A globular cluster in the Large Magellanic Cloud, a neighboring galaxy of the Milky Way, stands out among the other representatives of its kind. And leaves experts puzzling.
The globular cluster NGC 2005 is apparently an alien intruder in the Large Magellanic Cloud, a small neighboring galaxy to the Milky Way: The chemical composition of its stars differs significantly from that of the other star clusters in the dwarf galaxy, as observations show. Accordingly, it must be a remnant of a smaller galaxy that once merged with the Large Magellanic Cloud, writes an international research team in the journal Nature Astronomy. You see in this a confirmation of the “hierarchical scenario” of cosmology.
“After that, the large galaxies we are observing today achieved their enormous mass through merging with many small galaxies,” explain Alessio Mucciarelli from the University of Bologna in Italy and his colleagues. “But this process should also take place in the smaller satellite galaxies such as the Magellanic Clouds.” This can be seen in computer simulations of the cosmic development – but so far there have been no observations that prove such mergers in dwarf galaxies.
Mucciarelli and his colleagues examined 11 globular clusters in the Large Magellanic Cloud and 15 globular clusters in the Milky Way using their own observations and archive data from the Very Large Telescope of the European Southern Observatory Eso in Chile. Globular clusters are very dense collections of many thousands of stars. “Because of their density, they can survive mergers for billions of years,” say the researchers – while a smaller galaxy that has penetrated a larger one has already dissolved without a trace over such a period of time.
As expected, the chemical composition of the stars in the globular clusters of the Milky Way and the Large Magellanic Cloud differs significantly from one another, since both galaxies have gone through different evolutionary histories. Mucciarelli sees this as confirmation that globular clusters are well suited for studying the evolution of galaxies. But there is an outlier among the globular clusters of the Large Magellanic Cloud: NGC 2005 does not match the other clusters of the dwarf galaxy.
“The clear chemical differences between NGC 2005 and the other globular clusters show that there must be two completely different evolutionary paths here,” said Mucciarelli and his colleagues. NGC 2005 could not have formed in the same environment as the other star clusters. Rather, the lower proportion of heavy elements shows that NGC 2005 originated from a significantly smaller galaxy in which star formation took place more slowly than in the Large Magellanic Cloud.
So NGC 2005 is the only surviving witness of the merging of a small galaxy with the Large Magellanic Cloud. There is no longer any trace of the small galaxy itself. Accordingly, not only large galaxies, but also dwarf galaxies grow by merging.