A low Lyman Continuum escape fraction of < 10 per cent for extreme [O III] emitters in an overdensity at z ~ 3.5

Abstract

Recent work has suggested that extreme [OIII] emitting star-forming galaxies are important to reionization. Relatedly, [O III]/[O II] has been put forward as an indirect estimator of the Lyman Continuum (LyC) escape fraction (fesc) at z ≳ 4.5 when the opaque IGM renders LyC photons unobservable. Using deep archival U-band (VLT/VIMOS) imaging of a recently confirmed overdensity at z~3.5, we calculate tight constraints on fesc for a sample (N=73) dominated by extreme [OIII] emitters.We find no LyC signal (f escrel < 6.3-0.7+0.7 per cent at 1s) in a deep U-band stack of our sample (31.98 mag at 1σ). This constraint is in agreement with recent studies of star-forming galaxies spanning z ~ 1-4 that have found very low average fesc. Despite the galaxies in our study having an estimated average rest-frame EW([O III]λ5007) ~ 400 Å and [O III]/[O II] ~ 4 from composite SED fitting, we find no LyC detection, which brings into question the potential of [OIII]/[O II] as an effective probe of the LyC - a majority of LyC emitters have [OIII]/[OII] > 3, but we establish here that [O III]/[OII] > 3 does not guarantee significant LyC leakage for a population. Since even extreme star-forming galaxies are unable to produce the fesc ~ 10-15 per cent required bymost theoretical calculations for star-forming galaxies to drive reionization, there must either be a rapid evolution of fesc between z~ 3.5 and the epoch of reionization, or hitherto observationally unstudied sources [e.g. ultrafaint lowmass galaxies with log (M/M⊙) ~ 7-8.5] must make an outsized contribution to reionization.