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Astronomers Discover an Infrared Background Glow in the Universe

Donald Savage
Headquarters, Washington, DC                       January 9, 1998
(Phone:  202/358-1547)                 EMBARGOED UNTIL 8:30 AM EST

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone:  410/338-4514)

RELEASE:  98-3

ASTRONOMERS DISCOVER AN INFRARED BACKGROUND GLOW IN THE UNIVERSE
 
       Astronomers have assembled the first definitive detection 
of a background infrared glow across the sky produced by dust 
warmed by all the stars that have existed since the beginning of time.

       For scientists, the discovery of this "fossil radiation" is 
akin to turning out all the lights in a bedroom only to find the 
walls, floor and ceiling aglow with an eerie luminescence.

       The telltale infrared radiation puts a limit on the total 
amount of energy released by all the stars in the universe.  
Astronomers say this will greatly improve development of models 
explaining how stars and galaxies were born and evolved after the 
Big Bang.

       The discovery reveals a surprisingly large amount of 
starlight in the universe cannot be seen directly by today's 
optical telescopes, perhaps due to stars being hidden in dust, or 
being too faint or far away to be seen. 

       The discovery culminates several years of meticulous data 
analysis from the Diffuse Infrared Background Experiment aboard 
NASA's Cosmic Background Explorer (COBE), which was launched in 
1989.   The difficulty in making the discovery is analogous to 
listening for a faint background hum in a shopping mall full of 
people talking, music playing, and other noises.

       "This is another big step in bringing cosmology to a 
science based on observation as well as theory," said Michael 
Hauser of the Space Telescope Science Institute, Baltimore, MD, 
principal investigator on the Diffuse Infrared Background Experiment.  

       Reporting the results at the meeting of the American 
Astronomical Society in Washington, DC, Hauser added: "We set out 
to do this 23 years ago, and these results show it was worth it.  
Our discovery fulfills the third and final cosmology objective of 
the Cosmic Background Explorer mission."

LOST STARLIGHT

       The unexpected preponderance of far infrared light implies 
that many stars have "fallen between the cracks" in ultra-
sensitive visible light probes of the distant corners of the 
universe, such as the Hubble Deep Field.

       One possible explanation is that the universe is very 
dusty, with many stars hidden in blankets of dust. Alternatively, 
many stars may have been born in a flurry of activity in the very 
early universe but faded away at earlier times than yet reached 
with large telescopes.

       In either scenario, the existence of hidden stars is 
revealed by telltale dust which absorbs and re-radiates their 
light at infrared wavelengths, and so a permanent record of their 
existence is encoded in the infrared background.

UNCOVERING THE INFRARED BACKGROUND

       Data from two other instruments on the Cosmic Background 
Explorer  have already yielded the precise spectrum and a detailed 
map of another cosmological fossil (first discovered in the 
1960s), the microwave background radiation from the Big Bang.

       But finding the infrared background was not easy.  Unlike 
the cosmic microwave background, which at millimeter wavelengths 
outshines everything else in the universe, the infrared background 
is masked by infrared light from nearby dust in our solar system, 
stars and interstellar dust in the Milky Way Galaxy, and, for 
ground-based instruments, emission from the Earth's atmosphere and 
from the instrument  itself.  The COBE mission overcame the last 
two problems by observing from space using a small telescope and  
instrument cooled to within a few degrees of absolute zero.

       The COBE science team began by using the Diffuse Infrared 
Background Experiment to scan half the entire sky once a week, 
over a 10-month period from Dec. 1989 to Sept. 1990.  Astronomers 
then modeled and subtracted the infrared glow from foreground 
objects in our solar system, our galaxy's stars, and vast clouds 
of cold dust between the stars of our Milky Way.

       Solar system dust was fairly easy to identify in the data 
because its brightness changes from week to week as Earth orbits 
the Sun.  The interstellar dust of our Galaxy was identified in 
the data because it has structure, and so looks different across 
the sky.  Light from stars was removed using a detailed model 
based on counts of the many types of stars in the various parts of 
the Galaxy.

       When infrared light from these sources was subtracted from 
the all-sky maps, the astronomers found a smooth background of 
residual infrared light in the 240 and 140 micrometer wavelength 
bands in "windows" near the north and south poles of the Milky 
Way, which provided a relatively clear view across billions of 
light years.

       Astronomers next plan to probe the early formation of stars 
and galaxies using infrared telescopes on new space missions such 
as the Space Telescope Infrared Facility, Wide Field Infrared 
Explorer, Next Generation Space Telescope and the Far Infrared 
Space Telescope, and hope some day to make more infrared 
background  measurements using instruments launched deep into the 
solar system to escape the interplanetary dust.

                    - end -

EDITOR'S NOTE:  Images accompanying this release can be obtained 
from the internet at the following URLs:

http://oposite.stsci.edu/pubinfo/pr/1998/01.html 
http://oposite.stsci.edu/pubinfo/gif/dirbe.gif 
http://oposite.stsci.edu/pubinfo/jpg/dirbe.jpg