The Milky Way has been colliding with 12 galaxies since it formed billions of years ago, according to a new study published in the preprint server ArXiv.
Astronomers at the University of Heidelberg in Germany have concluded that for the past 12 billion or so years, the Milky Way galaxy has been merging with a dozen galaxies and three smaller ones.
By looking at globular clusters, the dense clusters of hundreds of thousands of stars that can be observed at the halo of the galaxy, the researchers have gained insight into the origin of the clusters and the galaxy itself.
Examining Globular Clusters
The researchers looked at 96 massive globular clusters orbiting the center of Milky Way and measured the age and metallicity of stars found among them, the latter referring to the abundance of heavy elements found in stellar material. Galaxies composed of stars rich in heavy elements are typically older than those with stars not as abundant in metals.
They have found that the Milky Way has been forming metal-rich galaxies since 12 billion years ago, which means it must have collided with large galaxies. The scientists have also discovered 35 globular clusters with less metal-rich stars, suggesting that the Milky Way also consumed two smaller galaxies.
It is, in fact, in the process of merging with a dwarf galaxy. Called Sagittarius, the unfortunate dwarf is one of the nine galaxies found to be orbiting the Milky Way. For the next 100 million years or so, Sagittarius will be moving right through the galaxy, where the strong gravitational pull from the collision will presumably tear it apart.
Interestingly, this is not the first time the dwarf galaxy has collided with Milky Way. Its orbit shows it has come in contact with the galaxy several times in the past and has surprisingly survived the collision. Researchers think Sagittarius may be harboring dark matter, which has kept it intact through the impacts.
New Research On Globular Clusters
The validity of the new findings may be at risk of being watered down owing to a recent study proposing that globular clusters may not be as old as once thought. In research published in the Monthly Notices of the Royal Astronomical Society, astronomers suggested that globular clusters may be 9 billion years old, not 13 billion years old as previously proposed.
This could complicate matters for the study done by the Heidelberg team, but study leader Diederik Kruijssen says the new finding on globular clusters only applies to faraway galaxies. For their research, they looked at clusters so near that they could study the elements of each star one by one.
"The major effect advertised in their press release only affects the globular cluster populations of other galaxies and does not apply to the globular cluster system of the Milky Way," Kruijssen says.
Consistent With Dark Matter Research
Kruijssen and his team's research are consistent with the data extracted from the Dark Energy Survey, a three-year study conducted by an international team of researchers launched in 2013.
The survey concludes that the Milky Way has gobbled up nearly a dozen galaxies, the remains of which can be observed rushing through space in what is called stellar streams.
The 570-megapixel Dark Energy Camera used for the survey detected 11 new stellar streams. By studying the positions and trajectories of these streams, the researchers were able to conclude that they were once part of galaxies that merged with the Milky Way.