Einstein once said: "The most incomprehensible thing in the universe is that the universe is comprehensible!" Since ancient times, the beautiful and vast Milky Way in the night sky has triggered countless imaginations and brave explorations, from the Three Kingdoms period "Xinghan is brilliant, like out of it" to the Song Dynasty "The Milky Way is like three thousand songs, and the bathing mallard flying heron is green"… It has pinned the expectations and yearning of generations of people for the Milky Way.
"Upturned" Milky Way
With the advance of observation technology, astronomers can gaze at the galaxy they dream of, and people’s understanding of the true face of the Milky Way is constantly improving. At present, the Milky Way we know is a galaxy about 200,000 light-years in diameter, with a box-shaped "nuclear ball" in the middle, surrounded by a flat "silver disc", and a huge "silver halo" looming above and below the silver disc.
About 70 years ago, scientists have known that the Milky Way is "warped". The edge region on one side is curved "up" from the plane of the Milky Way to the north, and the other side is curved "down". From a distance, it looks like the potato chips we usually eat. That is to say, when we look at the Milky Way from the side, we can see that the Milky Way has a S-shaped "body" that is convex and backward.
However, for a long time, due to the limitations of observation conditions, scientists could not vividly present the "potato chip-like" structure of the Milky Way in front of us with real and accurate star observation data.
In 2019, scientists in our country, Chen Xiaodian and others, first outlined the three-dimensional spatial distribution characteristics of the warping structure of the Milky Way through star counting, which provided direct observational evidence for humans to truly observe the structure of the Milky Way’s "potato chips".
The Milky Way, as a typical disk galaxy in the vast universe, has both universality and particularity. Why is the Milky Way so "warped"? What is the secret to maintaining an S-shaped body? For a long time, there have been various opinions and assumptions in the astronomical community, and it has become a hot topic of discussion among international astronomers.
Figure 2. Warp diagram of galaxy ESO 510-13, with tilting features at both left and right ends (Source: STScl/NASA)
"Upward" foresight
Some people think that the interaction between the Milky Way’s halo and the disk can produce a "warp structure"; some people think that gas falling into the Milky Way disk can give rise to a "warp structure" of the Milky Way disk; some people think that the Magellanic Cloud at the doorstep of the Milky Way or the collision of the neighboring Sagittarius galaxy is also a possibility; there is even a view that the Milky Way’s magnetic field also plays an important role in the formation of the "potato chip" structure and so on. For a long time, there have been new breakthroughs in the research on the source of warping, but it is difficult to reach a consensus.
Scientific theories need to be confirmed step by step by the measured data, and the ESA Gaia Survey presents an opportunity. Because the Gaia satellite can help us make unprecedented measurements of the three-dimensional speed of stars, astronomers use this new data to study and discuss warping increasingly.
In order to make it easier to understand, astrophysicists start from the macroscopic direction and simply classify the formation mechanism of the "potato chip" shape of the Milky Way disk into two categories:
The first type of reason is due to external attraction and influence, due to the attractive force process between the Milky Way disk and the surrounding non-luminous matter or the outer dwarf galaxy. Simply put, our Milky Way disk is like a star full of "idol baggage", attracting each other with the surrounding fans, and the "concave shape" has concave such a graceful figure.
The second type is considered to be a long-term evolution process that is not dominated by the interaction of attractive forces, and is simply called a non-attractive force process. For example, the influence of gas falling around the silver disk or magnetic field action caused the silver disk to be so "warped".
New knowledge of "warping"
If the second cause causes the "warp structure" of the galaxy, it is characterized by the bending strength of the young star family on the silver disk than that of the old star family. This is because if the non-attractive force process dominates, then the mechanism mainly acts on gas or young stars, and the old star has enough time to digest the influence of the external non-attractive force process on it, thus reflecting the distribution of bending strength of different stellar families that can be detected by humans.
Although we can use the massive data of the Gaia satellite to explore this, it currently measures the velocity of the star line of sight in a small range, and it cannot provide ideal star chemical information, age information, etc. Even the distance measurement of stars farther away from the galactic disk is not accurate enough.
Fortunately, our country’s major scientific and technological infrastructure, the Guo Shoujing Telescope (LAMOST), can better solve these shortcomings, and also provide us with an excellent opportunity to explore the origin of the galactic "warp" structure and inquire about its past, present and historical context.
Figure 3. LAMOST Telescope and the Milky Way (Source: Yuan Fengfang)
The international research team led by Wang Haifeng, a special young researcher of LAMOST, based on the rich information of stellar parameters in the LAMOST sky survey spectral data, cleverly constructed a probability distribution function, and traced the characteristics of the silver plate "potato chips" from a novel perspective.
They found that the bending strength of the "potato chip" form of the Milky Way disk decreases with the age of the star, and such a "warped" body can be maintained for a long time, but it is an unstable state. Based on scientific analysis, they reliably found evidence that the warped structure originated from a non-attractive force process represented by gas falling. Therefore, based on the advantages of LAMOST, the researchers found that the "warped" of the Milky Way disk is actually a structure that originated from a non-attractive force physical process and has been in an unstable state for a long time.
Evolution of galactic warping intensity, which decreases significantly with age, supports the conclusion that warping originates from non-attractive force processes
This discovery advances our understanding of the origin of warping and takes a whole new step towards understanding the warping and origin of the Milky Way’s disk.
In the future, the research team will make more efforts in terms of data and physics, gradually realizing the ideal of obtaining a complete physical image of the Milky Way, and then opening one mysterious and unknown door after another to restore the evolutionary history of the Milky Way and even the nearby universe.
About the author: Wang Haifeng, a postdoctoral fellow at the Southwest China Institute of Astronomy, Yunnan University, a distinguished young researcher at the Chinese Academy of Sciences LAMOST, and a Ph.D. in astrophysics at the National Astronomical Observatory of the Chinese Academy of Sciences. Research areas: chemical dynamics structure of the Milky Way, time-domain stellar physics, astronomical big data processing and analysis, etc.; Li Shuang, an engineer at the National Astronomical Observatory of the Chinese Academy of Sciences, is the publicity director of the LAMOST Operation and Development Center.