Radiative transfer in a clumpy universe: The colors of high-redshift galaxies

We assess the effects of the stochastic attenuation produced by intervening QSO absorption systems on the broadband colors of galaxies at cosmological distances. We compute the H I opacity of a clumpy universe as a function of redshift, including scattering in resonant lines, such as Lyα, Lyβ, Lyγ, and higher order members, and Lyman- continuum absorption. Both the numerous, optically thin Lyman-α forest clouds and the rarer, optically thick Lyman limit systems are found to contribute to the obscuration of background sources. We study the mean properties of primeval galaxies at high redshift in four broad optical passbands, Uₙ_, B, G, and R. Even if young galaxies radiated a significant amount of ionizing photons, the attenuation due to the accuphotoelectric opacity along the path is so severe that sources beyond z~3 will drop out of the Uₙ_ image altogether. We also show that the observed B - R color of distant galaxies can be much redder than expected from a stellar population. At z~3. 5, the blanketing by discrete absorption lines in the Lyman series is so effective that background galaxies appear, on average, 1 mag fainter in B. By z~4, the observed B magnitude increment due to intergalactic absorption exceeds 2 mag. By modeling the intrinsic UV spectral energy distribution of star-forming galaxies with a stellar population synthesis code, we show that the (B - R) AB_ ~ 0 criterion for identifying "flat-spectrum, " metal- producing galaxies is biased against objects at z > 3. The continuum blanketing from the Lyman series produces a characteristic staircase profile in the transmitted power. We suggest that this cosmic Lyman decrement might be used as a tool to identify high-z galaxies.