“Fossil galaxy” found hiding deep inside the Milky Way
Shane McGlaun - Nov 21, 2020

Scientists sorting through data gathered from the Sloan Digital Sky Survey’s Apache Point Observatory Galactic Evolution Experiment (APOGEE) have discovered what they call a “fossil galaxy” that’s tucked away deep inside the Milky Way. Scientists say that the proposed fossil galaxy could have collided with the Milky Way 10 billion years ago when our galaxy was still in its infancy. The fossil galaxy has been named Heracles.

Heracles’ remnants account for about a third of the Milky Way’s spherical halo. As for why no one noticed that there was a remnant of an ancient galaxy inside of our galaxy, it’s because of how deep inside the Milky Way it is. Researcher Ricardo Schiavon from Liverpool John Moores University says to find the fossil galaxy, researchers had to look at the detailed chemical makeup and motion of tens of thousands of stars.

Looking at that many stars is incredibly difficult in the center of the Milky Way because they are hidden from view by gigantic clouds of interstellar dust. APOGEE is perfect for this sort of investigation as it allows astronomers to peer through that dust and look deeper into the heart of our galaxy than ever before. APOGEE allows scientists to look through interstellar dust using near-infrared light, which isn’t obscured by dust the way visible light is.

Finding unusual stars in the heart of the Milky Way is likened to finding needles in a haystack. To separate stars belonging to Heracles from stars in the original Milky Way, the team used both chemical composition and velocity of stars as measured by APOGEE.

Researchers say out of the tens of thousands of stars investigated, a few hundred had strikingly different chemical compositions and velocities. Researchers say those stars were so different from the stars in the Milky Way that they could only have come from another galaxy. A detailed study could allow the researchers to trace the precise location and history of the fossil galaxy.