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Inner Workings: Astronomers track dwarf galaxies to better understand the Milky Way’s make-up and evolution

At the turn of this century, astronomers were confident they understood the Milky Way’s relationship with its galactic neighbors. Our home galaxy is the second-largest member of the Local Group, an assembly of more than 50 galaxies that spans roughly 10 million light-years. Many of the group’s smaller galaxies are satellites of the Milky Way—two of the closest companions are the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). Streams of gas and dust ripped from these dwarf galaxies seemed to show that they had made several trips around the Milky Way, bearing witness to the destruction wrought by our galaxy.



By studying the masses and movements of nearby galaxies, astronomers may collect details that rewrite the story of our own galaxy’s evolution. In this combined radio and visible-light image, a ribbon of gas called the Magellanic Stream (pink) stretches halfway around the Milky Way (light-blue band). Nearby galaxies known as Magellanic Clouds appear in the bottom right as white spots. Image courtesy of Nidever, et al. (Montana State University/NOAO), NRAO/AUI/NSF and Meilinger, Leiden-Argentine-Bonn Survey, Parkes Observatory, Westerbork Observatory, Arecibo Observatory.



But this neat picture was upended almost a decade ago when researchers used NASA’s Hubble Space Telescope to precisely measure the clouds’ motion. Suddenly, it became clear that the LMC and SMC are actually making their first orbit around the Milky Way. Since then, researchers have been trying to pin down the masses and movements of the clouds, details that could rewrite the story of our own galaxy’s evolution.

This year, the European Space Agency (ESA) space telescope Gaia has provided some vital clues in that quest. Gaia found that the LMC itself boasts several ultra-faint dwarf galaxies as satellites, observations that are helping constrain the masses of the cloud and the Milky Way. They also provide strong …