In one of the most exciting of the new mysteries unveiled by the new, second release of the European Space Agency’s Gaia spacecraft data, U.K. astronomers traced the origin of a hypervelocity star to the center of the Large Magellanic Cloud, the Milky Way’s largest satellite galaxy. It could mean only one thing: that the star was accelerated by whipping around a massive black hole at the center of the Large Magellanic Cloud. Full-size galaxies almost always harbor huge central black holes, which are of mysterious provenance. Their presence in some mini galaxies compounds the mystery.
Antoja’s paper, continues Wolchover, is one of a torrent following the long-awaited second data release from Gaia, which was launched in 2013 and has since charted the positions, brightnesses and colors of 1.7 billion Milky Way stars, and the velocities of 1.3 billion of those stars. (In September 2016, the Gaia team released its first map with only position and brightness measurements for 1.1 billion stars.) Astronomers, who had previously catalogued just 2.5 million of the brightest stars in the galaxy, are hailing a new era of precision astronomy. These are some of the most important discoveries to come from the Gaia data so far.
A team in France applied their preprepared STREAMFINDER algorithm to the Gaia data and immediately uncovered a rich network of “stellar streams,” or tributaries of stars flowing into and around the Milky Way. “The idea is to trace the streams backward in time along their orbits in order to contemplate the galaxy’s past and its formation history,” said Khyati Malhan of the University of Strasbourg, lead author of the paper detailing these “galactic archaeology” findings, in an email.
The profusion of stellar streams — believed to be remnants of small satellite galaxies and star clusters that were drawn in by gravity — could potentially resolve the “missing satellite problem,” which asks why only 50-odd satellite galaxies currently orbit the Milky Way, despite hundreds arising in computer simulations of galaxy formation. Another mystery is why the Milky Way’s satellites lie in a plane, even though simulations suggest they should have formed all around. Malhan and colleagues hope to either sharpen or resolve this plane-of-satellite problem via “statistical analysis of structure and dynamics of a large sample of streams,” he said.
Another group used the Gaia data to do a detailed study of the galaxy’s longest stellar stream. Some of its stars appear to have been perturbed by patches of invisible dark matter, suggesting the streams can be used to map dark matter substructure throughout the galaxy.
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