The universe may be a large, ever-expanding place, but gravity is relentless. Because of it, galaxies group into clusters and eventually cannibalize themselves. Astronomers know that the Milky Way, for example, grew that way, but now there is evidence that the Large Magellanic Cloud, a dwarf satellite galaxy of ours, also “devoured” other objects, such as globular clusters.
Globular clusters are not exactly galaxies, but they are more massive than open clusters. In a single “globe” there may be thousands or millions of stars, often with less metallicity than open clusters. In astronomy, metallicity refers to the amount of elements besides hydrogen and helium (yes, anything else is called “metallic” in astronomical jargon).
Measuring metallicity is important because stars are the only “factories” of heavier elements in the universe. This means that the older a star, cluster or galaxy is, the lower the metallicity index in the composition, while younger stars had the opportunity to form from clouds enriched by other elements created by previous stellar generations.
Want to catch up on the best tech news of the day? Access and subscribe to our new youtube channel, Canaltech News. Everyday a summary of the main news from the tech world for you!Not much is known about the processes of formation of globular clusters. Astronomers suspected that they would have formed from a single large cloud of gas, but there is evidence that, after this initial formation, they also collide with other clouds to trigger a new creation of stars. Anyway, these objects are pretty consistent, even after being absorbed by a galaxy.
A study led by Alessio Mucciarelli, professor at the Department of Physics and Astronomy at the University of Bologna, Italy, reports that a certain cluster in the Large Magellanic Cloud, called NGC 2005, has lower metallicity than 10 other clusters observed in this same galaxy. This could mean that NGC 2005 had a peculiar origin: it would be the remnants of a smaller galaxy that was “swallowed up” by the Large Magellanic Cloud.
Located about 750 light-years away from the center of the Large Cloud, the cluster was, billions of years ago, a galaxy with low star-forming efficiency and a mass similar to that of a dwarf spheroidal galaxy. Over billions of years, the small galaxy was separated and most of its stars scattered, but the core remained cohesive, virtually intact, and is now known as NGC 2005.he research team compared the metallicity of stars in NGC 2005 — over 200,000! — with another ten clusters from the Large Cloud and another 15 from the Milky Way. The amount of heavier elements in NGC 2005 indicates that their origin occurred in regions of star formation significantly less efficient than all the rest of the Large Magellanic Cloud. “This is typical of the Milky Way’s dwarf spheroidal satellites,” the authors said, referring to dwarf spheroidal galaxies like Sagittarius and Fornax, which orbit our own galaxy.
Specifically, Fornax is relatively compatible with NGC 2005 in metallicity, and is large enough to be a parent galaxy of something like this cluster. In other words, in addition to NGC 2005 being a “relic of a previous merger” with the Large Magellanic Cloud, it also appears to be the remains of an ancient spheroidal galaxy like Fornax. It is also the first time that a study has shown that dwarf galaxies neighboring the Milky Way might have formed from even smaller galaxies.