Figure 2.

An illustration of the so called “Lyman-break” or “UV-dropout” method for isolating galaxies at very high redshifts. This technique takes advantage of the sharp break expected at wavelengths <912 Å in the spectrum of a galaxy dominated (in the UV spectrum) by massive, young stars (A Top). The break is accentuated by the photoelectric absorption both in the galaxy hosting the stars (A Middle) and in the intervening intergalactic medium (A Bottom). The observed wavelength of the Lyman-break feature is found at 912(1 + z) Å, where z is the redshift. In the case illustrated here, the galaxy redshift is 3.15, bringing the feature into the optical window observable from the ground. Shown in A Bottom are broad-filter passbands that can be used effectively to isolate Lyman-break galaxies in the vicinity of redshift z = 3. (B) An example of small regions of charge-coupled device images taken through these filters. Note that the circled galaxy is seen clearly through the red and green filters, but it disappears completely through the UV filter. Only a few percent of all comparably faint galaxies will behave in this way, and such abrupt changes in spectral energy are not mimicked by anything except very distant galaxies. This procedure allows the efficient identification of large numbers galaxies in a prescribed range of redshifts at early epochs. The two highest redshift points in Fig. 1 were measured with this technique (17).