Astronomers still aren’t entirely sure how galaxies form, but they do know that collisions between galaxies are pretty common. In fact, our own Milky Way Galaxy is currently on a collision course with our neighbouring galaxy Andromeda, and astronomers predict that this major galaxy merger will occur 4.5 billion years in the future.
But according to new research, the Milky Way Galaxy has already gone through similarly-massive collisions in the past. By studying the chemical makeup of stars in the Milky Way, a team of astronomers discovered the remnants of a fossil galaxy called Heracles that collided with the Milky Way roughly 10 billion years ago, and currently accounts for about one third of the entire Milky Way halo.
The study was published last month in Monthly Notices of the Royal Astronomical Society, and was co-authored by Ted Mackereth, a Banting-CITA-Dunlap Fellow in Astrophysics at the University of Toronto.
Piercing through the dust
You might think that a galaxy remnant accounting for one third of the Milky Way’s halo (the region containing most of our galaxy’s mass) would be easy to spot. However, our vantage point from inside the Milky Way makes studying the rest of the galaxy tricky. Stars located near the Milky Way’s centre are particularly difficult to observe, mostly due to large clouds of dust and gas that block visible light from reaching our telescopes.
To get around this, a team of astronomers used infrared observations from the Sloan Digital Sky Survey (SDSS)’s Apache Point Observatory Galactic Evolution Experiment, or APOGEE. Infrared light, which is made up of longer wavelengths than visible light, can penetrate through the dusty clouds scattered throughout our galaxy, allowing astronomers to get a better view of regions that would be hidden otherwise.
“APOGEE lets us pierce through [the] dust and see deeper into the heart of the Milky Way than ever before,” said Ricardo Schiavon, a Reader in Astrophysics at Liverpool John Moores University (LJMU) and co-author on the study, in an interview.
The astronomers used infrared data from APOGEE to study the chemical compositions and velocities of tens of thousands of stars throughout our galaxy. While stars are mostly made up of hydrogen and helium, they can also contain trace amounts of other elements (calcium or sodium, for example) which astronomers refer to as metals. The types and amounts of different elements in a star can shed light on where and when that star formed, providing an archeological record of that star’s past.
A few hundred stars were “strikingly different”
As the team gathered more observations, they started to realize that a significant fraction of the stars they’d been studying were unusual compared to the rest of the Milky Way’s stellar population.
“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities,” said Danny Horta, a PhD student at LJMU and lead author of the study. “These stars are so different that they could only have come from another galaxy.”
By studying the velocities and trajectories of these unusual stars, the team was able to piece together their history. They realized that these stars all came from a different galaxy with its own unique chemical composition that must have collided with the Milky Way in the distant past.
“By studying [these stars] in detail,” said Horta, “we could trace out the precise location and history of this fossil galaxy.”
The fact that these stars make up one third of the Milky Way’s halo mass today means that the collision between the Milky Way and Heracles must have been a major event in our galaxy’s past. Collisions this large are unusual for young spiral galaxies, which makes the discovery even more interesting.
“As our cosmic home, the Milky Way is already special to us,” added Schiavon, “but this ancient galaxy buried within makes it even more special.”
Even more mysteries to uncover
While this discovery has provided astronomers with an unprecedented look into our galaxy’s past, it likely won’t be the last major result to be uncovered about our astronomical history.
APOGEE has already observed more than half a million stars throughout the Milky Way Galaxy that astronomers still have to study — and going forward, SDSS’s Milky Way Mapper will provide astronomers with observations of ten times more stars.