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

Abstract

Measuring the escape fraction, fesc, 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 escPDF, 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 escPDF = 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 escT = 1.40+0.80-0.42 and f esc 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 Ly C candidates selected based on photometric redshifts at z > 3.4, but find that all seven have overestimated photometric redshifts by Δz ∼0.2 making them unsuitable for Ly C measurements. This results likely due to a bias induced by our selection criteria.