Scientists working on data from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) Sloan Digital Sky Surveys (SDSS) have discovered a “fossilized galaxy” hidden in the depths of our Milky Way.
This result, published in the Monthly Notices of the Royal Astronomical Society, could shock our understanding of how the Milky Way grew into the galaxy we see today.
The proposed petrified galaxy could have collided with the Milky Way 10 billion years ago, when our galaxy was still in its infancy. Astronomers named it Heracles after the ancient Greek hero who, according to mythology, received the gift of immortality, and from the milk he drank the Milky Way was created.
The remains of Heracles make up about ⅓ of the spherical halo of the Milky Way. But if the gas and stars of Heracles make up such a large percentage of the galactic halo, why didn’t we know it before? The answer lies in its location deep in the Milky Way.
To find a fossilized galaxy like this, we had to look at the detailed chemical composition and motions of tens of thousands of stars. This is especially difficult for stars in the center of the Milky Way because they are obscured by clouds of interstellar dust. APOGEE allows us to cut through this dust and peer deeper into the heart of the Milky Way than ever before Says Ricardo Schiavon of the John Moores University of Liverpool (LJMU) in the UK, a key member of the research team.
APOGEE does this and simultaneously records star spectra in the near infrared rather than visible light, which is obscured by dust. Over its ten-year observation period, APOGEE has measured spectra for more than half a million stars across the Milky Way, including its core, which was previously obscured by dust.
Danny Horta, a LJMU graduate, lead author of the article explains: Examining so many stars is necessary to find unusual stars in the densely populated heart of the Milky Way, which is like looking for needles in a haystack.
To separate Heracles’ stars from those of the Milky Way, the team used both the chemical composition and the velocities of the stars measured by the APOGEE instrument.
Of the tens of thousands of stars we watched, several hundred had strikingly different chemical compositions and velocities. These stars are so different that they could only have come from another galaxy. Let us study them in detail so that we were able to trace the exact location and history of this fossilized galaxy Horta said.
Galaxies are formed by the merging of smaller galaxies over time, so remnants of older galaxies are often spotted in the outer halo of the Milky Way, the huge but very rare cloud of stars surrounding the main galaxy. Our galaxy was building from within, and finding the earliest connections requires looking at the most central parts of the Milky Way’s halo, which are buried deep within the disc and central bulge.
Heracles’ primordial stars now make up roughly the mass of the entire halo of the Milky Way – meaning that this newly discovered ancient collision must have been an important event in our galaxy’s history. This suggests that our galaxy may be unusual because most of the similar massive spiral galaxies had a much calmer early life.
As our cosmic home, the Milky Way is already special to us, but this ancient galaxy that is buried in it makes it even more special Says Schiavon.
Karen Masters, a spokesman for SDSS-IV, comments: APOGEE is one of the flagship reviews of Phase 4 SDSS and this result is an example of the amazing science anyone can do, now that we’ve almost completed our ten-year mission.
This new era of discovery will not end with the end of the APOGEE operation. The fifth phase of SDSS has already started collecting data, and its “mapping the Milky Way” will build on APOGEE’s success in measuring the spectra of ten times as many stars in all parts of the Milky Way using near infrared, visible light, and sometimes both.
In the picture: An artistic vision of what the Milky Way might look like seen from above. The colored rings show the approximate size of the petrified galaxy known as Heracles. The yellow dot shows the position of the sun. Image Credit: Danny Horta-Darrington (Liverpool John Moores University), NASA / JPL-Caltech and SDSS.