FEATURE ARTICLE
The Cosmic Web
Observations and simulations of the intergalactic medium reveal the largest structures in the universe
Robert Simcoe
In the Red
Intergalactic gas is so tenuous and dark (producing no light of its own) that you might well ask how astronomers can hope to observe it. The trick is to detect it indirectly, by seeing how it influences light coming from faraway sources. The most common object for these observations is a quasar, a special type of galaxy containing a supermassive black hole at its center. Gas around the black hole emits intense radiation, which often outshines the average galaxy by 100 or more times. Because quasars are so bright, we can observe them at great distances and so measure the effects of intergalactic gas over substantial portions of the universe.
Using the world's most powerful telescopes, we can collect photons from these distant beacons and sort them by their wavelengths into spectra (Figure 4). The strongest feature in such a record is an emission line that is produced by hydrogen atoms near the quasar's black hole. The electrons in these atoms are excited to a single quantum level above their ground state. When they settle back to ground, photons are emitted with the precise wavelength of 121.56701 nanometers—called the Lyman-α transition. Yet we observe the emission line at a much longer wavelength, 560 nanometers. This is because the quasar is racing away from us, carried by the general expansion of the universe (see "The Hubble Constant and the Expanding Universe," January–February 2003). The expansion is such that objects far from us recede proportionally faster than those that are close. As an object moves away from us, the light that it emits is stretched to longer wavelengths in much the same way that the Doppler effect lowers the pitch of a receding train whistle. Astronomers use the term redshift to describe this phenomenon, since the colors of ever-more-distant objects become systematically redder.
Now consider what happens to the light of a quasar when it is transmitted through the intergalactic medium. As light from the quasar heads toward the Earth, some of its photons will intercept hydrogen atoms along the way. If one of these photons has a wavelength of 121.56701 nanometers, it will be absorbed by the atom, which then has one of its electrons kicked out of the ground state. When the electron loses energy and falls back to the ground state, the photon is re-emitted, but in an arbitrary direction, which is not likely to be toward Earth (Figure 2). So a cloud of hydrogen atoms will absorb light at a very specific wavelength and scatter it away—we see this as a dark "hole" in the spectrum.
The intergalactic medium contains many hydrogen clouds at different distances from us. And because clouds at different distances have different redshifts, a quasar spectrum shows many absorption lines at different wavelengths. The wavelengths below the hydrogen emission line thus appear to be "eaten" away according to the location of each cloud between the quasar and us. In the past decade new instruments on large telescopes have allowed us to examine the spectra of quasars at very-fine-wavelength resolution and high signal-to-noise ratio. These "zoomed-in" views (Figure 4 bottom) resolve the intergalactic medium into individual clouds.
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