New Recipes for Dwarf Galaxies: Start With Leftover Gas
PASADENA, Calif. -- There is more than one way to make a dwarf galaxy, and NASA's Galaxy
Evolution Explorer has found a new recipe. The spacecraft has, for the first time, identified
dwarf galaxies forming out of nothing more than pristine gas likely leftover from the early
universe. Dwarf galaxies are relatively small collections of stars that often orbit around
larger galaxies like our Milky Way.
The findings surprised astronomers because most galaxies form in association with a mysterious
substance called dark matter or out of gas containing metals. The infant galaxies spotted by
the Galaxy Evolution Explorer are springing up out of gas that lacks both dark matter and
metals. Though never seen before, this new type of dwarf galaxy may be common throughout the
more distant and early universe, when pristine gas was more pervasive.
Astronomers spotted the unexpected new galaxies forming inside the Leo Ring, a huge cloud of
hydrogen and helium that traces a ragged path around two massive galaxies in the constellation
Leo. The cloud is thought likely to be a primordial object, an ancient remnant of material that
has remained relatively unchanged since the very earliest days of the universe. Identified about
25 years ago by radio waves, the ring cannot be seen in visible light.
"This intriguing object has been studied for decades with world-class telescopes operating at
radio and optical wavelengths," said David Thilker of Johns Hopkins University, Baltimore, Md.
"Despite such effort, nothing except the gas was detected. No stars at all, young or old, were
found. But when we looked at the ring with the Galaxy Evolution Explorer, which is remarkably sensitive
to ultraviolet light, we saw telltale evidence of recent massive star formation. It was really unexpected.
We are witnessing galaxies forming out of a cloud of primordial gas."
In a recent study, Thilker and his colleagues found the ultraviolet signature of young stars
emanating from several clumps of gas within the Leo Ring. "We speculate that these young stellar
complexes are dwarf galaxies, although, as previously shown by radio astronomers, the gaseous clumps
forming these galaxies lack dark matter," he said. "Almost all other galaxies we know are dominated
by dark matter, which acted as a seed for the collection of their luminous components--stars, gas and
dust. What we see occurring in the Leo Ring is a new mode for the formation of dwarf galaxies in material
remaining from the much earlier assembly of this galaxy group."
Our local universe contains two large galaxies, the Milky Way and the Andromeda galaxy, each with
hundreds of billions of stars, and the Triangulum galaxy, with several tens of billions of stars.
It also holds more than 40 much smaller dwarf galaxies, which have only a few billion stars. Invisible
dark matter, detected by its gravitational influence, is a major component of both giant and dwarf
galaxies with one exception-tidal dwarf galaxies.
Tidal dwarf galaxies condense out of gas recycled from other galaxies and have been separated from
most of the dark matter with which they were originally associated. They are produced when galaxies
collide and their gravitational masses interact. In the violence of the encounter, streamers of
galactic material are pulled out away from the parent galaxies and the halos of dark matter that
Because they lack dark matter, the new galaxies observed in the Leo Ring resemble tidal dwarf galaxies,
but they differ in a fundamental way. The gaseous material making up tidal dwarfs has already
been cycled through a galaxy. It has been enriched with metals--elements heavier than helium--
produced as stars evolve. "Leo Ring dwarfs are made of much more pristine material without metals,"
said Thilker. "This discovery allows us to study the star formation process in gas that has not
yet been enriched."
Large, pristine clouds similar to the Leo Ring may have been more common throughout the early universe,
Thilker said, and consequently may have produced many dark-matter-lacking, dwarf galaxies yet to be discovered.
The results of the new study reporting star formation in the Leo Ring appear in the February 19, 2009,
issue of the journal Nature.
Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations and
data analysis. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the mission and built
the science instrument. The mission was developed under NASA's Explorers Program managed by the
Goddard Space Flight Center, Greenbelt, Md. South Korea and France are the international partners
in the mission.
For images and information about the Galaxy Evolution Explorer, visit http://www.galex.caltech.edu/
For information about NASA and agency programs on the Internet, visit http://www.nasa.gov
Media contact: Whitney Clavin 818-354-4673/Rhea Borja 818-354-4673/354-0850
Jet Propulsion Laboratory, Pasadena, Calif.