Seven Dwarfs Discovered Orbiting the Milky Way

An international team of scientists has discovered seven dwarf galaxies orbiting the Milky Way. The galaxies were discovered as part of the Sloan Digital Sky Survey (SDSS-II), the most ambitious survey of the sky ever undertaken.

"These dwarf galaxies have been captured by the gravity of the Milky Way and most eventually will merge with our own galaxy," explained Donald Schneider, Penn State Professor of Astronomy and Astrophysics. "The discovery of these dwarf galaxies demonstrates the unexpected power of large astronomical surveys like the Sloan Digital Sky Survey, which originally was designed to investigate objects millions and billions of light years from Earth, not to discover faint companions of our own galaxy."

According to Daniel Zucker of Cambridge University in the UK, theories predict that there should be tens to hundreds more dwarf galaxies yet to be discovered in the "Local Group" of galaxies, which includes the Milky Way. "In less than a year, we have used SDSS-II data to find seven new Milky Way dwarf satellites," Zucker said. "We've just discovered an eighth new dwarf, but we're not yet sure this one is a Milky Way satellite." The SDSS team has found almost as many new Milky Way satellites as were detected in the previous 70 years. Only a handful were known before the SDSS-II survey.

Dwarf galaxies contain, at most, a few million stars. The new dwarfs have some unusual properties. "They're more like hobbits than dwarfs," commented co-investigator Vasily Belokurov, also of Cambridge, "since they are smaller and fainter than most previously known satellites. Several of the newly discovered systems appear to be on the verge of disruption -- probably by the tidal gravity of the Milky Way -- and the 'Ursa Major II' dwarf already seems to be in several pieces. "They look as though they're being ground up," noted Belokurov. Other members of the discovery team include Mark Wilkinson, Mike Fellhauer, and Gerry Gilmore of Cambridge University, and Jelte De Jong and Hans-Walter Rix of the Max Planck Institute for Astronomy.

Current theories of galaxy assembly suggest that many, perhaps all, of the stars in the halo and thick disk of the Milky Way originated in smaller dwarf galaxies, which were dissolved when they merged into the Milky Way itself. "The new dwarfs are really just the crumbs from the galactic feast," said Zucker. "Most of the merging happened early on, billions of years ago, and what we're seeing here are the leftovers."

The SDSS-II is a unique resource for finding Milky Way satellites because its deep, multi-color imaging allows detection of much fainter systems than previously were visible. The new objects are found using sophisticated computer algorithms that troll the digital data to find groupings of related stars. Because the SDSS-II covers only a fifth of the sky, astronomers expect that there are many more undiscovered dwarf galaxies in the universe.

The seven new Milky Way satellites all lie in the area of sky surveyed by the SDSS-II around the North Galactic Pole. There are two new dwarfs in the constellation of Canes Venatici, one in Bootes, one in Leo, one in Coma Berenices, one in Ursa Major, and one in Hercules.

The eighth and newest discovery may be the most intriguing. Named Leo T, it is about 1.4-million light years away, on the fringes of the Milky Way's gravitational influence. "It may be a 'free-floating' Local Group dwarf, rather than a satellite of the Milky Way," noted Sergey Koposov, of the Max Planck Institute for Astronomy in Heidelberg. In addition to its greater distance, Leo T is distinct from the previous seven discoveries in that it has both populations of fairly old stars (greater than 5 billion years old) and comparatively young populations (less than 1 billion years old). It also appears to have neutral hydrogen gas, an indication that its star-forming days may not be over.

Leo T could be the bellwether of a large population of faint galaxies that reside in the Local Group but are not closely associated with either the Milky Way or the Andromeda. It is too distant to be strongly influenced by the tidal force of the Milky Way. "Leo T probably has always been very faint, retaining its gas and slowly forming stars in relative isolation," commented Mike Irwin, a Cambridge University astronomer.

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