Where's the matter? Partially dimmed x-rays (dips in graph) from a flaring blazar (right) reveal two filaments of hot, diffuse matter in the vast spaces between galaxies.

The Case of the Missing Matter

Astronomers hot on the trail of ordinary matter in the universe--the stuff of people, planets, and stars--have found where half of it was hiding. An x-ray satellite has detected a web of hot gas lacing through space among galaxies, adding up to far more mass than the matter we see with our eyes. The tenuous clouds, predicted but not clearly revealed until now, bring astronomers closer to a census of everyday matter.

Cosmologists believe the universe is dominated by two unknown quantities: dark energy and dark matter. Ordinary matter accounts for less than 5% of the cosmic ingredients, according to studies of elements formed in the big bang and faint fluctuations in the big bang's radiation. Of that amount, just a tiny fraction exists in stars and galaxies, while cool clouds of gas account for a lot more. However, telescopes had never seen about half of the ordinary matter. Theorists suspected that it lurked within a web of rarefied gas spreading throughout space, sizzling at about 1 million °C. Hints of the hot gas had been found near galaxy clusters (ScienceNOW, 25 November, 2002), but astronomers needed more details to see the rest of the diffuse network.

Now, a team has illuminated part of that web by tracing a piercing beacon of light. Astronomer Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues monitored the galaxy Markarian 421, which contains a "blazar"--an active black hole that aims powerful jets of energy toward Earth. When the blazar flared to record levels in 2002 and 2003, NASA's Chandra X-ray Observatory studied its intense light. Two hot clouds of ionized carbon, nitrogen, oxygen, and neon atoms--located about 150 million and 370 million light-years away--absorbed some of the x-rays at specific energies, report the researchers in the 3 February issue of Nature.

The team found similar filaments by watching two other bright flares. Together, the results point to a widespread web of hot gas that gravity has not yet pulled into galaxies, says Nicastro. The amount of gas Chandra sees is consistent with the amount of "missing" ordinary matter, he notes.

The research demonstrates that x-ray satellites can chart the otherwise invisible gas, says astronomer J. Michael Shull of the University of Colorado in Boulder. "For the first time, this [detection] looks real," Shull says. "But it's just the tip of the iceberg." Future x-ray and ultraviolet telescopes will map the cosmic web more thoroughly, he predicts, shedding light on how gravity assembled hot gas into today's panoply of galaxies and stars.

Related sites
Abstract of paper, with link to full text
Chandra X-ray Observatory
FUSE satellite, used in similar studies of lower-temperature gas
Observations of Markarian 421 blazar
Shull's home page