Technical paper available here: PDF format or PS format
Thanks to the incredible tenuousness of gas in the vast spaces between galaxies, astronomers can see sources of light from billions of light-years away. However, these sources are very faint because they are so far away. If we want to study how the gas absorbs light, it makes sense to study the very brightest sources, namely quasars. Quasars lie at the centers of galaxies that are thought to harbor supermassive black holes weighing several billion times the mass of the Sun. As gas falls in toward the black hole, violent processes in the gas cause the quasar to emit intense light over a broad range of wavelengths (colors).
Observing distant quasars, and using the shadowing effect to study the intergalactic hydrogen lying between us and these quasars, is now a standard cosmological tool. Usually the absorption is studied only at large distances from the quasar, but we realized that the absorption had not been studied carefully in the region right near the quasar. Thus, we set out to construct a theoretical model in order to predict what could be learned about the quasar environment from such absorption.
At early times, large galaxies are still very rare and form only in the regions of highest density in the universe. As the matter that eventually forms the galaxy collapses toward these density peaks, a large surrounding region feels the strong gravitational pull of the accumulating mass. So when the galaxy finally forms along with the quasar inside it, gas from all around is in the process of falling toward the galaxy, and this gas is already much denser than typical intergalactic gas. This gas infall pattern should be reflected in the absorption pattern that the gas produces on background light.
In analyzing gas infall due to gravity, we must remember that the mass that produces this gravity is thought to consist mostly of "dark matter". Most of the matter in the universe is dark, with only about one-fifth in the form of the familiar chemical elements. In today's universe, galaxies are created at the centers of much more massive halos of this dark matter - an unknown substance that makes its presence felt by its strong gravitational effects. Just as the velocity and radius of the Earth's orbit allow us to measure the mass of the Sun, so the motion of stars and gas around the centers of nearby galaxies allow us to weigh those galaxies. On larger scales, the aggregate gravitational pull of dark matter affects the distribution of galaxies and the expansion rate of the universe.
2) The pattern of infall is determined by the total mass of the dark matter halo surrounding the quasar. Dark matter halos have been discovered and studied in the nearby universe, but it is much harder to study them in the early universe (which we can observe only at large distances). For example, in the local universe people study the motion of gas at the outskirts of galaxies, in order to infer the mass of the dark matter which is producing the motion, but the outskirts of distant galaxies are too faint to see. In fact, no-one has previously measured the mass of a dark matter halo (whether one containing a quasar or not), except in the nearby universe. Our detection of infall yields an estimate of the masses of halos when the universe was only around one billion years old (compared to its current age of about 14 billion years).
3) Since we did detect this effect around quasars, this also has important consequences for the nature of quasars. In the local universe, supermassive black holes (which shine as quasars if large amounts of gas fall onto them) are known to lie in the centers of galaxies, which in turn lie in the centers of dark matter halos. Typically, the dark halo contains around 5-10 times more mass than the galaxy, and the black hole contains only a fraction of one percent of the mass of the galaxy. Remember, all that was seen directly is a bright source of light corresponding to the quasar. We used the absorption effect to detect the large mass concentration around it. This suggests that the most distant quasars form similarly to those in the nearby universe.
It's important to note that gravitational effects are particularly interesting, since gravity is very well understood. Complicated gas processes often make it difficult to compare between theoretical models and observed data. On large scales, however, gravity dominates, potentially allowing a more direct test of the models.
More generally, the goal is to test models of how galaxies form, and look for surprises. For example, very little is known about dark matter other than its gravitational effects. If the dark matter interacted with itself through some other process, this would affect the number of dark matter halos that form. If we succeed in measuring many halo masses, they could come out either too high or too low, compared to the standard model, and any disagreement would provide a crucial clue about the nature of dark matter.
The links below were all active at the time. Many of them no longer work...
2) Harvard press release
6) UPI press release
7) NJ Star-Ledger
8) Troy Record
13) Science News (Not free online)
14) Astronomy.com (Inaccurate explanation of peaks)
16) Postech Korean site (English)
2) Ha'aretz newspaper
3) Ma'ariv newspaper
Israel21c.org (English; based on Jerusalem Post, Feb. 6, 2003 story)
2) www.panorama.it (Italian; bottom of page)
3) grani.ru (Russian)
4) www.heise.de (German)
5) www.tam.gov.tw (Chinese)