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Scientists pointed the Hubble Space Telescope at a distant quasar to find evidence of vast amounts of intergalactic hydrogen. Like a flashlight shining through a fog, the quasar illuminated clouds of oxygen, which imply the presence of much greater quantities of invisible hydrogen. (Image by John Godfrey) |
For the past decade astronomers have looked for vast quantities of hydrogen that was cooked up in the Big Bang but somehow managed to disappear into space.
Astronomers believe that at least 90 percent of the matter in the universe is hidden in a "dark" form that has not yet been seen directly. But until now they have not even been able to see most of the universe's ordinary, or baryonic, matter (normal protons, electrons and neutrons).
Now a team of scientists led by researchers at Princeton has used NASA's Hubble Space Telescope to uncover a large portion of this long-sought missing hydrogen, which could account for as much as half of the "normal" matter in the universe (the rest of the normal matter is locked up in myriad galaxies). The newly found material is roughly equivalent in mass to all the stars in the universe.
Previous observations have shown that billions of years ago hydrogen left over from the Big Bang formed vast complexes of hydrogen cloudsbut since then it had vanished. Now the keen eye of the Hubble telescope has found a telltale sign that it is still around: highly ionized oxygen between galaxies.
Since atoms won't interact with light without their electrons, scientists believe that the "missing" hydrogen was rendered invisible by extreme heat that stripped the hydrogen atoms of their electrons. Because oxygen is less susceptible to this stripping, it continues to be visible. Hydrogen is much more abundant than oxygen, so the oxygen serves as a "tracer," indicating that the spaces between galaxies contain huge quantities of hydrogen.
The results of the research by postdoctoral fellow Todd Tripp and Senior Research Astronomer Edward Jenkins, collaborating with Blair Savage of the University of Wisconsin, Madison, were published in the May 1 issue of Astrophysical Journal Letters.
Support for models
In addition to accounting for a vast amount of missing material, the discovery provides strong evidence that theories about the evolution of the universe are on the right track.
In particular, Tripp's observation supports models of the expanding universe developed in recent years by Senior Research Astronomer Renyue Cen and Charles A. Young Professor of Astronomy Jeremiah Ostriker. Starting with several sets of data about how the universe was composed in its earliest moments, Cen and Ostriker used supercomputers to simulate its evolution to its present structure.
These computational models predicted the existence of an intricate web of gas filaments, where hydrogen is concentrated along vast chain-like structures, and clusters of galaxies form where the filaments intersect. The models indicated that hydrogen clouds were heated to about a million degrees during the formation of the filamentary webwhich also rendered the hydrogen invisible.
The discovery of the missing hydrogen "is a major step," says Cen. "We are very excited and very glad it's a prediction confirmed, and it has profound implications for the basic gravitational instability picture of large-scale structure formation."
Telltale oxygen
Tripp and colleagues detected their oxygen tracer by using the light of a distant quasar to probe the space between the galaxies, like shining a flashlight beam through a fog. Looking into the eye of the flashlight, Hubble's Space Telescope Imaging Spectro-graph found the spectral "fingerprints" of intervening oxygen superimposed on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with telltale oxygen.
Tripp emphasizes that there are a number of uncertainties in the group's initial results, and he plans to conduct more observations with Hubble to develop better data. In addition to establishing the quantity and distribution of tracer oxygen, the scientists want better estimates of the ratio of hydrogen to oxygen. They are also planning to use NASA's Far Ultraviolet Spectroscopic Explorer to carry out supporting observations such as searching for highly ionized neon as well as nearby highly ionized oxygen.
Eventually, they hope to study how this highly ionized gas evolves with time, by looking at increasingly farther systems until the gap between what is seen from the ground and what is seen from space is filled in.
The Space Telescope Science Insti-tute is operated by the Association of Universities for Research in Astronomy Inc. for NASA, under contract with NASA's Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency.
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