Lyman-α Emitters and Lyman-break Galaxies at Z = 3− 6 in Cosmological Sph Simulations

We study the properties of Lyman-α emitters (LAEs) and Lyman-break galaxies (LBGs) at z = 3−6 using cosmological SPH simulations. We investigate two simple scenarios for explaining the observed Lyα and rest-frame UV luminosity functions (LFs) of LAEs: (i) the “escape fraction” scenario, in which the effective escape fraction (including the IGM attenuation) of Lyα photons is fLyα ≈ 0.1 (0.15) at z = 3 (6), and (ii) the “duty cycle” scenario, in which the fraction of LAEs that are turned on at z = 3 (6) is Cduty ≈ 0.07 (0.2) after correcting for the IGM attenuation. Our comparisons with a number of different observations suggest that the duty cycle scenario is preferred over the escape fraction scenario. We find that the mean values of stellar mass, metallicity and black hole mass hosted by LAEs are all smaller in the duty cycle scenario than in the escape fraction scenario. In our simulations, the galaxy stellar mass function evolves rapidly, as expected in hierarchical structure formation. However, its evolution is largely compensated by a beginning decline in the specific star formation rate, resulting in little evolution of the rest-frame UV LF from z = 6 to 3. The rest-frame UV LF of both LAEs and LBGs at z = 3 & 6 can be described well by the duty cycle scenario provided the extinction is moderate, E(B−V ) ≈ 0.15, for both populations, although our simulation might be overpredicting the number of bright LBGs at z = 6. We show that the correlation function of LAEs in the duty cycle scenario has a smaller correlation length than in the escape fraction scenario. The bias of LAEs relative to the dark matter distribution is higher at z = 6 than at z = 3 in both scenarios. The Lyα LFs at z = 6 in a field-of-view of 0.2 deg show a significantly larger scatter owing to cosmic variance relative to that in a 1 deg field, and the scatter seen in the current observational estimates of the Lyα LF can be accounted for by cosmic variance. Subject headings: cosmology: theory — stars: formation — galaxies: high-redshift — galaxies: evolution – galaxies: formation – methods: numerical