Astronomers still do not know much about the Milky Way, especially how the structure of our galaxy has evolved over billions of years. Seen from the inside, it’s difficult indeed to get an idea. A recent study, however, provides some answers.
For example, astronomers have long theorized that two groups of stars in the halo — around the center of our Galaxy and above and below the disk of the Milky Way — were once part of smaller galaxies. They would have later merged with the Milky Way. But according to a new study, it seems that before being expelled, these stars were in fact within the disk of the Milky Way.
As part of their study, researchers at the Max Planck Institute for Astronomy in Germany wanted to determine the chemical abundance patterns of 14 stars located in the galactic halo. To do this, they relied on data from the W.M. Keck Observatory, located on Mount Mauna Kea in Hawaii, United States. The observed stars were in two different halo structures – the Triangulum-Andromeda (Tri-And) and A13 – stellar currents that are about 14,000 light-years above and below the Milky Way disk.
“The analysis of chemical abundances is a very powerful test that allows, in a similar way to DNA pairing, to identify the parent population of the star,” says Margia Bergmann, a researcher at the Max Planck Institute.
“Different parent populations, such as the disk or the halo of the Milky Way, the dwarf satellite galaxies or the globular clusters, are known to have radically different chemical compositions. So once we know what the stars are made of, we can immediately connect them to their parent populations. ”
By comparing the chemical compositions of these stars with those found in other cosmic structures, the scientists noticed that the structures were almost identical. They were not only similar within the studied groups – and between them – but also corresponded closely to the star abundance patterns found in the outer disk of the Milky Way. The researchers then naturally concluded that these stellar populations in the galactic halo were formed in the Milky Way before being relocated above and below the galactic disk.
This phenomenon is called galactic eviction. “Star structures are pushed out of the Milky Way when a massive dwarf galaxy crosses the galactic disk,” notes Kate Van Nuys Page of Caltech (USA). “This passage causes oscillations that eject the stars from the disc, either above or below, depending on the direction in which the disturbing mass is moving.”
Astronomers plan to analyze additional star spectra to determine the masses and ages of these stars so that they can constrain the temporal limits of this galactic expulsion.