The Milky Way’s entourage of satellites just welcomed a few new members.
Two teams of astronomers independently discovered eight, or possibly nine, satellite galaxies around the Milky Way, adding to the 27 previously known. These galaxies, despite being tiny, hold clues about the formation of the Milky Way, the birth of the first galaxies, and the nature of dark matter, the elusive substance thought to hold galaxies together.
All the galaxies were found near the Large Magellanic Cloud, the largest Milky Way satellite. The nearest sits about 100,000 light-years away in the constellation Reticulum; the farthest, in the constellation Eridanus, is just over 1 million light-years from Earth — roughly half the distance to the Andromeda galaxy. The findings appear in four papers published online at arXiv.org on March 9.
Most of these satellites contain about a few thousand stars, far fewer than the Milky Way’s roughly 100 billion. But astronomers can see only several dozen of the brightest stars and have to infer the presence of the rest. “We literally run out of stars to study,” says Marla Geha, an astronomer at Yale University. “That’s why we’re so delighted to find more.”
Both groups found the satellites in images from the Dark Energy Survey, a five-year project at the Cerro Tololo Inter-American Observatory in Chile that is studying the accelerating expansion of the universe. Project scientists analyzed data from the first year of observations and turned up eight satellite candidates. At the same time, Sergey Koposov, an astronomer at the University of Cambridge in England, and colleagues mined publicly available data from the survey and found the same eight objects, plus one more.
“It’s exciting that there are two independent verifications of these objects,” says Beth Willman, an astronomer at Haverford College in Pennsylvania.
The discovery of a wealth of satellites in a relatively small patch of sky “was really, really surprising,” Koposov says. He suggests that the newly discovered galaxies are traveling with the Magellanic Clouds as part of a group that will be gobbled up by the Milky Way. Researchers will have to wait until the Dark Energy Survey looks at more of the sky, however, before coming to any conclusions.
For more than a decade, astronomers have noted that they see far fewer satellites than predicted, which led some researchers to wonder if theories about the evolution of galaxies were accurate. An explosion of satellite discoveries around 2005 indicated that there should be hundreds more, yet subsequent searches turned up nothing. “The discovery of these objects puts people a little more at ease,” says Alex Drlica-Wagner, an astrophysicist at the Fermi National Accelerator Laboratory in Batavia, Ill., who is part of the Dark Energy Survey.
Dwarf galaxies are good probes of dark matter, Drlica-Wagner adds. The stars and gas make up only about 1 percent of the mass of these dwarfs. The rest is dark matter. By measuring the speeds of stars within the satellites, he says, researchers can map out the dark matter, which could help them understand its role in forming galaxies.
To test ideas about the nature of dark matter, astronomers also can search the satellites for gamma rays, which some theories predict are created when dark matter particles collide. Alex Geringer-Sameth, an astrophysicist at Carnegie Mellon University in Pittsburgh, and colleagues claim that they see gamma rays coming from one of the newly discovered satellites. Geringer-Sameth and colleagues analyzed data from the Fermi space telescope to detect the gamma-ray signal, which they suggest comes from dark matter collisions. Researchers with the Dark Energy Survey, however, looked at the same data and report that they see no such signal.
Faint dwarf galaxies also give astronomers a peek at what the earliest galaxies were like. “These things turn out to be by far the oldest galaxies we know of,” Geha says. They ceased forming stars roughly a billion years after the Big Bang, unlike spiral galaxies like the Milky Way, which form stars throughout their lifetime. While images of very distant galaxies provide a snapshot of what the universe was like then, satellite galaxies give astronomers a closer look. Researchers can probe the conditions under which the first galaxies formed by analyzing the chemical makeup of the stars in these satellites, Willman says. “Those are the fingerprints of the gas from which these stars formed 12 billion years ago.”
Editor’s note: This story was updated March 16, 2015, to correct the spelling of the constellation Reticulum.