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.