  Based on cosmological simulations, we model Lyman continuum emission from a
sample of 11 high-redshift star forming galaxies spanning a mass range of a
factor 20. Each of the 11 galaxies has been simulated both with a Salpeter and
a Kroupa initial mass function (IMF). We find that the Lyman continuum (LyC)
luminosity of an average star forming galaxy in our sample declines from z=3.6
to 2.4 due to the steady gas infall and higher gas clumping at lower redshifts,
increasingly hampering the escape of ionizing radiation. The galaxy-to-galaxy
variation of apparent LyC emission at a fixed redshift is caused in
approximately equal parts by the intrinsic variations in the LyC emission and
by orientation effects. The combined scatter of an order of magnitude can
explain the variance in the far-UV spectra of high-redshift galaxies detected
by Shapley et al. (2006). Our results imply that the cosmic galactic ionizing
UV luminosity would be monotonically decreasing from z=3.6 to 2.4, curiously
anti-correlated with the star formation rate in the smaller galaxies, which on
average rises during this redshift interval.
