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

Abstract

Measuring the escape fraction, $f_{\mathrm{esc}}$, 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, $f_{\mathrm{esc}}^{\mathrm{PDF}}$, 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 $f_{\mathrm{esc}}^{\mathrm{PDF}}$ = 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 $f_{\mathrm{esc}}^{⟨T⟩}$ = 1.40${}_{-0.42}^{+0.80}$, and $f_{\mathrm{esc}}^{\mathrm{bias}}$ = 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 LyC candidates selected based on photometric redshifts at $z>3.4$, but find that all seven have overestimated photometric redshifts by $\hat{I}”z\sim 0.2$ making them unsuitable for LyC measurements. This results likely due to a bias induced by our selection criteria.

12 pages, 9 figues, 2 tables, accepted for publication in MNRAS