The impact of nebular Lyman-Continuum on ionizing photons budget and escape fractions from galaxies

Several Lyman Continuum (Ly C) emitters have been detected so far, but their observed ionizing spectra sometimes differ from attenuated stellar spectra predicted by stellar population synthesis modelling. This discrepancy may be due to a significant contribution of Ly C nebular emission. We aim to quantify the importance this emission in Ly C leakers: its contribution to the ionizing photons budget, and to measurements of Ly C escape. To estimate the nebular contribution to the Ly C spectra of galaxies, we run photoionization models with CLOUDY for a range of Binary Population and Spectral Synthesis templates, varying the column density of the surrounding gas, from density-bounded (N-H I = 10(16) cm(-2)) to ionization-bounded (N-H I = 10(19) cm(-2)) regimes. In the limits of very optically thin (f(esc) = 1), or thick configurations (f(esc) = 0), there is no nebular contribution to the emergent Ly C spectra. This contribution matters only at intermediate Ly C opacities (0 < f(esc) < 1), where it alters the shape of the Ly C spectrum chromatically, so that escape fractions estimates are highly sensitive to the wavelength range over which they are calculated. We propose a formula to estimate integrated escape fractions using F-lambda 700/F-lambda 1100 flux ratios, since this wavelength range is not affected by nebular emission. Regarding simulations, the boost of hydrogen ionizing photons escaping galaxies is inversely proportional to the stellar escape fractions, but since typical simulated escape fractions are low, Ly C photons escape is important. Nebular Ly C is a non-negligible additional source of ionizing photons from galaxies, whose contribution has been overlooked so far in observations and in cosmic reionization simulations.