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Abstract
We present estimates of the ultraviolet (UV) and hydrogen ionizing photon density contributed by galaxies of luminosities from 𝑀uv ≈ −25 to 𝑀uv = −5 at redshifts 5 ≤ 𝑧 ≤ 16 using a galaxy formation model that reproduces properties of local dwarf galaxies down to the luminosities of the ultra-faint satellites. We characterize the UV luminosity function (LF) of galaxies and their abundance as a function of the ionizing photon emission rate predicted by our model. Although the slope of the LF becomes gradually shallower with decreasing luminosity due to feedback-driven outflows, the UV LF predicted by the model is quite steep at the luminosities 𝑀uv ≲ −14, and remains approximately constant from 𝑀uv ≈ −14 to 𝑀uv = −5 before reionization. After reionization, the UV LF flattens at 𝑀uv ≳ −12 due to UV heating of intergalactic gas. We show that for a constant ionizing photon escape fraction the contribution of faint galaxies with 𝑀uv > −14 to the UV flux and ionizing photon budget is ≈ 40 − 60% at 𝑧 > 7 and decreases to ≈ 20% at 𝑧 = 6. The contribution of dwarf galaxies is larger if their ionizing photon escape fractions are larger than those of brighter galaxies. Our results thus imply that dwarf galaxies fainter than 𝑀uv = −14, beyond the James Webb Space Telescope limit, contribute significantly to the UV flux density and ionizing photon budget. We also explore different models of the ionizing photon escape fractions and assess the effects of dwarf galaxies on global reionization history. We show that a constant a model with the constant 10% escape fraction is consistent with existing observational constraints on the ionized fraction at different redshifts and the integral optical depth to Thompson scattering if the contribution of dwarf galaxies is taken into account. At the same time, the predicted reionization history depends sensitively on the assumptions about escape fraction dependence on galaxy properties.