Modeling Lyman Continuum Emission from Young Galaxies

Based on cosmological simulations, we model Lyman continuum emission from asample of 11 high-redshift star forming galaxies spanning a mass range of afactor 20. Each of the 11 galaxies has been simulated both with a Salpeter anda 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=3.6to 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-galaxyvariation of apparent LyC emission at a fixed redshift is caused inapproximately equal parts by the intrinsic variations in the LyC emission andby orientation effects. The combined scatter of an order of magnitude canexplain the variance in the far-UV spectra of high-redshift galaxies detectedby Shapley et al. (2006). Our results imply that the cosmic galactic ionizingUV luminosity would be monotonically decreasing from z=3.6 to 2.4, curiouslyanti-correlated with the star formation rate in the smaller galaxies, which onaverage rises during this redshift interval.Comment: 8 pages, 12 figures, ApJ, in pres