[...] Stars originally belonging to Heracles account for roughly one third of the mass of the entire Milky Way halo today -- meaning that this newly-discovered ancient collision must have been a major event in the history of our galaxy.
Over its ten-year observational life, APOGEE has measured spectra for more than half a million stars all across the Milky Way, including its previously dust-obscured core.
"That is especially hard to do for stars in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust.
fahrbot-bot shares a report from Phys.Org: Scientists working with data from the Sloan Digital Sky Surveys' Apache Point Observatory Galactic Evolution Experiment (APOGEE) have discovered a 'fossil galaxy' hidden in the depths of our own Milky Way.
To separate stars belonging to Heracles from those of the original Milky Way, the team made use of both chemical compositions and velocities of stars measured by the APOGEE instrument.