Astronomers know for many years that galaxies can be cannibalistic. Galaxies with large masses, such as our Milky Way, have absorbed smaller neighbors to gain mass.
It seems that smaller galaxies, such as the Large Magellanic Cloud, have also been able to feast on their smaller neighbors.
Astronomers discovered evidence that the Large Magellanic cloud (LMC) has absorbed globular clumps. Globular clusters don't look like galaxies. They are spherical clusters with thousands, or even millions, of stars. They are older and have a lower metallicity than open-clusters. Astronomers believe that globular clusters may remain partially coherent even after being absorbed into a larger galaxy such as the LMC.
A paper by an Italian and Dutch astronomer has been published. It presents evidence that the LMC is becoming more massive through its absorption of globular clusters. Title: A relic of a past merger in the Large Magellanic Cloud. Nature Astronomy published the paper.
Globular clusters (GCs), are a bit mysterious. Astronomers believed that they were collections or stars formed from one molecular cloud for a long time. It seemed that all stars in a cluster were the same age and had the same metallicity. As our observing power improved, we discovered that many clusters have multiple populations of stars with different ages and metalities. Astronomers now believe that the cluster might have encountered another large molecular cloud after it formed. This could have triggered another round star formation.
NASA/ESA Hubble Space Telescope photo of NGC 7006, a distant and compact globular star cluster. It is a Class 1 globular star cluster. This means that its stars are more concentrated than diffused and spread out. Image Credit: By NASA Hubble https://www.flickr.com/photos/[email protected]/49200475127/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=87407272
Fortunately, even though GCs may have many populations of stars, the centers of these stars can still hold together when they merge with a galaxy. This is crucial for this study.
This study was done by a team of researchers who looked at 11 different GCs within the Large Magellanic Cloud. NGC 2005 is one of these GCs. It lies approximately 750 light years from the LMC's center and has about 200,000 stars. NGC 2005 stars are more diverse than other stars in LMC because they have less zinc, copper and silicon than the other 10 LMC clusters.
Astronomers believe that NGC 2005 is a relic from a smaller galaxy that was destroyed by the LMC billions years ago. This smaller galaxy had a lower star formation efficiency than a dwarf spheroidal galaxie and a mass that was similar to one. The small galaxy split over billions of years and many of its stars were scattered around. The core was not destroyed. The core, which is known as NGC 2005, remained mostly intact after the rest of the small galaxy disintegrated.
In reality, we are seeing a remnant of an earlier merger.
Composite image of NGC 2005 (left), and the Large Magellanic Cloud. The chemical composition of NGC 2005's stars is different from the Large Magellanic Cloud stars. This is the first evidence that dwarf galaxies have merged outside of our Milky Way. (c) HLA/Fabian RR/ESO/VMC Survey/Astronomie.nl [CC BY-SA 3.0]
To identify stars with common origins, researchers use chemical tagging. This is done when there's no other evidence to support the claim. Chemical tagging, which is one of few methods that allows us to track completely dissolved satellites even if there are no kinematically coherent or spatially coherent relics, is one of the most useful.
The authors describe stars and clusters that were lost by their unusual chemical composition. This contrasts with the current environment.
This is difficult because it requires high-resolution spectrumcopy. This requires a thorough analysis of chemical abundances. These can be affected by slight differences in assumptions about the astronomical parameters of the objects being studied. The star's effective temperature is the most important parameter. If this parameter is not met, the rest of the results will likely be incorrect. This research team used the 11 GCs to trace these problems.
Because GCs can be used as tracers, even though one of them is destroyed in the larger galaxy with which they have merged, their cores remain intact. They will also keep records of the environment it was born in.
Researchers compared NGC 2005's metallicity with those of 10 other GCs from the LMC and 15 older GCs within the Milky Way. The table below clearly shows that NGC 2005's metallicity stands out.
NGC 2005's metallicity is distinct from the other GCs within the Large Magellanic Cloud or older GCs throughout the Milky Way. It contains less Silicon, Calcium and Copper than the other GCs in the Large Magellanic Cloud. The average weighted abundance ratios of [Fe/H],[Si/Fe],[Ca/Fe],[Cu/Fe], and [Zn/Fe] were calculated for the LMC and MW-old GCs. In brackets you will find the standard error and the dispersion for the weighted means. Image Credit: Mucciarelli et al, 2021.
Is there any other evidence that mergers occur?
According to the team, the abundances of metallicity in NGC 2005 and other GCs in LMC suggest that they were formed in areas with significantly lower star formation efficiency than the rest. This is typical for dwarf spheroidal satellites (dSph), of the Milky Way. It is therefore natural to search for a galaxy that is similar to NGC 2005's putative progenitor.
Fornax and Sagittarius are the only dwarf spheroidal galaxies around the Milky Way capable of forming GCs. Sagittarius metallicity has a similarity to the LMCs and is therefore not comparable to NGC 2005. Fornax, however, is more like a match in chemical abundance. It is also large enough that it could be a progenitor of something like NGC 2005. Fornax instead has a stellar mass of 2107 M, which is large enough to support 5 old GCs. Four of these GCs are in the same mass range of NGC 2005 (&1.3105M).
These findings convinced the team.
Davide Massari, coauthor of the University of Groningen paper and researcher at the INAF in Groningen, stated that we are actually witnessing a relic of an older merger. We have now shown for the first-time that smaller galaxies near our Milky Way have built up from smaller galaxies.
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