A cautionary tale of Ly C escape fraction estimates from high-redshift galaxies

Measuring the escape fraction, f(esc), of ionizing, Lyman continuum (Ly C) 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 Ly C escape fraction, f(esc)(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(esc)(PDF) = 0.51(-0.34)(+0.33) and compare this to the values computed assuming averaged IGM transmission with and without consideration of detection bias along average sightlines yielding f(esc)(< T >) = 1.40(-0.42)(+0.80) and f(esc)(bia)s = 0.82(-0.16)(+0.33) . Our results highlight the limitations of methods assuming average, smooth transmission functions. We also present MOSFIRE data for a sample of seven Ly C candidates selected based on photometric redshifts at z > 3.4, but find that all seven have overestimated photometric redshifts by Delta z similar to 0.2 making them unsuitable for Ly C measurements. This results likely due to a bias induced by our selection criteria.