The Milky Way and its nearest galactic neighbor, Andromeda, don't resemble each other much now, but a new study suggests the two shared similar beginnings and evolved in similar ways, at least over their first several billion years.
Researchers reached this surprising conclusion after surveying thousands of stars in Andromeda's halo and finding they are "metal-poor," or practically void of all elements other than hydrogen.
Astronomers had previously thought that Andromeda's stellar halo was metal-rich while the Milky Way's halo was metal-poor, said study team member Scott Chapman of the California Institute of Technology.
The researchers used the Keck-II telescope in Hawaii to measure the "radial velocity" of individual stars in Andromeda's halo. A star's radial velocity is the speed at which it is moving relative to Earth. Depending on whether a star is moving towards or away from Earth, it's light will appear more blue or red, respectively.
By analyzing a star's light spectrum, scientists can also determine its chemical makeup and the type of element it's burning as fuel.
Of the approximately 10,000 stars surveyed by the researchers, about 1,000 were located in Andromeda's halo. This halo is made up of mostly gas and stars and extends outward from the center of the galaxy by more than 500,000 light-years.
The researchers found that the stars in Andromeda's halo lack metals and thus probably formed early in the galaxy's evolution.
Stars near Andromeda's center, in contrast, contain more metals because they formed from heavy elements expelled into space by the burning of ancient stars and stellar explosions called supernovas.
The researchers think that both the Milky Way and Andromeda began forming within a half billion years of the Big Bang; over the course of three or four billion years, they began to gain mass and bulk up by gravitationally attracting matter to themselves with their huge stores of dark matter.
Dark matter is a substance believed to permeate galaxies, acting like a cosmic glue to prevent them from flying apart as a result of their rotations. Astronomers have yet to detect dark matter directly, although a recent study revealed some of its physical properties through indirect means.
According to current galaxy-formation theories, large concentrations of dark matter during the early period of the universe acted like "seeds" for today's galaxies by pulling in groups of stars as they passed by.
It's thought that galaxies like Andromeda and the Milky Way have each gobbled up about 200 smaller galaxies and clumps of interstellar matter over the last 12 billion years.
The new finding that our Milky Way and Andromeda shared similar evolutionary histories could lead to new insights about dark matter, said study team member Rodrigo Ibata of the Observatoire de Strasbourg in France.
"This is the first time we've been able to obtain a panoramic view of the motions of stars in the halo of a galaxy," Ibata said. "These stars allow us to weigh the dark matter, and determine how it decreases with distance."
The finding will be detailed in an upcoming issue of Astrophysical Journal.