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Faintest Spectra Ever Raise Glaring Question: Why do Galaxies inthe Young Universe Appear so Mature? (Forwarded)



 
 
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Old January 5th 04, 08:39 PM
Andrew Yee
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Default Faintest Spectra Ever Raise Glaring Question: Why do Galaxies inthe Young Universe Appear so Mature? (Forwarded)

Gemini Observatory
Hilo, Hawai`i

Media Contacts:
Peter Michaud, Gemini Observatory, Hilo, Hawai`i
Desk: 808/974-2510
E-mail:

Doug Isbell, NOAO (Tucson)
Desk: 520/318-8214
E-mail:


Science Contact:
Roberto Abraham, University of Toronto
Desk: 416/946-7289, Cell: 416/830-8172
E-mail:


Embargo Until January 5th, 2004, 9:20am EDT/4:20am HST

Faintest Spectra Ever Raise Glaring Question: Why do Galaxies in the Young
Universe Appear so Mature?

Until now, astronomers have been nearly blind when looking back in time to
survey an era when most stars in the Universe were expected to have formed. This
critical cosmological blind-spot has been removed by a team using the Frederick
C. Gillett Gemini North Telescope, showing that many galaxies in the young
Universe are not behaving as expected some 8-11 billion years ago.

The surprise: these galaxies appear to be more fully formed and mature than
expected at this early stage in the evolution of the Universe. This finding is
similar to a teacher walking into a classroom expecting to greet a room full of
unruly teenagers and finding well-groomed young adults.

"Theory tells us that this epoch should be dominated by little galaxies crashing
together," said Dr. Roberto Abraham (University of Toronto) who is a
Co-Principal Investigator of the team conducting the observations at Gemini. "We
are seeing that a large fraction of the stars in the Universe are already in
place when the Universe was quite young, which should not be the case. This
glimpse back in time shows pretty clearly that we need to re-think what happened
during this early epoch in galactic evolution. The theoreticians will definitely
have something to gnaw on!"

The results were announced today at the 203rd meeting of the American
Astronomical Society in Atlanta, Georgia. The data will soon be released to the
entire astronomical community for further analysis, and three papers have been
submitted for publication in Nature, The Astrophysical Journal, and The
Astronomical Journal.

These observations are from a multinational investigation, called the Gemini
Deep Deep Survey (GDDS), which used a special technique to capture the faintest
galactic light ever dissected into the rainbow of colors called a spectrum. In
all, spectra from over 300 galaxies were collected, most of which are within
what is called the "Redshift Desert," a relatively unexplored period of the
Universe seen by telescopes looking back to an era when the universe was only
3-6 billion years old. These spectra represent the most complete sample ever
obtained of galaxies in the Redshift Desert. By obtaining large amounts of data
from four widely separated fields, this survey provides the statistical basis
for drawing conclusions that have been suspected by past observations done by
the Hubble Space Telescope, Keck Observatory, Subaru Telescope and the Very
Large Telescope over the past decade.

Studying the faint galaxies at this epoch when the Universe was only 20-40% of
its current age presents a daunting challenge to astronomers, even when using
the light-gathering capacity of a very large telescope like Gemini North with
its 8-meter mirror. All previous galaxy surveys in this realm have focused on
galaxies where intense star formation is occurring, which makes it easier to
obtain spectra but produces a biased sample. The GDDS was able to select a more
representative sample including those galaxies which hold the most stars --
normal, dimmer, and more massive galaxies -- hat demand special techniques to
coax a spectrum from their dim light.

"The Gemini data is the most comprehensive survey ever done covering the bulk of
the galaxies that represent conditions in the early Universe. These are the
massive galaxies that are actually more difficult to study because of their lack
of energetic light from star formation. These highly developed galaxies, whose
star-forming youth is in fact long gone, just shouldn't be there, but are," said
Co-Principal Investigator Dr. Karl Glazebrook (Johns Hopkins University).

Astronomers trying to understand this issue might have to put everything on the
table. "It is unclear if we need to tweak the existing models or develop a new
one in order to understand this finding," said the survey's third Co-Principal
Investigator, Dr. Patrick McCarthy (Observatories of the Carnegie Institution).
"It is quite obvious from the Gemini spectra that these are indeed very mature
galaxies, and we are not seeing the effects of obscuring dust. Obviously there
are some major aspects about the early lives of galaxies that we just don't
understand. It is even possible that black holes might have been much more
ubiquitous than we thought in the early Universe and played a larger role in
seeding early galaxy formation."

What is arguably the dominant galactic evolution theory postulates that the
population of galaxies at this early stage should have been dominated by
evolutionary building blocks. Aptly called the Hierarchical Model, it predicts
that normal to large galaxies, like those studied in this work, would not yet
exist and would instead be forming from local beehives of activity where big
galaxies grew. The GDDS reveals that this might not be the case.

The spectra from this survey were also used to determine the pollution of the
interstellar gas by heavy elements (called "metals") produced by stars. This is
a key indicator of the history of stellar evolution in galaxies. Sandra Savaglio
(Johns Hopkins University), who studied this aspect of the research said, "Our
interpretation of the Universe is strongly affected by the way we observe it.
Because the GDDS observed very faint galaxies, we could detect the interstellar
gas even if partly obscured by the presence of dust. Studying the chemical
composition of the interstellar gas, we discovered that the galaxies in our
survey are more metal-rich than expected."

Caltech astronomer, Dr. Richard Ellis commented, "The Gemini Deep Deep Survey
represents a very significant achievement, both technically and scientifically.
The survey has provided a new and valuable census of galaxies during a key
period in cosmic history, one that has been difficult to study until now,
particularly for the quiescent component of the galaxy population."

Making observations in the Redshift Desert has frustrated modern astronomers for
the last decade. While astronomers have known that plenty of galaxies must exist
in the Redshift Desert, it is only a "desert" because we couldn't get good
spectra from many of them. The problem lies in the fact that key spectroscopic
features used to study these galaxies have been redshifted -- due to the
expansion of the Universe -- into a part of the optical spectrum that
corresponds to a faint, natural, obscuring glow in the Earth's nighttime atmosphere.

To overcome this problem, a sophisticated technique called "Nod and Shuffle" was
used on the Gemini telescope. "The Nod and Shuffle technique enables us to skim
off the faint natural glow of the night sky to reveal the tenuous spectra of
galaxies beneath it. These galaxies are over 300 times fainter than this sky
glow," explains Dr. Kathy Roth, an astronomer at Gemini who was also part of the
team and obtained much of the data. "It has proven to be an extremely effective
way to radically reduce the "noise" or contamination levels that are found in
the signal from an electronic light detector."

Each observation lasted the equivalent of about 30 hours and produced nearly 100
spectra simultaneously. The entire project required over 120 total hours of
telescope time. "This is a lot of valuable time on the sky, but when you
consider that it has allowed us to help fill in a crucial 20% gap in our
understanding of the Universe, it was time well spent," adds Dr. Glazebrook who
developed the use of Nod and Shuffle with Joss Hawthorn for faint galaxy
observations while at the Anglo-Australian Observatory a few years ago. A more
complete history and explanation of the technique, including its original
development in the mid 90's can be found on the Nod and Shuffle background page,
http://www.gemini.edu/project/announ...004-1-nod.html

Previous studies in the Redshift Desert have concentrated on galaxies that were
not necessarily representative of mainstream systems. For this study, galaxies
were carefully selected based upon data from the Las Campanas Infrared Survey in
order to assure that strong ultraviolet emitting starburst galaxies were not
oversampled. "This study is unique in that we were able to study the red end of
the spectrum, and this tells us about the ages of old stars," says Dr. Abraham.
"We undertook incredibly long observations with Gemini -- about ten times as
long as typical exposures. This let us look at much fainter galaxies than is
usually the case, and let us focus on the bulk of the stars, instead of just the
flashy young ones. This makes it a lot easier for us to work out how the
galaxies are evolving. We are no longer guessing at it by studying young objects
and assuming the old objects were not contributing much to the story of galaxy
evolution. It turns out that there are lots of old galaxies out there, but
they're really hard to find."

The GDDS was supported by a grant from the Packard Foundation and institutional
support from the United State's National Science Foundation, Canada's National
Research Council and the Natural Sciences and Engineering Research Council of
Canada, the United Kingdom's Particle Physics and Research Council and the GDDS
team-member institutions consisting of:

Roberto G. Abraham & R. G. Carlberg
Department of Astronomy & Astrophysics
University of Toronto
Canada

Karl Glazebrook & Sandra Savaglio
Department of Physics & Astronomy
Johns Hopkins University
Baltimore, MD

Patrick J. McCarthy
Observatories of the Carnegie Institution of Washington
Pasadena, CA

David Crampton & Richard Murowinski
Herzberg Institute of Astrophysics
National Research Council
Victoria, British Columbia, Canada

Inger Jørgensen & Kathy Roth
Gemini Observatory
Hilo, HI

Isobel M. Hook
Department of Astrophysics
Nuclear & Astrophysics Laboratory
Oxford University, England

Hsiao-Wen Chen
Center for Space Research
Massachusetts Institute of Technology
Cambridge, MA

Ronald O. Marzke
Dept. of Physics and Astronomy
San Francisco State University
San Francisco, CA

The Gemini Multi-Object Spectrograph used on the Frederick C. Gillett Gemini
Telescope on Mauna Kea to make the GDDS observations is one of two identical
instruments, which are used on both Gemini telescopes. The GMOS instruments were
built as a joint partnership between Gemini, the National Research Council of
Canada's Herzberg Institute of Astrophysics, the UK Astronomy Technology Centre
and Durham University, UK. Separately, the U.S. National Optical Astronomy
Observatory provided the detector subsystem and related software. GMOS is
primarily designed for spectroscopic studies where several hundred simultaneous
spectra are required, such as when observing star and galaxy clusters. GMOS also
has the ability to focus astronomical images on its array of over 28 million pixels.

The Gemini Observatory is an international collaboration that has built two
identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is
located on Mauna Kea, Hawai‘i (Gemini North) and the Gemini South telescope is
located on Cerro Pachón in central Chile (Gemini South), and hence provide full
coverage of both hemispheres of the sky. Both telescopes incorporate new
technologies that allow large, relatively thin mirrors under active control to
collect and focus both optical and infrared radiation from space.

The Gemini Observatory provides the astronomical communities in each partner
country with state-of-the-art astronomical facilities that allocate observing
time in proportion to each country's contribution. In addition to financial
support, each country also contributes significant scientific and technical
resources. The national research agencies that form the Gemini partnership
include: the US National Science Foundation (NSF), the UK Particle Physics and
Astronomy Research Council (PPARC), the Canadian National Research Council
(NRC), the Chilean Comisión Nacional de Investigación Cientifica y Tecnológica
(CONICYT), the Australian Research Council (ARC), the Argentinean Consejo
Nacional de Investigaciones Científicas y Técnicas (CONICET) and the Brazilian
Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq). The
Observatory is managed by the Association of Universities for Research in
Astronomy, Inc. (AURA) under a cooperative agreement with the NSF. The NSF also
serves as the executive agency for the international partnership.

Full-Resolution Images are at
http://www.gemini.edu/media/images_2004-1.html

Background Resources are at
http://www.gemini.edu/project/announ...4-1-links.html

 




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