The region known as the galactic halo lies outside the swirling spiral arms that form the Milky Way’s recognizable central disk and is rarely populated by stars. While the halo may appear to be mostly empty, it is assumed there is a lot of dark matter in it. It is believed that this mysterious and invisible substance makes up the bulk of the entire mass of the Universe.
The data for the new map comes from the ESA Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer – or NEOWISE, which operated from 2009 to 2013 under the name WISE. The study uses data collected by the spacecraft between 2009 and 2018.
The new map shows how a small galaxy – the Large Magellanic Cloud (LMC) – passed through the Milky Way’s galactic halo like a ship through water, leaving a gravitational trail in the stars behind it. LMC is located about 160,000 light-years from Earth and is less than a quarter of the mass of the Milky Way.
Although the interior of the halo has already been rendered with a high degree of accuracy, the new map is the first to give a similar picture of its outer regions – from 200,000 light-years to 325,000 light-years from the galactic center. Previous research has hinted at the existence of a “star trail,” but an all-sky map confirms its presence and offers detailed insight into its shape, size and location.