'Fossil galaxy' found hidden deep inside the Milky Way: study

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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."

RUNDOWN SHOWS:
1. Sloan Digital Sky Survey's Apache Point Observatory in New Mexico
2. Hercules galaxy collides and merges with the Milky Way
3. Locations of the Solar System and Heracles' remnants in the Milky Way
4. Depiction of the fossil galaxy being observed by a APOGEE
5. Stars observed to have unusual chemical compositions and velocities
6. Milky Way with remnants of the fossil galaxy highlighted on its spherical halo

VOICEOVER (in English):

"Scientists looking over data from the Sloan Digital Sky Survey's Apache Point Observatory Galactic Evolution Experiment, or APOGEE, discovered the remnants of what they call a 'fossil' galaxy buried in the inner depths of the Milky Way."

"The galaxy, which has been named Hercules, is thought to have collided with the Milky Way 10 billion years ago — 3.5 billion years after the Milky Way formed.

"This ancient collision is thought to account for about a third of the mass of the Milky Way's spherical halo. But no one could see it before, because stars inside the densely populated center of the Milky Way are hidden by gigantic clouds of interstellar dust."

"In their study, which was published last week in the Monthly Notices of the Royal Astronomical Society, scientists used data collected by APOGEE on more than half a million stars."

"Lead author Danny Horta, a graduate student at Liverpool John Moores University, explained the results in a press release provided by the Sloan Digital Sky Survey."

"'Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,' he said. 'These stars are so different that they could only have come from another galaxy.'"

"Because Hercules accounts for roughly one third of the mass of the entire Milky Way halo, the collision must have been an important event in the history of our galaxy."

"According to the Sloan Digital Sky Survey, this suggests our galaxy may be unusual, since most similar massive spiral galaxies have not had such traumatic early lives."

SOURCES: Sloan Digital Sky Survey, Monthly Notices of the Royal Astronomical Society
https://phys.org/news/2020-11-astronomers-fossil-galaxy-deep-milky.amp
https://academic.oup.com/mnras