Press Release

Distant Young Galaxy Hints at Gradual End to the Dark Ages

By SpaceRef Editor
June 3, 2004
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Astronomers have peered into the fog of the early universe and discovered
a young and extremely distant galaxy at a time when the Universe was
only about six percent of its present age. When compared with other
recent findings about the "dark ages" of the early universe, this discovery
suggests that this murky era could have lasted the better part of a billion
years.

These "dark ages" began about a million years after the Big Bang, when
most ordinary matter had condensed into a pervasive "fog" of neutral
hydrogen. They ended when the first stars formed in the earliest galaxies
and ionized the gas. However, much remains mysterious about this
transitional period in the cosmic history — when it took place, how it
occurred, where and when the first stars were formed, and what became of
them. The new observations suggest that the transition from the dark ages
to the modern Universe was a gradual and complex process.

This discovery is being reported today in Denver at the 204th meeting of
the American Astronomical Society.

The newly detected galaxy, designated LALA J142442.24+353400.2, was
discovered in the constellation Bootes using data from the Large Area
Lyman Alpha (LALA) survey, led by James Rhoads and Sangeeta
Malhotra of the Space Telescope Science Institute, Baltimore, MD. This
survey takes advantage of observations of the same portions of the
northern and southern skies as the NOAO Deep Wide-Field Survey, led by
Buell Jannuzi and Arjun Dey of the National Optical Astronomy
Observatory. Both surveys have been conducted using the National
Science Foundation’s 4-meter Mayall telescope at the Kitt Peak National
Observatory and 4-meter Blanco telescope at the Cerro Tololo Inter-
American Observatory. The nature of the newly discovered galaxy was
confirmed using a spectrum obtained at the international Gemini North 8-
meter telescope on Mauna Kea, Hawaii.

The galaxy was discovered by searching the sky for objects with strong
emission lines in their spectra. While starlight from galaxies has a
spectrum that is generally smooth (like the spectrum of an incandescent
light bulb), those galaxies where new stars are forming can emit several
percent of their light at a few discrete wavelengths, which appear as sharp
lines in their spectra. These emission lines are created when interstellar
hydrogen (or some other element) absorbs ultraviolet light from young
stars and re-emits the energy in specific lines. (Neon lights produce their
intensely colored glow by a similar process.)

The LALA survey works by taking images of the night sky through
special colored "narrowband" filters that admit only a narrow range of
color ("narrowband filters"). Star-forming galaxies with emission lines in
the admitted color range appear much brighter in the LALA survey image
than they do in parallel images of the same galaxy taken by the NOAO
Deep Wide-Field Survey.

Because light gets redder as it moves through the expanding Universe, the
chosen color of a narrowband filter determines the distances where
galaxies will be found by that filter. Moreover, the narrowband color can
be chosen at a special "window" where the night sky is particularly dark,
which allows faint, distant galaxies to be seen more easily and results in a
very efficient search.

The spectrum from the Gemini telescope showed a prominent hydrogen
emission line, which allowed the distance of the galaxy to be measured at
12.8 billion light years (redshift [z] =6.535), or about 850 million years
after the Big Bang.

The era of LALA J142442.24+353400.2 corresponds to the end of the so-
called "dark ages." Around this time, the ultraviolet light from the first
stars ionized the hydrogen gas that filled intergalactic space, and in the
process heated it from temperatures around 20 degrees above absolute
zero (-253 degrees C or -432 degrees F) to over 10,000 degrees C (18,000
degrees F), a process called "reionization".

The emission line seen in the LALA galaxy is the Lyman-alpha line and is
produced by neutral hydrogen. Prior to reionization, neutral hydrogen
filling the space between galaxies would act as a scattering "fog" for the
Lyman alpha line. "This fog would blur our view of the galaxy, effectively
hiding it from a survey like LALA," Rhoads says. "Thus, the detection of
this galaxy in the LALA narrowband image implies that reionization of
the universe was well under way by the era when we observed this
galaxy."

Previous studies of reionization based on spectra of luminous quasars
obtained by Sloan Digital Sky Survey consortium have concluded that
neutral gas in intergalactic space survived to a somewhat more recent
epoch (around redshift z=6.2). Another approach, using observations of
the cosmic microwave background radiation from NASA’s Wilkinson
Microwave Anisotropy Probe satellite, suggests that reionization may
have started as early as redshift 15 or 20.

"While much of the intergalactic gas was probably ionized already by the
time of the galaxy we found, a substantial minority could have remained
neutral," Rhoads explains. "By combining our observations with the Sloan
and WMAP results, we can assemble a more complete understanding,
which shows that reionization was a complex process lasting the better
part of a billion years."

The conclusions from LALA J142442.24+353400.2 are supported by a
handful of other line emitting galaxies at similar redshift that have been
published by Esther Hu and collaborators at the University of Hawaii and
by Keiichi Kodaira at the National Astronomical Observatory of Japan.

The result could be bolstered by finding substantially larger samples in
future, Rhoads adds, and further theoretical analysis of how Lyman alpha
photons travel in the early universe would also be beneficial. Finally, more
sensitive images of the starlight from LALA J142442.24+353400.2 are
planned using the Hubble Space Telescope, which will help provide data
to better understand its physical nature.

Co-authors of the paper include Chun Xu (STScI), Steve Dawson
(University of California, Berkeley), Arjun Dey (National Optical
Astronomy Observatory, Tucson, AZ), Sangeeta Malhotra (STScI),
JunXian Wang (The Johns Hopkins University, Baltimore, MD), Buell
Jannuzi (NOAO), Hyron Spinrad (U. C. Berkeley), and Daniel Stern
(NASA’s Jet Propulsion Laboratory, Pasadena, CA).

NOAO is operated by the Association of Universities for Research in
Astronomy (AURA) Inc., under a cooperative agreement with the
National Science Foundation.

SpaceRef staff editor.