Subaru Telescope | National Astronomical Observatory of Japan | 2019 Apr 29
Using the High Dispersion Spectrograph on the Subaru Telescope, astronomers have discovered a star in the Milky Way Galaxy with a chemical composition unlike any other star in our Galaxy (Figure 1). This chemical composition has been seen in a small number of stars in dwarf galaxies orbiting the Milky Way. This suggests that the star was part of a dwarf galaxy that merged into the Milky Way.
The Milky Way Galaxy has the so-called halo structure around the disk. The sparse structure of old stars has been formed in the early phase of the Galaxy formation. The structure includes stars formed in gas clouds in the growing large galaxy and also those formed in small stellar systems that have accreted onto the Galaxy. Such a scenario is predicted by computer simulation of galaxy formation assuming cold dark matter (Figure 2).
The small stellar systems accreted onto the Galaxy, that would be similar to dwarf galaxies found in the current universe, should have already been disintegrated in the space. However, individual stars should preserve the orbital motion in the Galaxy and their chemical abundance ratios that were determined before the accretion. The focus of current studies in this field is to find evidence for such a scenario of galaxy formation by observations of individual stars in the Milky Way.
The star formation in dwarf galaxies is relatively slow compared to larger galaxies, resulting in difference in chemical abundance ratios recorded in stars. For instance, the magnesium to iron (Mg/Fe) abundance ratios in stars of dwarf galaxies currently found around the Milky Way show clear difference from those of the bulk of Milky Way stars ...
Evidence for the accretion origin of halo stars with an extreme r-process enhancement ~ Qian-Fan Xing et al
- Nature Astronomy (online 29 Apr 2019) DOI: 10.1038/s41550-019-0764-5