New evidence of the annexation of dwarf galaxies by the Milky Way

Astronomers discovered a strange star in the Milky Way for the first time, and spectral analysis proved that it came from dwarf galaxies.

Guo Shoujing telescope. Photo courtesy of respondents

The formation and evolution of the Milky Way is a fascinating scientific problem. Astronomical circles previously believed that the Milky Way grew by constantly "annexing" nearby dwarf galaxies. Recently, a new discovery provides new evidence for the Milky Way to "annex" dwarf galaxies. The Sino-Japanese cooperative research team led by the National Astronomical Observatory of Chinese Academy of Sciences discovered a strange star in the Milky Way for the first time. By analyzing its "DNA", the research team found that this star is not an "aborigine" of the Milky Way, but an "alien" of a dwarf galaxy that was inhaled and disintegrated by the Milky Way.

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Analyzing stellar spectra and solving the mystery of "life experience"

Astronomers first discovered "alien immigrants" from dwarf galaxies in the Milky Way, which provided exact and reliable chemical evidence for the Milky Way to "annex" dwarf galaxies. This discovery will deepen our understanding of the formation and evolution of galaxies. The research results were published online by the international authoritative academic journal Nature Astronomy on April 30th, Beijing time.

Large galaxies, such as the Milky Way, are usually considered to be formed by the merger of dwarf galaxies. However, how many stars in the Milky Way come from dwarf galaxies? What are the characteristics of these stars? Astronomers still know very little about this.

Relying on the massive spectral data provided by the Guo Shoujing Telescope, the Sino-Japanese cooperative research team led by the National Astronomical Observatory of the Chinese Academy of Sciences discovered a special star about 22,000 light years away from the Earth in the galactic halo.

How did this star get noticed? Xing, the first author of the paper and an assistant researcher at the National Astronomical Observatory, said that the Guo Shoujing Telescope acquired the spectra of millions of stars, and the researchers measured the magnesium content of hundreds of thousands of them, from which more than 100 stars with low magnesium content were found.

Xing Qianfan further explained that stars largely retain the chemical composition of the environment in which they were born, and their origins can be traced by analyzing the chemical composition of stars. The spectrum of a star is like its DNA. Through the spectrum, astronomers can determine luminosity, temperature, chemical composition and so on. Solve the "mystery of life" of stars.

Star introduction, this star has a very high content of heavy elements such as silver, europium, gold and uranium. In particular, the content of europium greatly exceeds the average value of ordinary stars in the Milky Way. At present, only more than 30 stars of this type have been found in the silver halo.

The chemical composition is highly consistent with that of dwarfs.

"One high and one low, two characteristics are gathered in a star, which is very novel. This is also the first time in the world that a heavy element super-rich star with low magnesium has been discovered in the Milky Way. " Xing said.

"The content of magnesium is low, and we can basically judge that it comes from dwarf galaxies." Qian Fan explained that at the same time, the content of other elements (including silicon, calcium and titanium) in this star is extremely low, which is only one-fifth of that of ordinary stars in the Milky Way, while stars with similar chemical composition are common in dwarf galaxies near the Milky Way.

In order to fully confirm this conclusion, the research team determined the content of 24 elements in this star based on the joint observation of high-resolution spectrum of the 8-meter optical telescope, and made a detailed comparison with dwarfs and field stars in the silver halo.

It is found that the chemical composition of this star is highly consistent with that of a dwarf star, which is obviously different from that of a halo star in the Milky Way. This means that this star comes from a dwarf galaxy disintegrated by the Milky Way, which is a reliable chemical evidence of the merger of the Milky Way.

Xing Qian Fan said that astronomers have been trying to find evidence that dwarf galaxies were "swallowed up" by the Milky Way, and this work has made substantial research progress in this respect.

Further analysis shows that this star was formed after its original dwarf galaxy experienced an extremely rare neutron star merger event. Xing said that there are two speculations about the source of elements in the process of fast neutron capture. One thought it was caused by a supernova explosion, and the other thought it was caused by the merger of neutron stars. As a star with ultra-high content of heavy elements, its discovery also provides evidence support that neutron star merger is the main source of elements in the process of fast neutron capture.

■ dispel doubts

Will the Milky Way continue to get bigger?

The Milky Way contains hundreds of billions of stars. How did it form and evolve? Star Qian Fan said that the Milky Way is formed by the merger of dwarf galaxies (small galaxies), and dwarf galaxies are the weakest galaxies. At first, small galaxies and gas clouds gathered together and merged to form the original prototype of the Milky Way. A new star was born in the gas cloud of the Milky Way. This kind of star, similar to the "Pioneer of Xinjiang", is considered to be the "indigenous" of the Milky Way.

When the Milky Way grows into a large galaxy, dwarf galaxies that are in the evolution stage and then annexed are considered as "immigrants".

When the orbits of dwarf galaxies are close to or even intersect with the Milky Way, despite the constant struggle of dwarf galaxies, they can't escape the fate of being disintegrated and annexed by the Milky Way. The primitive stars of dwarf galaxies eventually "defected" to the Milky Way. "The most famous is the Sagittarius Star Stream, and the Milky Way is merging with the Sagittarius dwarf galaxy." Besides, the Andromeda galaxy is approaching the Milky Way, and some researchers predict that they will collide and merge to form a larger elliptical galaxy, Qian Fan said.

Xing Qian Fan said that dozens of surviving dwarf galaxies have been found around the Milky Way, so the Milky Way will be expanded by merging dwarf galaxies. However, neither the annexation of dwarf galaxies by the Milky Way nor the merger with other galaxies will have a great impact on the earth. "The distance between stars is much larger than their size, and the collision and merger of galaxies is not a real collision of stars."

■ Reveal the secret

Guo Shoujing can observe 4000 celestial bodies at the same time.

At Xinglong Observatory of the National Astronomical Observatory of the Chinese Academy of Sciences in Hebei Province, a huge white building stands at the southern foot of the main peak of Yanshan Mountain, pointing to the sky. This is a new type of large-aperture telescope with large field of view-Guo Shoujing telescope.

As a major national scientific project, Guo Shoujing Telescope started construction in September, 20001,costing 235 million yuan. April 20 10, officially named "Guo Shoujing telescope".

The completion of the Guo Shoujing Telescope has broken through the problem that astronomical telescopes cannot have a large field of view and a large aperture at the same time, and has become the largest large field of view telescope in the world, which can observe 4,000 celestial bodies at the same time.

2065438+In September, 2002, Guo Shoujing Telescope officially began its scientific survey of the sky. At present, the spectra of millions of stars have been obtained, which helps astronomers to "survey" the stars in the Milky Way, find those stars that originated from dwarf galaxies, and provide data support for studying the proportion, spatial distribution and kinematic characteristics of foreign stars in the Milky Way.