Galaxies merge all the time in the universe. These cosmic collisions are not cataclysmic blink-of-an-eye events but drawn out for billions of years. No matter the timescale, though, the effects of these mergers are long-lasting.
The Milky Way has merged with several small galaxies during its long life. One such event was discovered last October. Astronomers used the Gaia space observatory to uncover evidence of a past merger. The Milky Way cannibalized the much smaller Gaia-Enceladus galaxy some 10 billion years ago.
As reported in Nature Astronomy, a different team has now used more of Gaia’s data to perform further analysis. The researchers were able to determine the ages of nearly 600,000 stars. These stars are located in the thick disk, the region around the thin disk where the spiral arms are, and the halo, the spherical region that surrounds the whole galaxy.
The Gaia observatory has measured the precise position, velocity, and color of 150 million stars. This has allowed researchers to determine the existence of two distinct populations in the Milky Way’s halo. One is bluer in color while the other appears red.
These two populations appear to have the same age distribution, suggesting they stopped forming around the same time. What's surprising, though, is the difference in chemical composition. The red stars have more heavy elements suggesting that they formed in the more massive galaxy, the main progenitor of the Milky Way. The blue ones come from Gaia-Enceladus.
“Our age determination reveals that the stars... were among the first formed in the Milky Way, during approximately the first 3 billion years of its evolution, just before the merger with Gaia-Enceladus took place. We can date the merger as occurring about 10 billion years ago,” the authors write in their paper.
This merger led to the Milky Way we know today. Some of the stars from Gaia-Enceladus were thrown into the halo as the small galaxy came tumbling in. But stars from the original progenitor’s disk must have been thrown about too, with some ending up as the red sequence of stars seen in this study.
The team also found remarkable similarities between the stars in the thick disk and the red halo stars. They suggest that they formed together, and were thrown into their current orbit by the merger. The event also brought in more gas, fueling the formation of new stars. While Gaia-Enceladus is long gone, its effects are still impacting the Milky Way today.