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Optical images of the extra-Galactic sky show a variety of galaxies of diverse luminosities, shapes and distances. At the limit of what can be seen with photographic surveys, one finds as many as 18,000 faint galaxies per square degree. 1 With modern charge-coupled devices one can look farther out. During the past decade, ultradeep CCD imaging 2 over the wavelength range 0.3-1 micron has revealed a surprisingly dense and ubiquitous population of faint blue galaxies. Exposing CCD detectors sensitive to as little as one photon per pixel per minute at the 4-meter Interamerican Observatory telescope on Cerro Tololo in the Chilean Andes, we have found more than 300,000 of these faint blue background galaxies per square degree of sky. 3 This is the backdrop we exploit to study foreground concentrations of dark matter. Of course this previously unknown background of faint blue galaxies also deserves study for its own sake.

Figure 3 is a CCD color image of a randomly chosen 4.5 x 5.7 arcminutes of sky. These faint blue galaxies are, in general, too dim for spectroscopic measurement of their redshifts at existing telescopes. But several indirect indicators put their redshifts z between 0.5 and 3. We are probably seeing these galaxies early in their lives, when star formation was rampant. The resulting abundance of hot massive stars would shine brightly in the ultraviolet. Severely redshifted by a journey of billions of years, the ultraviolet from these young stars would now look like the blue spectra we see. The unusual blue color and low surface brightness of these background galaxies is particularly convenient for the use we make of them: They are easy to distinguish from the red galaxies of the foreground cluster whose dark matter we seek to map. We are seeing the latter at much more advanced ages, when most of their stars are old and red. Their star-forming gas has long since been swept away.

Figure 3

Figure 3. Ubiquitous background of distant faint blue galaxies shows up on this deep CCD survey image of a random 4.5 x 5.7-arcminute field of sky. Three different filters were used to produce the color image. The digital output reveals more than 300 000 such faint background galaxies per square degree of sky. Most of them have redshifts between 0.5 and 3.

How do we know that most of these faint galaxies have redshifts of less than 3? Ultraviolet light traversing hydrogen gas exhibits a sharp spectral cutoff at 912 Å, the hydrogen ionization threshold. At wavelengths below this "Lyman break," stellar photons are heavily absorbed by the interstellar gas of their galaxy of origin. With a terrestrial telescope one can't see the Lyman break for most galaxies, because ultraviolet wavelengths shorter than about 3200 Å don't get through our own atmosphere. But if the z of a galaxy in question is high enough, the Lyman break can be redshifted all the way up to accessible wavelengths. The fact that our observations of the faint blue galaxies do not show any clear sign of a short-wavelength cutoff 4 tells us that few if any of these background galaxies exceed a redshift of 3.

We can't, in general, measure the redshifts of individual faint blue galaxies. But we do have rough statistical limits. The faintest galaxies for which spectroscopic surveys are available show a redshift peak at about z = 0.4. But the bulk of the faint blue population is over 16 times fainter than these faintest spectroscopically surveyed galaxies. The strongly lensing foreground clusters have redshifts up to about 0.4. So we can conclude that most of the background galaxies lie beyond z = 1. Some are certainly as far away as z = 2. That is, their light began its journey to us when the universe was only a third of its present size.

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