If you are a small galaxy and want to shape new stars, come to the Milky Way – but don’t get too close if your aim is a long career as a star maker. New observations with the Gaia Space Telescope show that our galaxy is both friend and foe of the smaller galaxies that revolve around it.
Around 60 known galaxies orbit the Milky Way. About a dozen of these satellite galaxies are dark dwarf spheroidals, each emitting only 0.0005 to 0.1 percent as much light as the Milky Way (SN: 12/22/14). Their few stars are distant from each other, which gives the galaxies such a ghostly appearance that the first one found was originally believed to be a fingerprint on a photo plate.
But these ghostly galaxies once sparkled with young stars. A new study found that most of these galaxies lit up when they first entered our galaxy’s gravitational domain, as fresh stars formed. In most cases, however, the small galaxies soon stopped forming stars because the Milky Way cleared the dwarf galaxies of gas, the raw material for star formation.
Astronomer Masashi Chiba of Tohoku University in Sendai, Japan, and his then graduate student Takahiro Miyoshi studied seven of the spherical dwarf galaxies that orbit the Milky Way. The researchers used the European Space Agency’s Gaia spaceship, which measured the movements of the galaxies, to calculate their orbits around the center of the Milky Way. The orbits are elliptical, so the galaxies are approaching and then moving away from the center of our galaxy. The astronomers then compared these paths to the times when the galaxies formed their stars.
“We noticed that there was a very nice coincidence between the time the satellite first hit [toward the Milky Way] and the climax in the history of star formation, ”says Chiba. In work published online on arXiv.org on October 23, the astronomers attribute the outbreak of star formation in the small galaxies to the Milky Way. The encounter with the giant galaxy compresses the gas from the dwarf galaxies, causing this gas to collapse and create many new stars.
For example, the Draco dwarf galaxy first entered the domain of the Milky Way 11 billion years ago and formed numerous stars back then – but never again. More recently, the Leo I dwarf galaxy entered our galaxy’s realm just 2 billion years ago, a time that coincided with their last outbreak of the star birth. But today Leo I is not creating new stars and like Draco has no gas for it.
Dwarf galaxies that kept their distance also held their gas longer, the researchers found. The galaxies closest to the center of the Milky Way, such as Draco and Leo I, ceased star formation shortly after crossing the Milky Way’s boundary. However, the galaxies like Fornax and Carina, which entered the domain of our galaxy but stayed further away, fared better.
“These two galaxies kept their interstellar gas inside, so star formation continued,” says Chiba. Both galaxies managed to find new stars for billions of years after entering the realm of the Milky Way. Today, however, no galaxy has gas left.
“I think it all makes sense,” says Vasily Belokurov, an astronomer at Cambridge University who notes how important the Gaia spaceship was to the discovery. “It’s a nice demonstration of what we could never do before Gaia, and suddenly we can do these magical things.”