Detailed analyses of mankind’s deepest optical view of the universe, the
Hubble Ultra Deep Field (HUDF), by several expert teams have at last
identified what may turn out to be some of the earliest star-forming
galaxies. Astronomers are now debating whether the hottest stars in
these early galaxies may have provided enough radiation to “lift a
curtain” of cold, primordial hydrogen that cooled after the big bang.
This is a problem that has perplexed astronomers over the past decade,
and NASA’s Hubble Space Telescope has at last glimpsed what could be the
“end of the opening act” of galaxy formation. These faint sources
illustrate how astronomers can begin to explore when the first galaxies
formed and what their properties might be.

But even though Hubble has looked 95 percent of the way back to the
beginning of time, astronomers agree that’s not far enough. “For the
first time, we at last have real data to address this final frontier —
but we need more observations. We must push even deeper into the
universe, unveiling what happened during the initial 5 percent of the
remaining distance back to the big bang,” said Richard Ellis of the
California Institute of Technology in Pasadena, Calif.

In the past couple decades astronomers have amassed evidence that we
live in a reionized or “refried universe.” This so-called reionization
epoch was a critical watershed for the evolving universe. During that
early time cold hydrogen atoms drifting in space were pumped up with so
much energy from the ultraviolet starlight that they were stripped of
their electrons. The universe once again became transparent to light,
like the Sun burning off a morning fog. This early period is called
“reionization” because the primeval universe, which was hotter than our
Sun, was initially ionized as a soup of hydrogen nuclei and free-moving
electrons. As the universe cooled through the expansion of space,
these electrons were captured by hydrogen nuclei to make neutral
hydrogen. But the electrons were lost again when the first fiercely
bright stars fired up.

The epoch of reionization is thought to have ended 0.5 to one billion
years after the big bang. Constraints come from observations of quasars
located with the Sloan Digital Sky Survey, and recent measures of
polarization in the radiation emerging from the earliest phases of
cosmic history recorded by the Wilkinson Microwave Anisotropy Probe
(WMAP).

The major difficulty has been that galaxies at such a remote distance
are very faint and are very hard to find. Only the most luminous
galaxies can be relatively easily seen. Prior to the HUDF, astronomers
did not have the sensitivity to accurately constrain the numbers of very
distant sources at that epoch, and so there’s been a long-standing
debate whether normal galaxies were really capable of doing the
reionizing job.

The sensitivity of Hubble’s Advanced Camera for Surveys (ACS), combined
with the penetrating power of the Near Infrared Camera and Multi-Object
Spectrometer (NICMOS), finally revealed these long sough faint galaxies.
The HUDF shows that close to a billion years after the big bang the
early universe was filled with dwarf galaxies, but no fully formed
galaxies like our Milky Way. After careful analysis, they have been
sorted out as between 54 and 108 dim, red smudges sprinkled across the
HUDF image. From a hierarchical point of view, this means the universe
started out as a bunch of “mom & pop” stores, which merged into
businesses, and then into giant corporations — the majestic galaxies we
see today.

Five different teams of astronomers analyzing the HUDF results have
differed in their interpretation of what was happening in the early
universe. Some believe the objects that completed cosmic reionization by
redshift z=6 have been found, while others believe that other sources
than dwarf galaxies may be needed. Most groups believe that the action
started at even earlier times, which can only be reached with future
advanced cameras and telescopes. Different avenues of HUDF research are
being led by: Rodger Thompson (University of Arizona, Tucson, Ariz.),
Haojing Yan (Spitzer Space Center, California Institute of Technology,
Pasadena, Calif.) and Rogier Windhorst (Arizona State University, Tempe,
Ariz.), Massimo Stiavelli (Space Telescope Science Institute, Baltimore,
Md.), Andrew Bunker (University of Exeter and the University of
Cambridge, UK), and Sangeeta Malhotra and James Rhoads (Space Telescope
Science Institute). The teams used different techniques:

The Bunker team identified a list of 50 probable distant galaxies in the
Ultra Deep Field and distributed details of their work within a day of
the images becoming publicly available. They isolated their distant
sample using techniques developed with earlier, less sensitive, Hubble
images tested through spectroscopic observations undertaken with the
10-meter W.M. Keck observatory in Hawaii. Bunker’s team claims that the
combined ultraviolet light from the galaxies located in the Ultra Deep
Field is insufficient to reionize the universe. Perhaps the physics of
star formation was different at these early times, or a further, yet
more distant population is responsible.

The Stiavelli team shows that the same objects would be sufficient to
reionize the universe, if they possessed much fewer heavier elements —
anything heavier than helium — than those of present-day galaxies, and
if the early galaxies contained more massive stars. Both these
assumptions are reasonable at early epochs, since astronomers know that
stars make the metals that exist in the universe. Early on, when most of
the stars we see today hadn’t been formed, the amount of elements must
have been much lower.

The Yan and Windhorst team started from the objects that are seen, and
then carefully estimated the fraction of fainter galaxies that are not
seen, even in the Hubble Ultra Deep Field. They found that the number of
dwarf galaxies rapidly increases at fainter levels in the HUDF. This is
like a cosmic “stock-market chart” but with very few large corporations
and numerous “mom-and-pop corner stores.” Yan and Windhorst conclude
that this steep increase of the faint dwarf galaxy population
collectively generates enough ultraviolet light to finish reionizing the
universe by redshifts 6, even if the amount of heavier elements was
similar to that of present-day galaxies.

The HUDF NICMOS Treasury team (Thompson/Illingworth) has taken the
UDF data and other ACS survey data to get the best possible estimate of
the relative numbers of bright and faint galaxies around redshift 6,
only 900 million years after the big bang. The papers, led by Rychard
Bouwens, show that faint galaxies dominate at this epoch, compared to
more recent times, and are likely to have played a significant role in
the late stages of reionization. The team has also used the HUDF NICMOS
data to detect a small sample of galaxies at higher redshifts (at
z=7-8), 200 million years closer in time to the big bang. The amount of
reionizing light at redshifts 7-8 appears to be lower than what is seen
only 200 million years later at redshift 6.

The Malhotra and Rhoads team have found a “sheet” of galaxies in the
HUDF. They find that the galaxy density near redshift z=5.9 (look-back
time of 12.5 billion years) is four times the galaxy density in the rest
of the surveyed HUDF “core sample.” This supports theories of galaxy
formation which predict that dense regions should be the first sites of
galaxy formation. This evidence for an over density was bolstered by a
complementary study, undertaken by Malhotra, Rhoads, and JunXian Wang,
which uses the Cerro Tololo Inter-American Observatory to obtain a map
of galaxies over a much wider area than the HUDF. Even with its lower
sensitivity and more limited coverage in distance, this map shows that
“extra” galaxies are spread like a sheet, with the HUDF located near one
edge of the structure. “The presence of such structures doubtless
affected the reionization of the universe, because the ultraviolet light
that separated intergalactic hydrogen atoms into protons and electrons
would have been more intense where galaxies are more common. It is then
likely that reionization proceeded at different speeds in different
regions of the early universe,” says Rhoads. This Hubble team used
spectra to measure the distances of these galaxies very precisely.

The WFC3 built for Hubble is expected to see ten times as many distant
infrared galaxies as the NICMOS. When launched, the JWST will have the
light-gathering power to peruse an even earlier universe and actually
see the very first stars and star clusters, which remain beyond even
Hubble’s reach. These still hypothesized ultra-bright stars formed only
200 million years after the big bang (at redshifts z=20, and as deduced
from the WMAP image of the cosmic microwave background). They are
currently believed to have heated the universe so much back then, that
smaller, normal stars had to wait for the hydrogen gas to re-cool and
condense before they could form.

Electronic images, live webcast, and additional information are
available at:

http://hubblesite.org/news/2004/28

http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/28/video/a

For additional information, please contact:

Rogier Windhorst, Arizona State University, Tempe, AZ 85287,
(phone) 480-965-7143, (e-mail) rogier.windhorst@asu.edu

Haojing Yan, Spitzer Science Center, Caltech, Pasadena, CA 91125,
(phone) 480-965-0663, (e-mail) haojing.yan@asu.edu

Massimo Stiavelli, Space Telescope Science Institute, Baltimore, MD
21218, (phone) 410-338-4835, (e-mail) stiavelli@stsci.edu

Garth Illingworth, University of California/Lick Observatory, Santa
Cruz, CA 95064, (phone) 831-459-2843, (e-mail) gdi@ucolick.org

Andrew Bunker, University of Exeter, Exeter, EX4 4QL, UK, (phone)
44-1392-26-4118, (e-mail) bunker@astro.ex.ac.uk

Rodger Thompson, University of Arizona, Tucson, AZ 85721, (phone)
520-621-6527, (e-mail) rthompson@as.arizona.edu

Sangeeta Malhotra, Space Telescope Science Institute, Baltimore, MD
21218, (phone) 410-338-5097, (e-mail) san@stsci.edu.

The Space Telescope Science Institute (STScI) is operated by the
Association of Universities for Research in Astronomy, Inc. (AURA),
for NASA, under contract with the Goddard Space Flight Center,
Greenbelt, Md. The Hubble Space Telescope is a project of
international cooperation between NASA and the European Space
Agency (ESA).