A Cosmic Radiation Field Unveiled
University of California-Los Angeles
Contact:
Stuart Wolpert, stuartw@college.ucla.edu, (310) 206-0511
Harlan Lebo, hlebo@college.ucla.edu, (310) 206-0510
For Immediate Use: Jan. 12, 2000
AAS Paper #14.02
A COSMIC RADIATION FIELD UNVEILED
Astronomers from JPL, UCLA and UC Santa Cruz today announced the discovery of a new cosmic signal
that should lead to a better understanding about the creation of stars from the early universe. The new
signal, called the near-infrared background, is radiation with wavelengths four to six times longer than
the wavelengths of visible light (2.2 and 3.5 micrometers).
This discovery enables astronomers to set tight constraints on the star formation history of the
universe. Previously, only theoretical estimates were available of how much matter in the universe had
been converted to stars. Stars are born surrounded by dust, which absorbs visible light, so infrared
data are required.
Varoujan Gorjian, a National Research Council postdoctoral fellow at JPL; Edward L. Wright, a professor
at UCLA; and Ranga-Ram Chary, a postdoctoral fellow at UC Santa Cruz, found that the near-infrared
radiation measured by the Diffuse Infrared Background Experiment (DIRBE) on the Cosmic Background
Explorer (COBE) includes a significant contribution from distant galaxies throughout the universe.
This marks the end of a decade of cosmological discoveries coming from COBE, which started with the
announcement on Jan. 13, 1990, at the 175th AAS meeting that the cosmic microwave background
spectrum was extremely close to a blackbody with a temperature of 2.7 degrees above absolute zero.
Two years ago the DIRBE team, led by Michael Hauser of the Space Telescope Science Institute, reported
the discovery of a far-infrared background at much longer wavelengths: 140 and 240 micrometers. This
is radiation that has been absorbed by cold dust in distant galaxies and reradiated at long wavelengths.
The near-infrared radiation announced today is a direct detection of the short wavelength emission from
stars or active galactic nuclei that has been stretched (“redshifted”) by the expansion of the universe.
Now both the short and long wavelength parts of the Cosmic Infrared Background (CIRB) are known.
The total amount of radiation in this near-infrared background is about equal to that in the far-infrared
background. The sum of these infrared backgrounds amounts to 6 percent of the radiation in the cosmic
blackbody radiation which is left over from the Big Bang. This is an indication of the total amount of
power that has been produced in the lifetime of the universe by sources of any kind: stars, accreting
black holes or hypothetical exotic subatomic particles.
The near-infrared background was known to exist because a large number of faint, distant galaxies can
be measured at 2.2 micrometer wavelength using large telescopes. This provided a minimum level for the
background. But in 1998 the DIRBE team was not able to measure the near-infrared background
because of the confusing effect of the many stars in the Milky Way. But Gorjian, Wright and Chary
selected a region near the Big Dipper, which had very few stars that were bright in the infrared, and then
measured the brightness of these few stars using telescopes at the Lick Observatory on Mount Hamilton
near San Jose and the Palomar Observatory on Palomar Mountain. A second region of the sky was also
analyzed using data from NASA’s Two Micron All Sky Survey (2MASS).
When the measured brightness of these stars was subtracted from the DIRBE data, the near-infrared
background was seen at a level about three times higher than the minimum provided by observed faint
galaxies.
The cosmic background at 2.2 micrometers is about 10,000 times fainter than the dark night sky seen
from Earth. The cosmic background at 3.5 micrometers is about 2 million times fainter than the dark
night sky, which is very bright at this wavelength. Thus these observations must be done in space, above
the glow from the Earth’s atmosphere.
The CIRB is the full tally of all matter that has gone through luminous episodes. It includes the light that
has arrived to us directly from the stars in the near-infrared, and it contains the absorbed and
reradiated light from those stars in the far-infrared. Now with both ends of that spectrum known, a
better understanding can be gained about the creation of stars from the early universe to today.
A paper describing this work in detail has been submitted to the Astrophysical Journal, and a preprint is
available at http://arXiv.org/abs/astro-ph/9909428 .
Gorjian and Chary were graduate students at UCLA working with Wright and obtained their Ph.D.s in
1998 and 1999.
Another new measurement of the near-infrared background is reported in a paper by Wright and Erik
Reese. Reese, who is now a graduate student at the University of Chicago, was an undergraduate at
UCLA when the first draft of this paper was written. The results of this second measurement of the
near-infrared background fully support the Gorjian, Wright and Chary results. Wright and Reese’s paper
was submitted to the Astrophysical Journal last month, and a preprint is available at
http://arXiv.org/abs/astro-ph/9912523 .
For more information, contact:
Edward L. Wright, wright@astro.ucla.edu, (310) 825-5755
Varoujan Gorjian, Varoujan Gorjian@jpl.nasa.gov, (818) 354-2068
Ranga-Ram Chary, rchary@ucolick.org, (831) 459-3828
