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

TL;DR

This paper investigates how nebular Lyman-Continuum emission influences the measurement of ionising photon escape fractions from galaxies, revealing its significant role at intermediate opacities and proposing a new method to estimate escape fractions unaffected by nebular emission.

Contribution

It quantifies nebular LyC emission's impact on escape fraction measurements and introduces a flux ratio-based formula to estimate escape fractions independent of nebular effects.

Findings

Nebular LyC emission significantly affects escape fraction estimates at intermediate opacities.

A flux ratio method (f_lambda700/f_lambda1100) can reliably estimate escape fractions unaffected by nebular emission.

Nebular LyC contributes notably to ionising photon budgets, impacting cosmic reionisation models.

Abstract

Several Lyman Continuum (LyC) emitters have been detected so far, but their observed ionising spectra sometimes differ from attenuated stellar spectra predicted by stellar population synthesis modelling. This discrepancy may be due to a significant contribution of LyC nebular emission. We aim to quantify the importance this emission in LyC leakers: its contribution to the ionising photons budget, and to measurements of LyC escape. To estimate the nebular contribution to the LyC spectra of galaxies, we run photoionisation models with Cloudy for a range of BPASS templates, varying the column density of the surrounding gas, from density-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=16) to ionisation-bounded (log(NH$_{\rm{stop}}$/cm$^{-2}$)=19) regimes. In the limits of very optically thin (f$_{\rm{esc}}$ = 1), or thick configurations (f$_{\rm{esc}}$ = 0), there is no nebular contribution to the emergent LyC spectra. This contribution matters only at intermediate LyC opacities ($0 <$ f$_{\rm{esc}}$ $< 1$), where it alters the shape of the LyC 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 ionising photons escaping galaxies is inversely proportional to the stellar escape fractions, but since typical simulated escape fractions are low, LyC photons escape is important. Nebular LyC is a non-negligible additional source of ionising photons from galaxies, which contribution has been overlooked so far in observations and in cosmic reionisation simulations.