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"Scientific American" recently published a cover article entitled "new vision of the galaxy" jointly written by senior astronomer of Harvard Smithsonian astrophysics center, academician of the American Academy of Sciences, mark Reid, and Professor Zheng Xingwu of the school of astronomy and space science of Nanjing University. It summarized the two of them and K. menden of the radio astronomy Institute of Max Planck, Germany The international team led by Professor Menten has been working on the structure of the galaxy for 15 years. The team used the very long baseline interferometry technology to accurately measure the distance and self motion of nearly 200 massive star forming regions on the silver disk, obtained the structure of the spiral arm of the galaxy, the position of the solar system and its rotation speed around the center of the galaxy, and drew a new galactic structure map with a scale of 100000 x 100000 light-years. This is the most accurate structural map of the Milky way so far. It clearly shows that the Milky way is a rod spiral galaxy with four spiral arms. It completely solves the major scientific problem of how many spiral arms the Milky way has in astronomy.
Science American, Vol. 322, No. 4 (2020) cover
The structure of the galaxy is one of the most important scientific problems in astronomy. The main reason is that the diameter of the Milky way is estimated to be about 100000-180000 light-years. Our solar system is far away from the center of the Milky way, and its position is close to the central plane of the Galactic channel. Therefore, the spiral arms we see are all overlapped and projected on the celestial sphere and cannot be distinguished, just as "we do not know the true face of Lushan Mountain, but only because we are in this mountain". However, if we can accurately measure the distance from enough celestial bodies on the spiral arm to the sun, we can completely sketch the spiral arm structure of the galaxy from the inside to the outside. However, there are dense dust and gas distributed on the silver disk, which causes very serious extinction to the optical radiation of the celestial bodies on the spiral arm of the galaxy. Even with the use of large optical telescope, we can only see the celestial bodies on the silver disk within 6000 light-years, which is far smaller than the size of the galaxy. The real extensive and in-depth observation and study of the structure of the Milky way is due to the rise of radio and infrared astronomy in the 1950s. Astronomers use radio and infrared telescopes to find that in addition to the objects with optical radiation in the formation area of massive stars on the spiral arm of the galaxy, there are more radio and infrared objects that are not visible in the optics, as well as the molecular gas and dust that breed them. They can all be used as the tracers of the structure of the galaxy. What's more, radio and infrared radiation can penetrate the dense dust on the Galactic surface, allowing us to reach the edge of the galaxy. After nearly half a century of observation and research, astronomers generally believe that the Milky way is a rod spiral galaxy. However, due to the inaccuracy of measuring the distance of young objects in the massive star forming region, some basic problems of the structure of the Milky way are still unsolved, such as how many spiral arms the Milky way has, the exact position and movement of the sun in the Milky way, etc.
At the beginning of the 21st century, 22 astronomers from 8 countries including China (6), the United States (3), Germany (3), Italy (3), the Netherlands (3), South Korea (2), Japan (1) and Poland (1) planned to establish the bar and spiral structure legacy Survey, abbreviated as "Bessel" and Bessel) major scientific programs in Chinese and English. Bessel plans to use the very long baseline array with the equivalent aperture size of the earth diameter to directly measure the distance and self motion of the maser source in the massive star forming region by the geometric triangle parallax method (maser source is a cosmic laser point source with a bright temperature of more than 100 million degrees or even trillions of degrees). Since 2003, the research team has measured the distance and self motion of maser sources in 163 massive star forming regions on the silver disk. Combined with 37 maser sources measured by other international clusters, the team has obtained the distance and self motion of nearly 200 massive star forming regions in the galaxy. The distribution of these massive star forming regions on the silver plate clearly delineates four main spiral arms, namely, the Perseid arm, the Centaur ship bottom arm, the cunchi arm and the shield Centaur arm. Combined with the observation data of infrared, carbon monoxide and a large number of young celestial bodies, the structure of the Milky way is finally mapped out. It is a rod spiral galaxy with four spiral arms. Among the brighter extragalactic systems, rod vortex is a large family. Our galaxy is an ordinary galaxy, but also a beautiful spiral arm symmetry galaxy.
The solar system we live in is not on the four main arms, but very close to a local arm independent of the four main arms. This local arm, located between the Perseus arm and the humanoid - ship bottom arm, is about 20000 light-years long, much larger than previously estimated. Its shape and abundant massive star forming area can be compared with the other four main spiral arms. The local arm may not be an isolated arm segment, but it is most likely a part of the adjacent fairy arm and humanoid bottom arm. The distance between the sun and the galactic center is 26000 light-years, and the rotation speed around the galactic center is 236 km / s, that is to say, it takes about 212 million years to make a circle. The sun is almost in the central plane of the silver plate, and the vertical distance from the central plane is about 20 light-years, smaller than the previous estimate of 82 light-years.
Science, 311,54 (2006) cover
The Chinese members of the Bessel project research team are from Nanjing University, Zijinshan Observatory, Shanghai Observatory and the national time service center. In the past 15 years, the Chinese team has played an important role in the Bessel plan based on the principles of CO initiation, CO organization, complementary resources and sharing of achievements.
In 2003, Dr. Xu ye and Prof. Zheng Xingwu from the school of astronomy and space science of Nanjing University, Dr. Reid from Harvard Smithsonian Center of Astrophysics in the United States and Prof. Mendon from Max Planck Institute of radio astronomy in Germany cooperated to measure the trigonometric parallax and self motion of methanol (CH3OH) maser in w3oh in the great mass star forming region of Yingxian arm successfully for the first time by using very long baseline interference technology The distance of massive star forming area is 6360 ± 40 light-years, and the measurement accuracy is as high as 2%. This is the most accurate distance measurement for such a distant celestial body in history. From the self and apparent velocities of the maser, the three-dimensional motion of the massive star forming region in the galaxy can be obtained, and the kinematic properties of the spiral arm of the galaxy can be studied. This work indicates the feasibility of directly measuring the structure and motion of the spiral arm in the galaxy. This achievement was published in science as a cover paper in January 2006, which is the pioneering work of the Bessel plan. Since then, Chinese members have proposed and participated in the pilot experimental observation, and the research results have laid a solid foundation for the implementation of the Bessel plan.
By the end of 2019, Chinese astronomers have observed and analyzed 85 of the 163 target maser sources planned by Bessel. Bessel project has published 35 papers in internationally renowned astronomical and astrophysical journals, including 16 papers by Chinese astronomers. By participating in this major scientific program, the development of VLB astrometry in China has been promoted, and young astronomers have been trained, so that they are at the forefront of VLB astrometry in the world.
Mark Reid & Xing Wu Zheng, sciame, vol.322, No.4, p.31 (2020)
Copyright 2019 by Zheng Xingwu and mark J. Reid
Source: Nanjing University News Network
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