2018ApJ...868..115Y

Ultra-faint galaxies are hosted by small dark matter halos with shallow gravitational potential wells, hence their star formation activity is more sensitive to feedback effects. The shape of the faint end of the high-z galaxy luminosity function (LF) contains important information on star formation and its interaction with the reionization process during the Epoch of Reionization. High-z galaxies with M_UV≳-17 have only recently become accessible thanks to the Frontier Fields (FFs) survey combining deep HST imaging and the gravitational lensing effect. In this paper we investigate the faint end of the LF at redshift >5 using the data of FFs clusters Abell 2744 (A2744), MACSJ0416.1-2403 (M0416), MACSJ0717.5+3745 (M0717), and MACSJ1149.5+2223 (M1149). We analyze both an empirical and a physically motivated LF model to obtain constraints on a possible turnover of LF at faint magnitudes. In the empirical model the LF drops fast when the absolute UV magnitude M_UV is much larger than a turnover absolute UV magnitude M_UV^T. We obtain M_UV^T≳-14.6 (15.2) at the 1 (2)σ confidence level (C.L.) for z ∼ 6. In the physically motivated analytical model, star formation in halos with circular velocity below v_c^* is fully quenched if these halos are located in ionized regions. Using updated lensing models and new additional FFs data, we re-analyze previous constraints on v_c^* and f_esc presented by Castellano et al. using a smaller data set. We obtain new constraints on v_c^* ≲59 km s^–1 and f_esc <= 56% (both at 2σ C.L.) and conclude that there is no turnover detected so far from the analyzed FFs data. Forthcoming JWST observations will be key to tightening these constraints further.