The Cosmic Web

Observations and simulations of the intergalactic medium reveal the largest structures in the universe

Robert Simcoe

A Mighty Wind

It had long been assumed that the intergalactic medium was chemically pristine and that the production and distribution of new elements took place only within galaxies themselves. But astronomers also noticed that a few weak absorption features in quasar spectra appear redward of the hydrogen emission line. These other lines arise from different elements—in the case of Figure 2, carbon and silicon—whose characteristic wavelengths are redder (longer) than hydrogen's 121.56701 nanometers.

The absorption lines of these heavy elements are observed within regions that also contain a considerable amount of hydrogen. These zones correspond to gaseous halos around the first galaxies, whose stars were thought to supply the chemicals. However, in the early 1990s, quasar spectra taken by Lennox Cowie and Antoinette Songaila on the newly commissioned Keck telescopes revealed heavy elements far removed from any galaxy. Their discovery suggested that the chemical pollution of intergalactic space was much more efficient than originally believed.

The concentration of heavy elements in the intergalactic medium is very low: For example, only about one carbon atom can be found for every million (mostly hydrogen) atoms. So a box of intergalactic space that is 100 meters on a side would contain just a single carbon atom! Yet even this tiny amount reveals that some heavy elements were mixed throughout the cosmic web early in the history of the universe. How did they get out there—so far from the stars and galaxies in which they were made?

The evidence suggests that they were blown out into intergalactic space by violent galactic winds. These streams of matter flow out of galaxies where stars are actively forming. In all galaxies, the most massive stars burn brightly, and rapidly produce new elements. These stars burn so fast that they quickly exhaust their nuclear fuel and can no longer continue fusing light elements into heavier ones. When the reactor in a massive star turns off, the star ends its life in a tremendous explosion known as a supernova. The blast energy of a typical supernova rivals the simultaneous detonation of 10³¹ atom bombs, and the remnants of the dead star—including its newly fused heavy elements—are launched into surrounding space.

Despite its explosive power, a single supernova cannot pollute the intergalactic medium because the gravitational force from the star's galaxy traps the expanding debris before it can escape. However, galaxies occasionally experience bursts of unusually vigorous star formation where stars are born and die 10 to 50 times faster than usual. During these starbursts, multiple supernovae can be triggered in near succession. Their collective energy drives debris outward, like a rocket boosted by several stages, breaching the gravitational barrier and expelling heavy elements into the intergalactic medium. This phenomenon has been observed in a number of nearby galaxies (Figure 7).

Although we can study nearby starbursts and the resulting outflows in exquisite detail, these galaxies are the rare exception in the local universe. Most galaxies quietly go about forming stars and manage to retain the heavy elements they produce. But in the early universe, the situation was quite different. New observations of distant galaxies by Max Pettini at the University of Cambridge and his colleagues have revealed that outflows were extremely common when the universe was about 15 percent of its present age. This has two important implications. Nearly every galaxy we see today underwent some period of intense star formation in its past. And large quantities of heavy elements were launched into the intergalactic medium very early in the life of the universe. There was thus plenty of time for this material to coast out to large distances and mix with the chemically pristine intergalactic gas.

Studies of early galaxies and their feedback on the intergalactic medium define an important frontier of our knowledge about the first stars and cosmic structures. Several important questions remain open. For example, exactly when and where did the first stars form? Do heavy elements pervade the entire universe, or is there still chemically pristine gas left over from the Big Bang? Were the stars that triggered reionization the same stars that produced the observed intergalactic heavy elements?

For the past few years I have been investigating some of these questions with Wallace Sargent at the California Institute of Technology and Michael Rauch at the Carnegie Observatories. We have been measuring heavy-element concentrations in the early cosmic web to learn whether there are pristine corners of the universe that have not yet been reached by the galactic winds. So far we have detected heavy elements throughout all of the strands of the cosmic web, but it is still not clear whether the winds' sphere of influence extends beyond the filaments and into the intergalactic voids. In these remote regions the expected heavy-element densities are so low that even our most sensitive observations cannot reveal their absorption lines directly. Nevertheless, our results show that debris from galactic winds must have dispersed into most of the mass in the universe before the cosmos was a mere 20 percent of its present age.

We have also compared our observations with different models of star formation and chemical production to determine whether the stars that reionized the universe were the same ones that polluted the intergalactic medium. Our results suggest that the earliest stars did not produce most of the heavy elements, most likely because their heyday was too short (Figure 8). Instead, we believe that galaxy feedback occurred in a series of waves. The first generation of stars reionized the universe, and later generations progressively enriched the intergalactic medium with chemicals.

On the theoretical front, the most advanced cosmological simulations are just beginning to incorporate realistic models of galactic winds and the chemical enrichment of the universe. The physics of star formation and galaxy outflows is so complex that even the most sophisticated numerical models must make broad, simplifying assumptions to make the problem computationally tractable. The subject continues to progress rapidly, as both the observations and the theory evolve.

There are, of course, many details to be refined. Exactly how and when did the first stars form? How do galaxies and the intergalactic medium interact? And, perhaps most importantly, what is the nature of dark matter? Yet, when sufficient time has passed to offer a historical perspective, the past decade may well be remembered for the emergence of a standard model of the cosmos that ties all we know about galaxies and the intergalactic medium into a single package.

Bibliography

Cowie, L. L., and A. Songaila. 1998. Heavy-element enrichment in low-density regions of the intergalactic medium. Nature 394:44–46.
Padmanabhan, T. 1998. After the First Three Minutes: The Story of Our Universe. New York: Cambridge University Press.
Pettini, M., et al. 2001. The rest-frame optical spectra of Lyman break galaxies: star formation, extinction, abundances, and kinematics. The Astrophysical Journal 554:981–1000.
Rauch, M. 1998. The Lyman alpha forest in the spectra of QSOs. Annual Review of Astronomy and Astrophysics 36:267–316.
Simcoe, R. A., W. L. W. Sargent and M. Rauch. 2002. Characterizing the warm-hot intergalactic medium at high redshift: a high-resolution survey for O VI at redshift 2.5. The Astrophysical Journal 578:737–762.
Simcoe, R. A., W. L. W. Sargent and M. Rauch. In press. The distribution of metallicity in the intergalactic medium at redshift 2.5: OVI and CIV the spectra of 7 QSOs. The Astrophysical Journal.