A tiny galaxy has given astronomers a glimpse of a
time when the first bright objects in the universe formed,
ending the dark ages that followed the birth of the
Astronomers from Sweden, Spain and Johns Hopkins used
NASA's Far Ultraviolet
Spectroscopic Explorer satellite to make the first
direct measurement of ionizing radiation leaking from a
dwarf galaxy undergoing a burst of star formation. The
result, which has ramifications for understanding how the
early universe evolved, will help astronomers determine
whether the first stars — or some other type of
object — ended the cosmic dark age.
The team presented its results Jan. 12 at the American
Astronomical Society's 207th meeting in Washington, D.C.
Considered by many astronomers to be relics from an
early stage of the universe, dwarf galaxies are small, very
faint galaxies containing a large fraction of gas and
relatively few stars. According to one model of galaxy
formation, many of these smaller galaxies merged to build
up today's larger ones. If that is true, any dwarf galaxies
observed now can be thought of as "fossils" that managed to
survive — without significant changes — from an
Led by Nils Bergvall of the Astronomical Observatory
in Uppsala, Sweden, the team observed a small galaxy known
as Haro 11 that is located about 281 million light-years
away from Earth in the southern constellation of Sculptor.
The team's analysis of FUSE data produced an important
result: Between 4 percent and 10 percent of the ionizing
radiation produced by the hot stars in Haro 11 is able to
escape into intergalactic space.
Ionization is the process by which atoms and molecules
are stripped of electrons and converted to positively
charged ions. The history of the ionization level is
important to understanding the evolution of structures in
the early universe because it determines how easily stars
and galaxies can form, according to FUSE team member B-G
Andersson, a research scientist in the
Henry A. Rowland
Department of Physics and Astronomy at Johns
"The more ionized a gas becomes, the less efficiently
it can cool. The cooling rate in turn controls the ability
of the gas to form denser structures, such as stars and
galaxies," Andersson said. The hotter the gas, the less
likely it is for structures to form, he said.
The ionization history of the universe therefore
reveals when the first luminous objects formed, and when
the first stars began to shine.
The Big Bang occurred about 13.7 billion years ago. At
that time, the infant universe was too hot for light to
shine. Matter was completely ionized: Atoms were broken up
into electrons and atomic nuclei, which scatter light like
fog. As it expanded and then cooled, matter combined into
neutral atoms of some of the lightest elements. The imprint
of this transition today is seen as cosmic microwave
The present universe is, however, predominantly
ionized; astronomers generally agree that this
re-ionization occurred between 12.5 billion and 13 billion
years ago, when the first large-scale galaxies and galaxy
clusters were forming. The details of this ionization are
still unclear but are of intense interest to astronomers
studying the so-called "dark ages" of the universe.
Astronomers are unsure if the first stars or some
other type of object ended those dark ages, but FUSE
observations of Haro 11 provide a clue.
The observations also help increase understanding of
how the universe became re-ionized. According to the team,
likely contributors include the intense radiation generated
as matter fell into black holes that formed what we now see
as quasars, and the leakage of radiation from regions of
early star formation. But until now, direct evidence for
the viability of the latter mechanism has not been
"This is the latest example where the FUSE observation
of a relatively nearby object holds important ramifications
for cosmological questions," said George Sonneborn,
NASA/FUSE project scientist at NASA's Goddard Space Flight
Center, Greenbelt, Md.
This result has been accepted for publication by the
European journal Astronomy and Astrophysics.
The FUSE project is a NASA Explorer mission developed
in cooperation with the French and Canadian space agencies
by Johns Hopkins; the University of Colorado, Boulder; and
the University of California, Berkeley. The mission is
operated out of Johns Hopkins' Homewood campus.
For more information about the FUSE mission, go to