The Johns Hopkins Gazette: August 20, 2001
August 20, 2001
VOL. 30, NO. 42


FUSE Data Unveils Space Cobweb

By Michael Purdy
Johns Hopkins Gazette Online Edition

The Far Ultraviolet Spectroscopic Explorer, an orbiting observatory operated for NASA by Johns Hopkins, made scientific news twice in the past two weeks, contributing key data to a study in Science on Aug. 10 and to a paper in Nature on Aug. 16.

In the study reported in Science, astronomers used FUSE and the Hubble Space Telescope to analyze the light of a distant quasar for the fingerprints of tenuous intergalactic clouds of helium gas that the radiation passed through on its way to Earth. The results gave them one of their most detailed looks yet at the cobwebs of gas laced through the vast spaces between galaxies.

In the Nature study, astronomers used FUSE to analyze the contents of a disk of gas and dust around a nearby young star. What they found strongly suggested the possibility that the cloud of gas, which astronomers believe is developing into a solar system, may contain comets.

The findings published in Science are particularly significant for FUSE team members. Learning more about the tenuous material between galaxies, known as the intergalactic medium, is a primary objective for FUSE. Much of the intergalactic medium was created in the big bang that began the universe, and astronomers believe the material may hold many clues to what was happening in the early universe.

In addition, the study continues the work of and was authored in part by Arthur Davidsen, a longtime leader in the Physics and Astronomy Department, who died last month.

In 1977, Davidsen used a rocket-mounted spectroscope to become the first astronomer to look at the ultraviolet spectrum of radiation from a quasar. The new Science study essentially did the same thing but used a much fainter and more distant quasar. With the more advanced instrumentation on FUSE and Hubble, and the more stable observing perch provided by both observatories, scientists were able to gather much more detailed information.

Davidsen was also the principal investigator for the Hopkins Ultraviolet Telescope, an observatory that flew twice aboard the space shuttle. Based on observations taken during a shuttle flight in 1995, Davidsen and his colleagues were the first to suggest that the gas between galaxies was not smooth as astronomers had expected but was instead lumpy.

Data from the new study in Science, led by Jerry Kriss, an astronomer at the Space Telescope Science Institute, supports the work of theorists who have proposed that the intergalactic medium's lumpy structure stretches between the galaxies like a cobweb. Scientists believe stars, galaxies and galaxy clusters form along the junctures of this cobweb.

For the study, Kriss and colleagues at Hopkins and the University of Colorado used FUSE and the Hubble Space Telescope to observe a quasar approximately 10 billion light-years from Earth. Quasars are thought to be distant galaxies with highly active black holes in their centers. As the black holes pull in gas and dust, those materials become superheated, causing intense radiation emissions.

While that radiation traveled toward Earth, the clouds of hot helium gas that it passed through between galaxies absorbed characteristic wavelengths of light. Because Earth and the quasar are moving away from each other as the universe expands, these spectral fingerprints are shifted to many different wavelengths within a region of the spectrum known as the far ultraviolet. FUSE specializes in analysis of this region.

By comparing the FUSE data with complementary observations of the quasar taken through the Hubble Space Telescope in other regions of the ultraviolet spectrum, Kriss says, the authors also were able to probe the likely causes of an event cosmologists often call the "cosmic renaissance."

The "renaissance" occurred in two stages during the universe's first billion years, when expansion of the universe had dissipated energy from the big bang and the first atoms began forming. These events produced a period known as the "cosmic dark ages," during which much of the matter in the universe blocked transmission of light.

Near the close of the first billion years, enough energy had been pumped into the universe to re-energize and restore electric charge first to the hydrogen and later to the helium in the early intergalactic medium.

Scientists once thought most of the energy that caused those changes came from quasars, but the data in the Science study and other observations in the past decade have astronomers thinking hydrogen may have received a significant energy boost from regions of intense star formation in galaxies known as starburst galaxies.

For the Nature study, FUSE's target was the much closer star Beta Pictoris. Alain Lecavalier des Etangs of the Astrophysics Institute of Paris led the study of the star, which is located about 60 light-years from Earth and thought to be a young star only about 20 million years into its several-billion-year lifespan.

Astronomers are keenly interested in the possibility that a disk of gas and dust surrounding Beta Pictoris may be developing into a solar system. Many stars are surrounded by similar disks, but Beta Pictoris is close and oriented in a way that gives earthbound astronomers an ideal, nearly edge-on view of the disk. Astronomers have found a number of clues that suggest the system may be forming planets.

When Lecavalier des Etangs and colleagues at Hopkins, NASA's Goddard Space Flight Center and the Astrophysical Laboratory of Marseille pointed FUSE at Beta Pictoris in March of this year, they found that the disk seems to have almost no molecular hydrogen (H2). That was surprising not only because of the general pervasiveness of H2, which is the most common molecule in the universe, but also because FUSE's specialties include detection of this molecule.

In addition, the same team had previously used the Hubble Space Telescope to show that the disk contains carbon monoxide.

"Carbon monoxide is broken down by starlight relatively quickly, so for it to show up in a disk surrounding a star that's 20 million years old, it has to be steadily released from a shielded reservoir of some kind," says Aki Roberge, a Hopkins graduate student who was an author on the paper.

Carbon monoxide is typically seen only in association with molecular hydrogen in our galaxy's clouds of gas and dust, suggesting that the molecular hydrogen in the Beta Pictoris system must be locked up somewhere. When astronomers look to our solar system for reservoirs that could contain abundant carbon monoxide and small amounts of molecular hydrogen, the prime candidates are the thousands and perhaps millions of comets that travel the outer edges of our solar system and occasionally plunge into the inner solar system.

"If a comet swarm similar to our own surrounds Beta Pictoris, the comets would still be warm enough to slowly release carbon monoxide but far too cold to release molecular hydrogen, which would remain locked up as water ice (H2O)," says Paul Feldman, an author on the paper and chair of Hopkins' Physics and Astronomy Department.

Feldman notes that previous studies of the Beta Pictoris system using the Infrared Space Observatory detected evidence for a much larger amount of molecular hydrogen than expected based on the FUSE result. One factor that could account for this seeming discrepancy would be if the molecular hydrogen was not uniformly dispersed throughout the system but instead gathered in clumps. Those clumps could be left over from the formation of the star, or they could be gas giant planets in the process of forming, according to Feldman.