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 left over from
the early universe. Dwarf galaxies are relatively small
collections of stars that often orbit around larger
galaxies such as 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
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,"
Thilker said. "This discovery allows us to study the star
formation process in gas that has not yet been
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 issue of the journal
Caltech leads the Galaxy Evolution Explorer mission and is
responsible for science operations and data analysis.
NASA's Jet Propulsion Laboratory manages the mission and
built the science instrument. The mission was developed
under NASA's Explorers Program managed by the Goddard Space
Flight Center. South Korea and France are the international
partners in the mission.
For images and information about the Galaxy Evolution
Explorer on the Internet, go to