The "hierarchical model of galaxy formation" describes, somewhat less technically, the principle of "big eats small": Large galaxies grow by merging with smaller galaxies. Researchers led by Alessio Mucciarelli from the University of Bologna have now used the Large Magellanic Cloud, a satellite galaxy of our own Milky Way, to test whether the principle of "big eats small" also applies to "small eats small". Their conclusion: Yes, even initially smaller galaxies like the Large Magellanic Cloud grow by ingesting one or the other dwarf galaxy. The study appears in the journal »Nature Astronomy«.
The Large Magellanic Cloud is more familiar to the inhabitants of the southern hemisphere, who can see it with the naked eye between the constellations Swordfish and Table Mountain. It is about 163,000 light years away from us. It achieved some astronomical prominence when a supernova exploded in it in 1987, the next supernova explosion observed in modern times.
Afterwards, astronomers can understand whether a galaxy has once merged with another. For example, indications provide star streams, i.e. a group or rather loose accumulation of stars that move in similar directions at similar speeds. In 2018, researchers showed that neighboring galaxy grew into our Milky Way around ten billion years ago. The necessary data for the analysis had previously been collected by the GAIA mission. For their examination of the large Magellan cloud, the group around Mucciarelli could not access star streams or Gaia data. Instead, she examined objects that can survive a galactic collision with globular clusters. In addition, the chemical composition and the element frequency of ball star heaps are quite typical of their home galaxy. An once swallowed galaxy should therefore differ from a globe star heap, which was created in the large Magellan cloud itself.
The NGC 2005 globular cluster offers proof of a galaxy merger.
The astronomers examined a total of 13 global star clusters in the large Magellan cloud on their element frequencies to find out whether our satellite galaxy has swallowed another galaxy. The otherwise unspectacular bumper heap NGC 2005, which differs from the other globular clusters in the large Magellan cloud: the composition of his elements suggests that it can hardly have been created in the large Magellan cloud. Due to modeling, the researchers consider it likely to be the last remnant of a former dwarf galaxy, which had a lower starry rate than the large Magellan cloud.
When exactly and how these two galaxies merged, the results can give no information. In fact, NGC 2005 provides no other clues to its extragalactic origins: its radial velocity is similar to that of other globular clusters nearby. Thus, only its composition reveals that it did not originally originate in this satellite galaxy of the Milky Way, according to the researchers. They interpret their result as an indication that the model of hierarchical galaxy evolution applies not only to spiral galaxies such as the Milky Way, but also to smaller irregular dwarf galaxies such as the Magellanic Clouds.
