The MUSE Hubble Ultra Deep Field Survey XIV. The evolution of the Lya emitter fraction from z=3 to z=6
Abstract
The Lya emitter (LAE) fraction, XLAE, is a potentially powerful probe of the evolution of the intergalactic neutral hydrogen gas fraction. However, uncertainties in the measurement of XLAE are still debated. Thanks to deep data obtained with MUSE, we can measure the evolution of XLAE homogeneously over a wide redshift range of z~3-6 for UV-faint galaxies (down to M1500~-17.75). This is significantly fainter than in former studies, and allows us to probe the bulk of the population of high-z star-forming galaxies. We construct a UV-complete photo-redshift sample following UV luminosity functions and measure the Lya emission with MUSE using the second data release from the MUSE HUDF Survey. We derive the redshift evolution of XLAE for M1500 in [-21.75;-17.75] for the first time with a equivalent width range EW(Lya)>=65 A and find low values of X LAE<~30% at z<~6. For M1500 in [-20.25;-18.75] and EW(Lya)<~25 A, our XLAE values are consistent with those in the literature within 1sigma at z<~5, but our median values are systematically lower than reported values over the whole redshift range. In addition, we do not find a significant dependence of XLAE on M1500 for EW(Lya)>~50 A at z~3-4, in contrast with previous work. The differences in XLAE mainly arise from selection biases for Lyman Break Galaxies (LBGs) in the literature: UV-faint LBGs are more easily selected if they have strong Lya emission, hence XLAE is biased towards higher values. Our results suggest either a lower increase of XLAE towards z~6 than previously suggested, or even a turnover of XLAE at z~5.5, which may be the signature of a late or patchy reionization process. We compared our results with predictions from a cosmological galaxy evolution model. We find that a model with a bursty star formation (SF) can reproduce our observed XLAE much better than models where SF is a smooth function of time.