NASA's Far Ultraviolet Spectroscopic Explorer (FUSE) satellite has given astronomers their best glimpse yet at the ghostly cobweb of helium gas left over from the big bang, which underlies the universe's structure. The helium is not found in galaxies or stars but spread thinly through the vastness of space.
The observations help confirm theoretical models of how matter in the expanding universe condensed into a web-like structure pervading all of the space between galaxies. The helium traces the architecture of the universe back to very early times. This structure arose from small gravitational instabilities seeded in the chaos just after the big bang.
"Visible galaxies are only the peaks in the structure of the early universe. The FUSE observations of ionized helium show us the details of the hills and valleys between the mountain tops," says Gerard Kriss, leader of the FUSE observing team and astronomer at the Space Telescope Science Institute in Baltimore, MD.
The FUSE observations also bolster evidence that the early universe was re-energized by torrents of radiation from black holes in active galaxies, and a firestorm of star birth.
"The observed absorption by intergalactic helium agrees extremely well with theoretical predictions made at the University of Colorado of an intergalactic medium ionized by both quasars and starburst galaxies," says Michael Shull, member of the FUSE team and professor at Colorado.
The observation was accomplished by using the distant light from a quasar (a brilliant, active nucleus of a galaxy) to allow FUSE to peer across 10 billion light-years of seemingly empty space to make new and precise measurements of the universe's hidden structure.
The observations were made by collecting the light of a distant quasar for a total of twenty days, during two observing campaigns in August and October 2000. Along the trajectory to Earth intervening clouds containing hot helium gas modified the quasar's light. As light passes through intergalactic clouds, helium atoms in the gas absorb specific colors of the light in the far-ultraviolet range of the spectrum.
Complementary, simultaneous observations using NASA's Hubble Space Telescope showed the brightness of the quasar at longer ultraviolet wavelengths where the spectrum is unaffected by helium. The spectrum allows Kriss and co-investigators to trace how helium, which was opaque to radiation in the early universe, grew more transparent as the early universe expanded and was "re-ionized" by a flurry of quasar and galaxy formation, like an early-morning fog is burned off by the rising sun.
The helium nuclei were forged in the first few minutes of the big bang. As the universe expanded the nuclei captured electrons to form a cool gas of neutral atoms. This gas was then reheated and ionized by a "fireworks show in reverse" as torrents of radiation poured into space from the powerful black holes at the centers of some newly formed galaxies and from the firestorm of star birth in other galaxies.
Astronomers have pondered what exactly energized the early universe. By comparing the absorption caused by intergalactic hydrogen, which is visible in spectra from ground-based telescopes like the Keck Observatory, to the helium absorption seen with FUSE, astronomers are able to achieve a better understanding of the energy source.
Though more abundant, intergalactic hydrogen is less easily detected because it is so highly energized (ionized). Even when ionized, helium manages to retain an electron. This etches the light from the quasar with a "forest" of spectral absorption features.
Because the universe is expanding, these absorption features are found at many different wavelengths depending on the distances of the intergalactic clouds from Earth. The FUSE comparison of helium to hydrogen absorption favors an energy source that is a mix of quasars powered by supermassive black holes and the light from newly formed stars. Quasars, historically, have been the preferred power source to heat the early universe. The FUSE observations support other recent suggestions that star formation is also important.
"This is a very exciting discovery. The search for the spectral signatures of a forest of ionized helium gas in the early universe was one of the major objectives of the FUSE mission, and it has been fulfilled spectacularly," says Dr. George Sonneborn, FUSE Project Scientist at NASA's Goddard Space Flight Center in Greenbelt, MD.
Studying the intergalactic medium in ultraviolet light is one of the top tasks for FUSE. In the 1990s astronomers probed the distant universe using the ultraviolet capabilities of the Hubble Space Telescope and the Hopkins Ultraviolet Telescope (HUT) on the ASTRO-2 Space Shuttle mission.
The HUT observations, led by the late Arthur Davidsen of the Johns Hopkins University, gave the first inkling that the intergalactic medium was not a smooth distribution of gas between the galaxies. FUSE has the necessary combination of far-ultraviolet sensitivity and spectral resolution to make the definitive observation of structure in the intergalactic medium traced by ionized helium.
The Johns Hopkins University had the lead role in building FUSE under the direction of principal investigator Warren Moos, and they now operate the satellite for NASA. The FUSE results are being published in the August 10, 2001 issue of the journal Science.
The team next plans to use FUSE to look at other quasars to trace the universe's structure.
The FUSE is a NASA Origins mission developed and operated by The Johns Hopkins University in collaboration with NASA's Goddard Space Flight Center, the Centre National d'Etudes Spatiales (France), the Canadian Space Agency, the University of Colorado, and the University of California, Berkeley. FUSE was launched on June 24, 1999 on a three-year mission to obtain high-resolution spectra in the far ultraviolet wavelength region (905-1185 Angstroms) of faint galactic and extragalactic objects. For further information about FUSE, visit the mission web site at http://fuse.pha.jhu.edu.
NOTE TO EDITORS: An illustration is available on the Web at http://oposite.stsci.edu/pubinfo/pr/2001/27 and via links http://hubble.stsci.edu/go/news, and http://oposite.stsci.edu/pubinfo/pictures.html.
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).
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