Researchers working on data from Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE) have discovered a “fossil galaxy” hidden in the depths of our own The Milky Way.
This result, published today in the Monthly Notices of the Royal Astronomical Society, may shake our understanding of how the Milky Way grew into the galaxy we see today.
“APOGEE lets us penetrate that dust and look deeper into the heart of the Milky Way than ever before.” – Ricardo Schiavon
The proposed fossil galaxy may have collided with the Milky Way ten billion years ago when our galaxy was still in its infancy. Astronomers called it Hercules after the ancient Greek hero who received the gift of immortality when the Milky Way was created.
The remains of Heracles account for about a third of the Milky Way’s spherical halo. But if stars and gas from Heracles make up such a large percentage of the galactic halo, why did we not see it before? The answer lies in its location deep inside the Milky Way.
“To find a fossil galaxy like this, we had to look at the detailed chemical composition and motions of tens of thousands of stars,” said Ricardo Schiavon of Liverpool John Moore’s University (LJMU) in the UK, a key member of the research team. “It is especially difficult to do for stars in the middle of the Milky Way because they are hidden from view by clouds of interstellar dust. APOGEE lets us penetrate that dust and look 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 being obscured by dust. Over the course of its ten-year observation life, APOGEE has measured spectra of more than half a million stars across the Milky Way, including its former dust-obscured core.
Graduate student Danny Horta of 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. ”
To distinguish stars belonging to Heracles from the stars in the original Milky Way, the team used both chemical compositions and velocities of stars measured using the APOGEE instrument.
“Of the tens of thousands of stars we looked at, a few hundred had markedly 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 were able to trace the exact location and history of this fossil galaxy. ”
This movie shows a computer simulation of a galaxy like the Milky Way. The film winds through simulated time from 13 billion years ago to today. The main galaxy grows as many small galaxies merge with it. Heracles resembles one of the smaller galaxies that merged with the Milky Way early in the process. Credit: Video built by Ted Mackereth based on the EAGLE simulations
Because galaxies are built through the fusion of smaller galaxies over time, the remnants of older galaxies are often seen in the outer halo of the Milky Way, a huge but very sparse cloud of stars that encloses the main galaxy. But since our Galaxy is built from the inside out, it requires looking at the most central parts of the Milky Way’s halo to find the earliest fusions, which are buried deep inside the disk and bulge.
Stars originally belonging to Heracles account for approx. a third of the mass of the entire Milky Way halo today – meaning that this newly discovered ancient collision must have been an important event in our Galaxy history. This suggests that our galaxy may be unusual, as most similar massive spiral galaxies had much calmer early lives.
“Like our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried inside makes it even more special,” says Schiavon.
Karen Masters, spokeswoman for SDSS-IV, comments, “APOGEE is one of the flagship studies in the fourth phase of SDSS, and this result is an example of the amazing science that anyone can do now that we’re almost finished our ten- years of mission. ”
And the discovery of this new age does not end with the end of APOGEE observations. The fifth phase of SDSS has already begun to take data, and its “Milky Way Mapper” will build on the success of APOGEE in measuring spectra ten times as many stars in all parts of the Milky Way using near-infrared light, visible light, and sometimes both.