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

Abstract

Measuring the escape fraction, 𝑓 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 PDF esc , 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 PDF esc = 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 𝑇 esc = 1.40 +0.80 −0.42 , and 𝑓 bias esc = 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 Δ𝑧 ∼ 0.2 making them unsuitable for LyC measurements. This results likely due to a bias induced by our selection criteria.