University of Hawaii Astronomers Gain New View of Helium Gas Leftover from the Big Bang

University of Hawaiʻi
Posted: Aug 9, 2001

NASA‘s FUSE satellite (Far Ultraviolet Spectroscopic Explorer) has given astronomers their best glimpse yet at the ghostly cobweb of helium gas leftover from the big bang, and underlying our universe‘s structure. The helium is not found in galaxies or stars but is spread thinly through the vastness of space.

The observations help confirm theoretical models of how matter in the expanding universe coagulated into a sponge-like structure pervading all of the space between galaxies. The helium traces the architecture of the universe back to very early times.

University of Hawaiʻi astronomers Antoinette Songaila Cowie and Lennox L. Cowie were part of the investigative team, whose observations will be published in the August 10 issue of the journal Science.

"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.

The FUSE observations bolster evidence that the early universe was re-energized by both radiation from black holes in active galaxies and by a firestorm of starbirth.

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 20 days, during two observing campaigns in August and October 2000. Along the trajectory to Earth, intervening clouds of hot helium gas modified the quasar‘s light. As light passes through clouds of intergalactic helium, helium atoms in the gas absorb specific colors of the light in the far-ultraviolet range of the spectrum. Simultaneous observations using the Hubble Space Telescope showed the brightness of the quasar at longer ultraviolet wavelengths where the spectrum is unaffected by helium.

The spectrum allowed investigators to trace how helium, which was very dense and opaque to radiation in the early universe, grew thinner as the early universe expanded and was "re-ionized" by a flurry of quasar and galaxy formation, like an early-morning fog is lifted by the rising sun. The new FUSE observations show details in this thin web of helium in unprecedented detail.

What exactly caused the fog to lift in the early Universe — that is, what sources provided the ionizing radiation — is one of the important unknowns. Clues to what the sources were can be found by comparing the effect of their radiation on the helium in the early universe with the effect on the much more abundant, but more highly ionized, hydrogen. But the hydrogen absorption is visible from the ground whereas the helium absorption occurs at ultraviolet wavelengths that are blocked by the Earth‘s atmosphere. Until now, the level of detail available on the hydrogen absorption has not been matched by the helium observations. However, obtaining just such a high precision measurement of early helium was one of the major drivers of the FUSE project. The team compared the FUSE helium spectrum with a highly precise spectrum of the hydrogen in the same direction obtained by Songaila Cowie at the Keck 10m telescope on Mauna Kea and confirmed that the energy source must be a mix of active galactic nuclei (AGNs) powered by billion solar mass black holes and the light from newly formed stars. AGNs 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.

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 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

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