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. The proposed fossil galaxy may have collided with the Milky Way ten billion years ago, when our galaxy was still in its infancy. Astronomers named it Heracles, after the ancient Greek hero who received the gift of immortality when the Milky Way was created. The remnants of Heracles account for about one third of the Milky Way’s spherical halo. But if stars and gas from Heracles make up such a large percentage of the galactic halo, why didn’t we see it before? The answer lies in its location deep inside the Milky Way. “To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” says Ricardo Schiavon from Liverpool John Moores University (LJMU) in the UK, a key member of the research team. “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. APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before.” APOGEE does this by taking spectra of stars in near-infrared light, instead of visible light, which gets obscured by dust. 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. 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. […] 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. That suggests that our galaxy may be unusual, since most similar massive spiral galaxies had much calmer early lives. The findings have been reported in Monthly Notices of the Royal Astronomical Society.
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