Upper limits on the escape fraction of ionizing radiation from galaxies at 2 ≲ z < 6

Abstract

ABSTRACT In this work, we investigate upper limits on the global escape fraction of ionizing photons ($f_{\rm esc/global}^{\rm abs}$) from a sample of galaxies probed for Lyman-continuum (LyC) emission characterized as non-LyC and LyC leakers. We present a sample of nine clean non-contaminated (by low-redshift interlopers, CCD problems, and internal reflections of the instrument) galaxies that do not show significant (>3σ) LyC flux in the range 880 < λrest < 910 Å. The nine-galaxy stacked spectrum reveals no significant LyC flux with an upper limit of $f_{\rm esc}^{\rm abs} \le 0.06$. In the next step of our analysis, we join all estimates of $f_{\rm esc}^{\rm abs}$ upper limits derived from different samples of 2 ≲ z < 6 galaxies from the literature reported in last ∼20 yr and include the sample presented in this work. We find the $f_{\rm esc}^{\rm abs}$ upper limit ≤0.084 for the galaxies recognized as non-LyC leakers. After including all known detections from literature $f_{\rm esc/global}^{\rm abs}$ upper limit ≤0.088 for all galaxies examined for LyC flux. Furthermore, $f_{\rm esc}^{\rm abs}$ upper limits for different groups of galaxies indicate that the strongest LyC emitters could be galaxies classified as Lyman α emitters. We also discuss the possible existence of a correlation among the observed flux density ratio $(F_{\nu }^{\mathrm{ LyC}}/F_{\nu }^{\mathrm{ UV}})_{\rm obs}$ and Lyman α equivalent width EW(Lyα), where we confirm the existence of moderately significant correlation among galaxies classified as non-LyC leakers.