'Fossil galaxy' found hidden deep Inside the Milky Way

LIVERPOOL, ENGLAND — Scientists have discovered the remnants of what they call a "fossil" galaxy buried in the inner depths of the Milky Way, according to a study published last week in the Monthly Notices of the Royal Astronomical Society.

The galaxy, which has been named Hercules, is thought to have collided 10 billion years ago with the Milky Way, which itself is about 13.5 billion years old.

This ancient collision is thought to account for about a third of the mass of the Milky Way's spherical halo. It had not been observed before because stars inside the dense center of the Milky Way are hidden by massive clouds of interstellar dust.

In their study, scientists used data collected by the Sloan Digital Sky Survey's Apache Point Observatory Galactic Evolution Experiment, or APOGEE, on more than half a million stars.

"Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities," said lead author Danny Horta, a graduate student at Liverpool John Moores University. "These stars are so different that they could only have come from another galaxy."