Super-Dim Galaxy May Be One of Hundreds Orbiting the Milky Way

By Ken Croswell

Published on New Scientist (October 4, 2016)

Astronomers have spotted one of the dimmest galaxies ever seen. The discovery of this super-faint galaxy – a satellite of the Milky Way – bolsters current theories of how galaxies arise.

Giant galaxies like the Milky Way are thought to form after smaller galaxies smash together. That suggests that hundreds of satellite galaxies would orbit our own, leftovers of the ones that formed our Galaxy. But, so far, astronomers have found only about 50.

The new galaxy, named Virgo I, is the latest satellite to be discovered. It appeared as a team led by Daisuke Homma and Masashi Chiba of Tohoku University in Sendai, Japan, searched the sky with a new camera on the giant 8.2-metre Subaru Telescope in Hawaii.

“Virgo I might be the faintest galaxy,” Chiba says. It emits about half as much light as Segue 1, another satellite of the Milky Way and the previous faint-galaxy champ. A single bright star in our Galaxy outshines all of Virgo I’s stars put together.

“Tiny little puny things”

“It continues to be interesting how faint you can go and still be a galaxy,” says Erik Tollerud of the Space Telescope Science Institute in Baltimore, Maryland. “They’re such tiny little puny things.” The new galaxy radiates so little light because it has so few stars, which give off only about 180 times as much light as the Sun.

Virgo I is just a few hundred light-years across, which nonetheless indicates it is a galaxy, Chiba says. A star cluster this dim would be even smaller.

Still, the galaxy’s luminosity is somewhat uncertain, because astronomers can only see its brightest stars, says Vasily Belokurov of the University of Cambridge, whose team discovered Segue 1 in 2006. “All these luminosities should be taken with a great pinch of salt,” he says. “So I would not really play this game of which one is the faintest.”

Such a galaxy is so dim it can lose most of its light when just one of its red giant stars becomes a white dwarf.

Last year, a less luminous object turned up in the constellation Cetus. But that system is so compact it may be a star cluster rather than a galaxy.

Virgo I definitely surpasses its dim competitors in one regard: it’s much farther, lying 280,000 light-years from Earth, nearly twice as far as the Milky Way’s brightest satellite galaxy, the Large Magellanic Cloud.

If Virgo I has always been so remote, the Milky Way didn’t make it dim by stealing its stars. Instead, these galaxies likely started out as “incredibly faint systems”, says Josh Simon of the Carnegie Observatories in Pasadena, California. “They didn’t become that way later on.”

The great distance explains why earlier searches, which employed smaller telescopes, never found the galaxy. Many more dim and distant satellites may emerge as the astronomers continue their search, which so far has covered only 1/400th of the sky.

“I’m hoping they’re going to find hundreds of these,” Belokurov says. “It’s going to be fun.”

Ken Croswell earned his Ph.D. in astronomy from Harvard University and is the author of The Alchemy of the Heavens and The Lives of Stars.

"An engaging account of the continuing discovery of our Galaxy...wonderful." --Owen Gingerich, The New York Times Book Review. See all reviews of The Alchemy of the Heavens here.

"A stellar picture of what we know or guess about those distant lights."--Kirkus. See all reviews of The Lives of Stars here.