CANUCS: UV and ionizing properties of dwarf star-forming galaxies at z ~ 5–7

The epoch of reionization (EoR) progressed through the emission of ionizing photons from galaxies to their local intergalactic medium. In this work, we characterize the dwarf star-forming galaxies as candidates for the source of ionizing photons that drove EoR. We investigate the ionizing properties and star formation histories of star-forming dwarf galaxies at the last stages of EoR at 4.8< z < 7 using observations from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The magnification due to gravitational lensing allows us to probe large dynamic ranges in stellar mass (2× 10⁶≤ M_*/ M_ ≤ 5× 10⁹) and ultraviolet (UV) magnitudes (-22.68≤ MUV≤ =-15.95). We find a median UV slope β ₁₅₀₀\, of -2. 56± 0.23 and the production efficiency of ionizing photons logξ ᵢₒₙ\,=25.39± 0.6 for the full sample (4.8< z < 7) with a median stellar mass of 6.3± 0.5× 10⁷ \, M_. We find both β ₁₅₀₀\, and ξ ᵢₒₙ\, are marginally correlated with the stellar mass of the galaxy, indicating a possible greater contribution of dwarf galaxies to the reionization of the Universe. We find that on average, galaxies in our sample are experiencing a recent rise/burst of star formation which translates to a higher scatter in ξ ᵢₒₙ\, and a large scatter in Hα equivalent widths (EWs). Finally, we investigate the trends of Hα and [O iii] + Hβ EWs with UV magnitude and find MUV is correlated between Hα but not with [O iii] + Hβ EWs indicating low metallicities and recent burst in the UV faint galaxies.