The expected abundance of Lyman-alpha emitting primeval galaxies. I. General model predictions

We present model calculations for the expected surface density of Ly-α emitting primeval galaxies (PGs) at high redshifts. We assume that elliptical galaxies and bulges of spiral galaxies (=spheroids) formed early in the universe and that the Ly-α emitting PGs are these spheroids during their first burst of star formation at high redshift. One of the main assumptions of the models is that the Ly-α bright phase of this first starburst in the spheroids is confined to a short period after its onset due to rapid formation of dust. The models do not only explain the failure of early surveys for Ly-α emitting PGs but are also consistent with the limits of new surveys (e.g. the Calar Alto Deep Imaging Survey - CADIS). At faint detection limits S lim ≤ 10 −20 Wm −2 the surface density of Ly-α emitters is expected to vary only weakly in the redshift range between z = 3a nd z = 6 with values >10 3 /deg 2 /∆z = 0.1 reaching its maximum at z0 ≈ 4. At shallower detection limits, S lim ≥ 3 × 10 −20 Wm −2 the surface density of high-z Ly-α emitters is expected to be a steep function of redshift and detection limit. This explains the low success in finding bright Ly-α galaxies at z > 5. We demonstrate how the observed surface densities of Ly-α emitting PGs derived from recent surveys constrain the parameters of our models. Finally, we discuss the possibility that two Ly-α bright phases occur in the formation process of galaxies: An initial - primeval - phase in which dust is virtually non-existant, and a later secondary phase in which strong galactic winds as observed in some Lyman break galaxies facilitate the escape of Ly-α photons after dust has already been formed.