A ‘fossil galaxy’ hidden in the depths of our own Milky Way

It may have collided with the Milky Way ten billion years ago.


A new study may revolutionize our understanding of how the Milky Way grew into the galaxy we see today. It suggests that a “fossil galaxy” is hidden in the depths of our own Milky Way.

The outcomes come from data observed from the Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE).

According to astronomers, this fossil galaxy might have collided with the Milky Way ten billion years ago. Astronomers dubbed this fossil galaxy as Heracles after the ancient Greek hero received the gift of immortality when the Milky Way was created.

The remnants of Heracles represent around one-third of the Milky Way’s spherical halo. Yet, on the off chance that stars and gas from Heracles make up such a large percentage of the galactic halo, why did we miss it? The answer lies in its location somewhere inside the Milky Way.

Ricardo Schiavon from Liverpool John Moores University (LJMU) in the UK, a vital member of the research team, said, “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. That is especially hard to do for stars in the Milky Way center 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.”

Graduate student Danny Horta from LJMU, the lead author of the paper announcing the result, explains, “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.”

The team used chemical compositions and velocities of stars measured by the APOGEE instrument- to separate stars belonging to Heracles from those of the original Milky Way.

Horta said, “Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities. 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.”

Stars that belong to this fossil galaxy account for roughly one-third of the entire Milky Way halo mass. It means this newly-discovered ancient collision must have been a major event in the history of our galaxy.

Schiavon said, “As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within makes it even more special.”

Karen Masters, the Spokesperson for SDSS-IV, comments“APOGEE is one of the flagship surveys of the fourth phase of SDSS, and this result is an example of the amazing science that anyone can do, now that we have almost completed our ten-year mission.”

Journal Reference:
  1. Evidence from APOGEE for the Presence of a Major Building Block of the Halo Buried in the Inner Galaxy,” Danny Horta et al., 2020 Nov. 20, Monthly Notices of the Royal Astronomical Society: arxiv.org/abs/2007.10374