Shooting through sheets of galaxies.
Like a searchlight slicing through the mist, light from a bright, distant quasar can illuminate whatever lies directly between the quasar and Earth. Intervening galaxies, hydrogen clouds and intergalactic material intercept the light, absorbing certain wavelengths while allowing others to pass through unhindered. But interpreting the resulting spectra is no simple matter. Sometimes, the spectra are so complicated there's no telling where the light has been.
In 1987, astronomers noticed striking similarities between the spectra from two separate quasars, designated Tol 1037 and Tol 1038, approximately 4 billion light-years from Earth. As seen in the sky, these quasars appear separated by an angle equivalent to two-thirds the diameter of the full moon, or by a distance of at least 15 million light-years at the quasar locations. The closely matching spectra suggested that the line of sight to each quasar apparently cuts through four separate, strongly absorbing regions located at the same positions along both lines. Because the probability of finding so many matching absorption regions by chance is small, a number of astronomers concluded that the two lines of sight intercept a single supercluster of galaxies at least 15 million light-years across and 45 million light-years deep.
A fresh look at the light from the two quasars, however, now reveals fine details in the spectra, clearly indicating a less-than-perfect match and the presence of additional light-absorbing regions. Laura Ferrarese and J. Christopher Blades of the Space Telescope Science Institute in Baltimore and Stefano Cristiani of the University of Padua in Italy report that although the positions of the light-absorbing regions are roughly comparable along both lines of sight, the patterns of light absorption are significantly different in three of the four cases. The team tentatively interprets these observations to mean that light from the two quasars traverses three or more "sheets" of galaxies, with each line of sight poking through different galaxies or clouds in the same cluster. The sheets appear to sit about 12 million light-years apart along both lines of sight.
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|Date:||Jun 23, 1990|
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