Evidence for a low Lyman Continuum Escape fraction in three Massive, UV-bright galaxies at z > 7

Abstract

Although low-mass star-forming galaxies are the leading candidates of the reionisation process, we cannot conclusively rule out high-mass star-forming galaxies as candidates. While most simulations indicate the former is the best candidate some models suggest that at z > 6 massive, UV-bright galaxies - "oligarchs" - account for at least 80% of the ionising budget. To test this hypothesis we target massive (log10 (M*[Msol]) > 10), UV-bright (MUV ~ -22) Lya emitters at z > 7 in archival data, observed with similar resolution spectrographs (VLT/X-shooter and Keck/MOSFIRE). To increase the reliability of our conclusions we stack all spectra and obtain a deep-stacked spectrum of 24.75 hrs. The stacked Lya profile displays a clear asymmetric red peak and an absence of a blue peak. We additionally estimate the intrinsic stacked Lya profile of our targets by correcting for IGM transmission using a range of neutral hydrogen fractions, finding no significant change in the profile. We measure a velocity offset Vred > 300 km/s and an asymmetry in our red peak A ~3. Using various models and estimators such as the peak separation, the asymmetry of the red peak, the ratio between Lya and Hb and the beta slope, we conclude that the escape fraction in these three UV bright, massive (10^10 Msol), z > 7 galaxies is fesc(LyC) < 10%.