A large international team of astronomers and astrophysicists has discovered some evidence that the Small Magellanic Cloud is not a single galaxy - it is actually two galaxies, one behind the other. The group wrote a paper describing their work and posted it to the arXiv preprint server.
For many years, the Magellanic Clouds were thought to be two irregular dwarf galaxies that could be seen very close to each other on the Southern Hemisphere's celestial sphere. Based on their size, they are also named the Large Magellanic Cloud and the Small Magellanic Cloud. In the late 1980s, some evidence suggested that there were not one, but two dwarf galaxies in the Small Magellanic Cloud (SMC). In this new study, researchers found more evidence that the Small Magellanic Cloud is indeed two small dwarf galaxies.
To learn more about the SMC, the team first studied data from ESA's Gaia observatory, through which they estimated the average speed of stars in each part of the SMC. Next, they studied data from the Galaxy Australian Square Kilometer Array Pathfinder, a radio telescope array in Western Australia, to learn more about the interstellar medium of the SMC and LMC. They also analyzed APOGEE survey data from the Sloan Foundation Telescope and the New Mexico State University Telescope's Bidirectional 300 Fiber Spectrograph, both at the Apache Point Observatory in New Mexico.
Taking all the evidence together, the researchers found that the two "parts" of SMC are chemically distinct. They also found that the two parts had different velocities, with the part closer in the galaxy appearing to move faster. They also found that the two parts had roughly the same mass, and that both parts were interacting with the LMC.
Taken together, the researchers concluded that the evidence strongly suggests there are two unique galaxies, with one located nearly behind the other relative to Earth. They note that this positioning explains why it was only recently noted that the SMC is likely to be two galaxies.
The team also calculated that the closer of the two galaxies is about 199,000 light-years away from Earth, while the more distant one is about 215,000 light-years away.