Just as we use several bricks to create a wall, the galaxies that populate our Universe are thought to have been built from the merger of smaller ones. The "Milky Way building" is the result of this assembly process and, recently, several building blocks of our galaxy have been discovered.
Smaller galaxies are expected to be built following the same architectural plan, using smaller bricks. One way to test this assumption is to investigate the most massive galaxy among the Milky Way's small satellite galaxies: the Large Magellanic Cloud (LMC). To separate the stars that originally made up this satellite galaxy from those that were accreted from another, smaller system, the chemical composition of all the clusters of this parent galaxy must be studied to identify systems with different chemistry.
To achieve this, an international group of researchers led by Alessio Mucciarelli, an academic at the Department of Physics and Astronomy of the University of Bologna, Italy, studied the chemistry of eleven old star clusters of the LMC, concluding that one of them was not born in this galaxy and thus demonstrating that galaxies of any size are built following the same scheme. This research was published this October 18 in the prestigious journal Nature Astronomy.
In this study, chemical abundances were measured in stars of globular clusters of the LMC and the Milky Way, using high-resolution optical spectra obtained with the MIKE spectrograph installed on the Magellan Telescope of Las Campanas Observatory, and the UVES and FLAMES spectrographs on the Very Large Telescope of the European Southern Observatory (ESO).
According to the researchers, the NGC2005 cluster, identified as the "intruder" based on its chemical peculiarities, was born in a galaxy that formed its stars at a much slower rate than the LMC. Therefore, it is very likely that its progenitor was one of the small satellite galaxies of the LMC itself, which was completely disrupted by the accretion process. This former satellite galaxy dissolved and NGC2005 would be the only surviving witness of this system, preserving in its chemical composition the memory of the chemical evolution of the parent galaxy.
"Previous work only focused on the chemistry of a few globular clusters in the LMC, which did not allow us to identify an intruder. In contrast, our work investigated for the first time almost all ancient globular clusters homogeneously, which allowed us to directly compare their chemical composition. In particular, the comparison between the chemistry of NGC2005 and that of LMC clusters with similar metallicity clearly reveals the distinct nature of the former," explains Alessio Mucciarelli.
The astronomer states that this discovery was possible thanks to the use of high-resolution spectra collected at the Magellan telescope of Las Campanas Observatory and ESO's Very Large Telescope. "We have shown, for the first time, that the process of hierarchical galaxy assembly also took place in a small nearby satellite of our own Milky Way, thus demonstrating that galaxies of any size are built following the same scheme," he says.
This result opens a new window in the study of stellar populations in Milky Way satellites, providing a powerful tool for reconstructing the assembly history of small Local Group galaxies. This can be obtained by investigating the chemical DNA of stars, which preserves the memory of the parent galaxy.
Image credit: A. Mucciarelli et al.