Mapping the Distant Universe

A new technique, in combination with the powerful light--gathering power of the new generation of ground--based optical/infrared telescopes, is allowing one to map out the structure traced by galaxies on large scales, when the universe was only 10% of its current age. The simplicity and reliability of this technique for isolating the extremely distant galaxies from among the much more numerous foreground, less distant ones, allows true mapping of the galaxies' large--scale distribution for the first time.

The illustration above shows a small portion of three images of the same piece of sky, taken with the 200-inch Hale Telescope at the Palomar Observatory in California. The images were taken through red, green, and ultraviolet filters specially designed for finding high redshift galaxies. The object at the center of the circle is clearly present in both the red and the green image, but disappears in the UV image. The characteristic UV ``dropout'', the disappearance of the galaxy in the ultraviolet filter, is due to absorption by hydrogen gas that can only be seen when the galaxy is at an extreme distance, when the redshift is large enough that the absorption edge, which occurs in the far ultraviolet, is shifted all the way to wavelengths that are not blocked by the earth's atmosphere. Candidate high redshift galaxies are isolated using such images (the full-size images contain 100-150 of these candidates) and then are confirmed using a spectrograph on the 10-meter W. M. Keck Telescope in Hawaii. The technique is sensitive to galaxies with redshifts in the range 2.6-3.4, or distances of about 12.5 billion light years for a Universe 14 billion years in age.

This is an example of a full image obtained with a CCD camera on the Palomar Hale Telescope, with all of the galaxies having "ultraviolet dropouts" circled. Spectra of these objects are then obtained with the W.M. Keck 10-meter telescopes in Hawaii, to measure the precise redhift, or distance, to each galaxy. There are about 4000 galaxies in the image, of which about 150 are candidate extremely distant galaxies.

After redshifts are determined, they can be placed into "bins" in distance, where large "spikes" indicate significant concentrations of galaxies in space. In combination with the image of the sky above, this allows the reconstruction of the 3-dimensional structure traced by galaxies in the distant universe. The green histogram shows the number of galaxies at each distance; the purple histogram shows the expected distribution if the galaxies were arranged randomly in space. The difference illustrates the very strong clustering of the very distant galaxies. The presence of such strong clustering at early times for the most luminous objects represents strong support for the idea that galaxies form at the highest peaks in the distribution of matter; these high peaks are themselves expected to be strongly correlated in space. Over the age of the universe, gravity then amplifies these dense regions, so that by the present day, they are likely to be the same regions where rich clusters of galaxies are found. The details of the distribution of early galaxies, and the nature of the galaxies themselves, are providing critical tests of theories of galaxy and structure formation. They are also showing that the relationship of visible galaxies to the overall distribution of matter in the universe can be quite complicated, and may be varying as a function of time; this relationship must be quantified in order to fully understand what the distribution of galaxies is telling us about the overall structure of the universe. Thus, it is necessary to understand how galaxies form in order to use them to trace the large scale structure of the universe. A great deal of progress is being made in these areas, with much more progress expected in the near future.