A Cautionary Tale of LyC Escape Fraction Estimates from High Redshift Galaxies

Abstract

Measuring the escape fraction, $f_{\rm esc}$, of ionizing, Lyman Continuum (LyC) radiation is key to our understanding of the process of cosmic reionization. In this paper we provide a methodology for recovering the posterior probability distribution of the LyC escape fraction, $f_{\rm esc}^{\rm PDF}$, considering both the observational uncertainties and ensembles of simulated transmission functions through the intergalactic medium (IGM). We present an example of this method applied to a VUDS galaxy at $z=3.64$ and find $f_{\rm esc}^{\rm PDF}$ = 0.51$^{+0.33}_{-0.34}$ and compare this to the values computed assuming averaged IGM transmission with and without consideration of detection bias along average sightlines yielding $f_{\rm esc}^{\langle T \rangle}$ = 1.40$^{+0.80}_{-0.42}$, and $f_{\rm esc}^{\rm bias}$ = 0.82$^{+0.33}_{-0.16}$. Our results highlight the limitations of methods assuming average, smooth transmission functions. We also present MOSFIRE data for a sample of seven LyC candidates selected based on photometric redshifts at $z > 3.4$, but find that all seven have overestimated photometric redshifts by $\Delta z \sim 0.2$ making them unsuitable for LyC measurements. This results likely due to a bias induced by our selection criteria.