ON THE CONTRIBUTION OF DWARF GALAXIES TO REIONIZATION OF THE UNIVERSE

Abstract

We present estimates of the ultraviolet (UV) and Lyman continuum flux density contributed by galaxies of luminosities from MUV ≈ −25 to MUV = −4 at redshifts 5 ≤ Z ≤ 10 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 and present accurate fitting functions describing them. Although the slope of the LF becomes gradually shallower with decreasing luminosity due to feedback-driven outflows, the UV LF predicted by the model remains quite steep at the luminosities MUV ≲ −14. After reionization, the UV LF flattens at MUV ≳−12 due to UV heating of intergalactic gas. However, before reionization, the slope of the LF remains steep and approximately constant from MUV ≈ −14 to MUV = −4. 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. Before reionization, even ultra-faint galaxies of MUV > −10 contribute ≈ 10 − 25% of ionizing photons. If the escape fraction increases strongly for fainter galaxies, the contribution of MUV > −14 galaxies before reionization increases to ≈ 60 − 75%. Our results 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 before reionization alleviating requirements on the escape fraction of Lyman continuum photons.