Our home galaxy is a cannibal. Recent evidence strongly suggests the Milky Way swallowed up a smaller galaxy, known as Gaia-Enceladus, during its early years, incorporating the galaxy's stars, gases and dust into its orbit. However, an exact date for this "accretion event", as the astronomers say, has been hard to pin down. New research, from a collaboration of scientists in Europe, examined the ages of the stars across the Milky Way to more accurately determine how the galaxy formed.
The research, published in journal Nature Astronomy on July 22, utilized a huge data set accumulated by the European Space Agency's , which provides accurate measurements of the position, brightness and movement of stars. The research team, led by Carme Gallart of the Instituto de Astrofísica de Canarias in Spain, was particularly interested in the Milky Way's "halo", a huge, spherical region of space surrounding the galaxy, and its "thick disk", a region full of stars that contains most of a spiral galaxy's mass.
Previous research, published in Nature in 2018, found evidence that a huge population of stars within the halo region of the Milky Way were moving in a different direction to the rest of the galaxy's billions of stars. The study concluded that this only made sense if a galactic collision occurred during the Milky Way's formative years.
Gallart's team built on that research, determining accurate ages for hundreds of thousands of stars in the halo and the disk. "Knowing how these components have formed and evolved over time," Gallart says, "implies knowing most of the galaxy's history." By dating the stars, a host of tantalizing new information allowed Gallart and her team to conclude that two distinct systems -- Gaia-Enceladus and the Milky Way -- were evolving separately 13 billion years ago. Moreover, studying the distribution of heavy elements in the systems, they found that the Milky Way was maturing more quickly than Gaia-Enceladus.
Then they collided.
The merger put the brakes on star formation for Gaia-Enceladus and threw the Milky Way into a period of chaos. The energy of the collision caused stars in the Milky Way to be flung out into the galactic halo. In addition, the extra gases and dust that passed through the Milky Way provided the galaxy with more material to form new stars in the inner disk.
"Apart from allowing us to date these early events, our analysis also indicated that the merger further stoked star formation in the Milky Way thick disk, which had been continuously forming stars since the earliest time," says Gallart.
All of these events occurred in tandem, around 10 billion years ago, leading the researchers to develop a far more accurate timing of the merger and paint a much clearer picture of how our galaxy formed.
The next step for the astronomers is to extend the timeframes even further which should provide an even more accurate determination of how the Milky Way's disk came to be.
"Now we are working on the full evolution of the disk as a function of time, till the present, using the same technique to derive ages and chemical compositions of the stars," she says.