Way: The Big Bang 13.8 billion years ago, the thick disk appeared 13 billion years ago, the silver halo formed 11 billion years ago, the thick disk grew significantly, the formation of the thick disk stopped 8 billion years ago, the thin disk began to form and continues to this day, image source: Yu Jingchuan
Figure 7: Cover of the March 24 issue of Nature – Starcatcher's Guide to the Galaxy, Image Source: Literature[2]
About the Author
Shuang Li, Director of Publicity at LAMOST Operations and Development Center; Dan Wang, Director of the Office of LAMOST Operations and Development Centers.
bibliography:
[1] Kant, Critique of Practical Reason
[2] “A time-resolved picture of our Milky Way’s early formation history”, Maosheng Xiang and Hans-Walter Rix,DOI: 10.1038 /s41586- 022-04496-5
[3] “The merger that led to the formation of the Milky Way's inner stellar halo and thick disk”, Helmi, A. et al., Nature, Volume 563, Issue 7729, p.85-88
[4] “Co-formation of the disc and the stellar halo”, Belokurov, V. et al., Monthly Notices of the Royal Astronomical Society, Volume 478, Issue 1, p.611-619
[5] ” Evidence from the H3 Survey That the Stellar Halo Is Entirely Comprised of Substructure”, Naidu, Rohan P., et al. The Astrophysical Journal, Volume 901, Issue 1, id.48, 32 pp.
[6] “The Milky Way’s Most Recent Meal was a Galaxy it Gobbled up 8-10 Billion Years ago”, BRIAN KOBERLEIN, Universe Today-Space and astronomy news,2022.1
https://www.universetoday.com/154007/the-milky-ways-most-recent-meal-was-a-galaxy-it-gobbled-up-8-10-billion-years-ago/.
National Astronomical Journal of China, March 2022
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