Astronomers Find ‘Fossil Galaxy’ Hidden inside our Milky Way Dec 12, 2020 14:00:21 GMT 10
Post by Wayne Smith on Dec 12, 2020 14:00:21 GMT 10
Astronomers Find ‘Fossil Galaxy’ Hidden in Depths of Milky Way
A small satellite galaxy, named Heracles, collided with our Milky Way Galaxy about 10 billion years ago and its remnants account for about one third of the Milky Way’s spherical halo, according to an analysis of data gathered by Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE).
“To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” said Dr. Ricardo Schiavon, an astronomer at Liverpool John Moores University.
“That is especially hard to do for stars in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust.”
“APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before.”
“APOGEE does this by taking spectra of stars in near-infrared light, instead of visible light, which gets obscured by dust.”
Over its ten-year observational life, APOGEE has measured spectra for more than half a million stars all across the Milky Way, including its previously dust-obscured core.
“Examining such a large number of stars is necessary to find unusual stars in the densely-populated heart of the Milky Way, which is like finding needles in a haystack,” said Danny Horta, a graduate student at Liverpool John Moores University.
To separate stars belonging to Heracles from those of the original Milky Way, the astronomers made use of both chemical compositions and velocities of stars measured by the APOGEE instrument.
“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,” Horta said.
“These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy.”
The team found that Heracles’ stars account for roughly one third of the mass of the entire Milky Way halo today, meaning that this newly-discovered ancient collision must have been a major event in the history of our Galaxy.
That suggests that the Milky Way may be unusual, since most similar massive spiral galaxies had much calmer early lives.
“As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within makes it even more special,” Dr. Schiavon said.
The findings are published in the Monthly Notices of the Royal Astronomical Society.