THE JOHNS HOPKINS UNIVERSITY
OFFICE OF NEWS AND INFORMATION
901 S. Bond Street/Suite 540
Baltimore, Maryland 21231

February 18, 2009
FOR IMMEDIATE RELEASE
Johns Hopkins Media Contact: Lisa De Nike
443-287-9960, Lde{at}jhu.edu

JPL/NASA Media Contacts: Whitney Clavin/Rhea Borja
818-354-4673/354-0850
Whitney.clavin{at}jpl.nasa.gov/rhea.r.borja{at}jpl.nasa.gov

NEW RECIPE FOR DWARF GALAXIES: START WITH LEFTOVER GAS

There is more than one way to make a dwarf galaxy, and NASA's Galaxy
Evolution Explorer has found a new recipe. It 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.

Led by David Thilker of the Henry A. Rowland Department of Physics
and Astronomy at The Johns Hopkins University, a team of 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 Thilker,
a research scientist. "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 team 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 surround them.

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 Feb. 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 on the
Internet, visit http://www.nasa.gov/centers/jpl/missions/galex.html.
For information about NASA and agency programs on the Internet, visit
http://www.nasa.gov.

Digital images of Thilker are available. Contact Lisa De Nike at
Lde{at}jhu.edu or 443-287-9960.
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