Researchers have revealed galactic master merger events that occurred in the distant past

Recently, researchers at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences have used a new model independently developed – the “stellar family-orbital superposition model” and high-resolution two-dimensional integral field of view spectral data to find that the lens galaxy NGC 1380, which is 0.6 billion light-years away from the Earth, underwent a major merger about 10 billion years ago, and revealed that the mass of the satellite galaxy accretion of this merger is about 40 billion solar masses. This is the oldest and most massive galaxy merger event that astronomers have ever revealed in the near universe. The results were published in Astronomy & Astrophysics.

At present, astronomers generally believe that galaxies are formed by mergers, and the merger between small galaxies forms large galaxies. But because most of the process of galaxy mergers occurred in the distant past, it is challenging for astronomers to detect key features of merger events (such as the timing of the merger and the mass of accretion of satellite galaxies) from the structures of galaxies that can be observed today. Ancient mergers mainly leave two clues: on the one hand, the metal abundance of stars in galaxies carries information about the age of the stars and their birth environment, and if stars are born in relatively small mass galaxies and later merged into new galaxies, then the metal abundance of these stars will be relatively low; On the other hand, the stars in the galaxy follow a certain trajectory, and the merger will cause some stars to be on a special trajectory, forming a halo structure dominated by radial motion. Previously, astronomers were able to directly distinguish the properties of a single star in the Milky Way by looking at the far plane, find out this low metal abundance and have a special trajectory of the star, so as to quantitatively portray the merger that has been experienced between galaxies, thus finding that the Milky Way has undergone a merger about 10 billion years ago, and the mass of the merged satellite galaxy is about 400 million solar masses, but for most of the nearest neighboring galaxies, astronomers have not yet been able to directly observe the nature of the single star in it.

The researchers collaborated with the international team Fornax 3D project to obtain high-resolution integral-field spectral data on the early-type galaxy NGC 1380 in the Virgo Cluster 0.6 billion light-years away from Earth, which was observed by MUSE, an integral field of view spectrometer on VLT. By analyzing this data, the collaborative team obtained the distribution of the galaxy’s accumulated motion information and chemical abundance in the direction of the line of sight in the two-dimensional sky plane. To reconstruct the three-dimensional trajectory and corresponding chemical abundance of stars in galaxies, the research team developed a new three-dimensional “stellar-orbital superposition model” and applied this model to the study of the early galaxy NGC 1380. Using this model, the researchers successfully fitted all of the galaxy’s 2D plane observations.

By further analyzing the composition of the orbital structure of the stars inherent in this three-dimensional family-orbital model, the researchers found that NGC 1380 has an internal halo structure composed mainly of stars in radial orbits, and this internal halo structure is old and has low metal abundance. The researchers believe that this halo structure is a relic of the ancient main merger that can be used to quantitatively characterize the mass of accretion satellite galaxies. The mass of the halo structure of NGC 1380 indicates that it once merged with a satellite galaxy with a mass of more than 40 billion solar masses, accounting for 1/5 of its total mass today. NGC 1380 still has a large amount of gas remaining on the disk after the merger to continue to form stars, and the disk of stars formed before the merger will at least be partially destroyed. Based on comparisons of the age distribution of stars on the disk and in other structures, the researchers speculate that the main merger of NGC 1380 occurred about 10 billion years ago.

Figure 1 To date, astronomers have only quantitatively characterized the main merger events of three near-neighbor galaxies. Including the Milky Way, which accretrates a satellite galaxy about 10 billion years ago with a mass of about 400 million solar masses (Helmi+2018, Belokurov+2018); The Andromeda galaxy, which underwent a major merger about 2 billion years ago, accretion of satellite galaxies with a mass of about 20 billion solar masses (D’Souza +2018); NGC 1380, which underwent a major merger about 10 billion years ago, accretion of satellite galaxies with a mass of about 40 billion solar masses.

Figure 2 Schematic view of NGC 1380 and its internal stellar trajectory. A galaxy is made up of hundreds of millions of stars that move along certain trajectories. By constructing a stellar-orbital model, the researchers revealed the distribution of star trajectories within the galaxy, as well as the age and metal abundance of the corresponding stars (the figure above is characterized by different colors). The yellow dots in the figure represent old and metallic stars whose trajectories are dominated by radial motion, and these stars form the halo structure and are the remnants of the ancient merger of galaxies. The blue dots in the figure represent younger stars, whose trajectories are dominated by tangential rotations that form a disc-like structure in galaxies.

Prior to this work, studies to quantitatively characterize ancient merger events relied heavily on observations of the properties of a large number of individual stars, so that the scope of study was limited to the two galaxies closest to Earth, the Milky Way and the Andromeda galaxy. The study is the first to push the study of ancient merger events to distant galaxies with integral field of view spectral data, NGC 1380, which is 0.6 billion light-years away, thus opening a new window for quantitative study of the history of galaxy mergers. (Source: Shanghai Astronomical Observatory, Chinese Academy of Sciences)

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