Wait, "fossil galaxies" are a thing?

I had no idea there was such a thing as fossil galaxies, remnants of older galaxies that exists within a current galaxy. Apparently such a fossil galaxy has been discovered in the center of the Milky Way.

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.

This result, published today in Monthly Notices of the Royal Astronomical Society, may shake up our understanding of how the Milky Way grew into the galaxy we see today.

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.

Read the rest of Phys.org.

Image: Danny Horta-Darrington (Liverpool John Moores University), NASA/JPL-Caltech, and the SDSS