Identification of intergalactic immigrants in the Milky Way by chemical DNA

There have been many astronomical events recently. Some time ago, the first black hole photo was released, and now astronomers in China have found chemical evidence that the Milky Way swallowed the surrounding dwarf galaxies!

In the early morning of April 30th, 20 19, Beijing time, an important discovery led by astronomers in China was published online by the international scientific journal Nature Astronomy. The Sino-Japanese cooperative research team led by Zhao Gang, a researcher from the National Astronomical Observatory of Chinese Academy of Sciences, first revealed the mystery of the "origin" of stars with ultra-high element content in the process of fast neutron capture in the Milky Way, providing accurate and reliable chemical evidence for the merger of the Milky Way and providing new clues for identifying "intergalactic immigrants" in the Milky Way based on chemical DNA.

After reading the above words, do you feel a little bit?

Inscrutable? In the fog?

Keep looking down.

The source of all things: the DNA of stars

Astronomers have found that stars largely retain the chemical composition of their birth environment. Their chemical composition, like DNA, provides astronomers with important clues to reveal the origin of stars.

Then, the stars are so far away from us and the starlight is so weak. How do astronomers know the mysteries of these stars? The spectrum of a star is like the fingerprint of a star. With spectra, astronomers can determine the chemical composition of stars and trace their origins.

Stellar spectrogram (from Baidu Encyclopedia)

Guo Shoujing Telescope (LAMOST) independently developed by Chinese astronomers is the astronomical telescope with the highest spectrum acquisition rate in the world. After 7 years of measurement, 65,438+065,438+250,000 spectra were obtained. These "rainbows" in the starlight provide the most powerful data support for astronomers to explore the mysteries of frontier science such as the formation and evolution of the Milky Way and galaxy physics.

Strange discovery: the "vault" in the galaxy

The formation and evolution of the Milky Way has always been a hot issue explored by astronomers. It is generally believed that the Milky Way was formed by constantly devouring nearby dwarf galaxies. To this end, astronomers have been trying to find evidence that dwarf galaxies are "swallowed up" by the Milky Way. Know which of the existing stars in the Milky Way are the original "aborigines" of the Milky Way and which are "intergalactic immigrants" from neighboring dwarf galaxies. Thereby promoting the study of the formation and evolution of the Milky Way.

LAMOST and starry sky (photo courtesy of National Astronomical Observatory)

Relying on the spectral data of LAMOST telescope, a major scientific and technological infrastructure in China, Dr. Xing, the first author of this paper and the National Astronomical Observatory, and his collaborators discovered a heavy metal star with ultra-high element content in the process of fast neutron capture in the silver halo. The europium-iron ratio of this star is 10 times that of the sun, which greatly exceeds the average value of similar stars. It carries a lot of heavy elements, including gold, and is called a "treasure house" in the Milky Way.

At present, astronomers have only found more than 30 such stars in the silver halo. Such a rare star aroused the strong curiosity of astronomers. Where do these stars come from? Are they "primitive inhabitants" of the Milky Way or "intergalactic immigrants" from neighboring dwarf galaxies? A series of problems prompted astronomers to move on. In order to trace the origin of stars, it is necessary to continue to study the composition of DNA-chemical elements of stars.

DNA identification: revealing true identity

The dwarf galaxies swallowed by the Milky Way are similar in mass to the surviving dwarf galaxies, so their member stars also have similar chemical characteristics. By studying the chemical composition of the members of dwarf galaxies near the Milky Way, we can know the chemical characteristics of the stars in the dwarf galaxy family, so as to screen out the stars from the dwarf galaxies in the Milky Way like DNA identification.

The researchers found the content of α elements (magnesium, silicon, calcium and titanium, etc.). ) is significantly lower than the average value of the stars in the silver halo among the member stars of dwarf galaxies near the Milky Way. Based on this, the feature of "low content of α element", like a DNA life code, has become an effective criterion for screening alien "immigrants" in the Milky Way, helping us to find out the stars originally from the silver halo of dwarf galaxies.

According to this clue, Xing et al. carried out DNA identification of α element content of this suspicious star with ultra-high heavy metal element content. Coincidentally, the alpha content of this star is extremely low, only one-fifth of that of similar stars, and stars with similar chemical composition are common in dwarf galaxies near the Milky Way. Therefore, it is preliminarily inferred that the star in this silver halo is an "alien", probably an "immigrant" brought about by the merger of the Milky Way and dwarf galaxies.

The picture shows the "intergalactic migration" of the newly discovered galaxy (drawn by Ma Yuwei of China National Astronomy).

Astronomers have always been rigorous. In order to further confirm their alien "immigrant" status, a high-resolution spectral observation was carried out with the help of the Japanese 8-meter optical telescope, and the finer chemical composition of this star was determined, and the contents of 24 elements were obtained, which were carefully compared with dwarf stars and silver halo stars respectively. By comparison, it is found that the chemical composition of this star is highly consistent with that of a dwarf star, and obviously different from that of a halo star in the Milky Way. Thus, through the above-mentioned DNA paternity, the "life story" and "family history" of this strange star finally surfaced, and it really came from a dwarf star disintegrated by the Milky Way.

Further analysis shows that the ultra-high content of heavy metal elements in this star is due to the rare neutron star merger event in its original dwarf galaxy. The merger of neutron stars can quickly produce a large number of elements in the fast neutron capture process, which can significantly increase the content of elements in the fast neutron capture process in the environment.

At this point, the star's true identity was exposed: it was an "intergalactic immigrant" from a neighboring dwarf galaxy, and came to the Milky Way with "heavy money".

On March 28th this year, the National Astronomical Observatory released the data of LAMOST survey for seven years to domestic astronomers and international collaborators, with a total of 1 1.25 million spectra, marking the official entry of LAMOST spectra into the era of tens of millions. We expect LAMOST to bring you more discoveries and reproduce wonderful stories in the starry sky.

About the author: Xing Qianfan, assistant researcher of National Astronomical Observatory of Chinese Academy of Sciences, and outstanding backbone of LAMOST in Astronomical Research Center of China Academy of Sciences. The main research fields are the abundance of stellar elements and the evolution of the Milky Way. Shuang Lee, an engineer at the National Astronomical Observatory of Chinese Academy of Sciences, is the propaganda director of LAMOST Operation and Development Center.

Rotating editor: Deng (researcher of National Astronomical Observatory)