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

Abstract

ABSTRACT 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 $4.8\lt z \lt 7$ 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 ($2\times 10^{6}\le {M}_*/{\rm M}_\odot \le 5\times 10^{9}$) and ultraviolet (UV) magnitudes ($-22.68\le M_{\rm UV}\le =-15.95$). We find a median UV slope $\beta _{1500}\,$ of $-2. 56\pm 0.23$ and the production efficiency of ionizing photons $\log$$\xi _{\rm {ion}}\,$$=25.39\pm 0.6$ for the full sample ($4.8\lt z \lt 7$) with a median stellar mass of $6.3\pm 0.5\times 10^{7} \, {\rm M}_\odot$. We find both $\beta _{1500}\,$ and $\xi _{\rm {ion}}\,$ 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 $\xi _{\rm {ion}}\,$ and a large scatter in H$\alpha$ equivalent widths (EWs). Finally, we investigate the trends of H$\alpha$ and [O iii] + H$\beta$ EWs with UV magnitude and find $M_{\rm UV}$ is correlated between H$\alpha$ but not with [O iii] + H$\beta$ EWs indicating low metallicities and recent burst in the UV faint galaxies.