Newly discovered traces of a galaxy that merged with the Milky Way; Her name is “Shakti” and “Shiva”. sorae universe portal website

The early Milky Way is said to have formed from the merger of several small galaxies. In recent years, large-scale data on the positions and directions of star movement have become available, making it possible to specifically determine the effects of galaxy mergers.

A research team led by Khayati Malhan and Hans-Walter Rex of the Max Planck Institute for Astronomy will combine and analyze data from Gaia, which records a large number of stars, and the Sloan Digital Sky Survey, and we have looked for traces of the galaxy. Results,Traces of two galaxies that are estimated to have merged with the Milky Way at a very early stage, about 12 to 13 billion years ago.I succeeded in discovering. They called these galaxies after Hindu mythology.Shakti(Shakti)” and “Shiva(Shiva)”.

■ آثار اصطدامات مجرية تركت في حركة مجرة ​​درب التبانة

فمجرة درب التبانة التي تنتمي إليها الأرض أكبر من المجرات المحيطة بها. النظرية السائدة حاليًا هي أن المجرات الكبيرة مثل مجرة ​​درب التبانة تشكلت عن طريق دمج عدة مجرات أصغر. قبل الاندماج، يكون لكل مجرة ​​نجومها الخاصة وغاز الهيدروجين. عندما تندمج المجرات، تمتزج النجوم ببعضها البعض، وقد تتولد نجوم جديدة من غاز الهيدروجين.

At first glance, it seems impossible to see the effects of mergers that occurred billions of years ago, because the merger of galaxies disrupts the assembly of stars and gas. However, galaxies that appear crowded with stars at first glance are actually so sparse that they can be compared to “two watermelons floating in the Pacific Ocean,” so stars whose direction of motion and speed are disrupted when they merge are just galaxies. Few stars, most of them, retain such mechanical properties even after galaxy mergers.

In addition, stars that precede or were born shortly after merger, an event that occurred billions of years ago, tend to be older. Stars contain metals(※1)The smaller the amount of metal, the older it is estimated to be, so if a group of 'low metallicity stars' is found with a low amount of metal, the entire group can be assumed to have an older origin. In other words,If we find a large group of stars that are consistent in motion and age, they could be the remnants of a merged galaxy.

However, now, several billion years after the merger, the star clusters that were once separate galaxies are now moving almost in the direction of the Milky Way's rotation, and their original dynamical properties have partially disappeared. There are also groups of stars, such as star clusters and stellar streams, which are much smaller than galaxies but have a uniform age and direction of motion. Therefore, in order to find traces of large-scale clusters such as merging galaxies, it is necessary to obtain and analyze a large amount of stellar data.

It would not have been possible to conduct such research in the past, but the space telescope launched by the European Space Agency (ESA) in 2013Gaia” became possible. Gaia continues to collect the properties of stars belonging to the Milky Way Galaxy, and currently has data on approximately 1.5 billion stars.

Analysis of the Gaia star catalog reveals thisGaia sausage(gaia sausage)” and “Pontus StreamTraces of galaxies thought to have merged more than 8 billion years ago, such as the Pontus Stream, have been discovered one by one. In addition, at the center of the Milky Way,Poor old heart(Poor old heart).(※2)There is a group of ancient stars, and the current Milky Way Galaxy may have formed as a result of this group merging with other galaxies.

*1…The term “metal” in a star is a general term for elements other than hydrogen and helium, and also includes chemically non-metallic elements such as carbon and oxygen.

*2…Poor means that there are few metals, old means that the star is old, and hearty means that this cluster is located in the center of the Milky Way Galaxy. .

■The discovery of the “Shakti” and “Shiva” galaxies, which merged in a very ancient era

In order to search for traces of these ancient galaxies, the Malhan and Rex team conducted observations using the latest version (data release 17) of SDSS (Sloan Digital Sky Survey), another comet-hunting project, and the data was added to the Gaia star catalogue. And analyze it. Gaia and SDSS not only observe differences in stars, but also sometimes collect different data on the same star. The researchers selected approximately 5.8 million stars from the data in the two catalogs and analyzed them.

As a result, two populations were found, both of which were older and had similar directions and speeds of movement. Each of them is located relatively far from the center of the Milky Way.Traces of galaxies that merged about 12 to 13 billion years agoThis age is expected to be very old compared to other known merger effects.It may be the remains of the first galaxy that merged with the Poor Old Heart.In this analysis, 1,719 stars in the first group and 5,607 stars in the second group were counted. However, before the merger, it was estimated that both were dwarf galaxies with a mass of about 0.001% of the mass of the Milky Way (about 10 million times the mass of the Sun).

Due to the age and large size of the merger, both men named the first group “Shakti“And the second group is called”Shiva“Shakti and Shiva are both Hindu words. Shiva is one of the main gods in Hinduism and is the ruler of destruction and creation. On the other hand, Shakti often refers to a goddess who is considered the wife of Shiva, or a symbol of energy or power. Since the two merged around the same time, in Early in the history of the Milky Way, the name can be said to symbolize that they were a couple involved in the “creation and destruction” of the Milky Way.

■ Research to find the effects has only just begun

The discovery of Shakti and Shiva is an important discovery in the investigation of the formation process of the Milky Way and galaxies of similar size. If you observed the universe since ancient times, you might find small galaxies that existed before the merger. If we study the properties of the stars belonging to Shakti and Shiva in detail, we can compare their properties with those of these galaxies, and we may be able to understand the situation more accurately before and after the merger.

In addition, research is being conducted to look for traces of colliding galaxies like these in parallel all over the world, and they are being discovered one by one based on Gaia observational data alone. By combining observational data from many other comet observation projects, the researchers hope that this discovery will accelerate further, and that it will not be long before we can see the history of the merger and formation of the Milky Way. Maso.

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Written by Riri Aya