The absence of any continuous Lyα opacity from the intergalactic medium at z = 3-4.5 can be understood if a significant population of star-forming galaxies is present at these redshifts. We show that such galaxies can be present within the galaxy populations at 22 < B < 27. We analyze theoretically the expected ionization produced during galaxy formation in terms of highly model-independent metal production arguments and conclude that (if such galaxies are dust-free and transparent to ionizing photons) then at least 4(H_0_/50 km s^-1^ Mpc^-1^)^3^% of galaxy formation would have to have occurred in this redshift range. Finally we note that some part of the "blue" or "flat-spectrum" galaxy population that appears in galaxy counts at B > 23 could be a prime candidate for the producer of the ionizing flux at z = 3. We speculate that these objects may also be the source of the diffuse X-ray background and also suggest that the ionized surfaces of the damped Lyα clouds may be close to detectability.
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Songaila et al. (1990) studied this question.