STScI Logo

The Science Potential of a 10-30m UV/Optical Space Telescope
Project C - Mapping the Cosmic Web

The story of the assembly of galaxies is believed to be found in the connection between the cosmic web of gas and the known galaxies, at a variety of redshifts. The gaseous web is generally invisible except by probing the gas using distant QSOs as background sources for the study of the UV absorption lines of H I, O VI and C II, for instance. The map of the cosmic web does not require high angular resolution, as do the first two problems, but does a require a very large aperture because the number of points to sample (distant QSOs) is so large. The Sloan Digital Sky Survey provides 1 million QSOs over 10,000 square degrees, virtually all the sources needed for this study. They are largely too faint for HST and at any rate it is not twos and threes but two hundreds and three hundreds of sources that are needed, even for a "small " study, never mind a key project. Hundreds of each of several types of objects (galaxies, groups of galaxies, clusters of galaxies) need to be probed in tens of angularly proximate, background QSOs for each foreground galaxy, group, or cluster, to map the web and its connections to galaxies, in detail.