The MUSE Hubble Ultra Deep Field Survey

XIV. Evolution of the Lyα emitter fraction from z = 3 to z = 6

¹ Observatoire de Genève, Université de Genève, 51 chemin de Pégase, 1290 Versoix, Switzerland
e-mail: haruka.kusakabe@unige.ch
² Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, UMR5574, 69230 Saint-Genis-Laval, France
³ Tomonaga Center for the History of the Universe (TCHoU), Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
⁴ Hiroshima Astrophysical Science Center, Hiroshima University 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁵ Gemini Observatory/NSF’s OIR Lab, Casilla 603, La Serena, Chile
⁶ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁷ ESO Vitacura, Alonso de Córdova 3107,Vitacura, Casilla 19001, Santiago de Chile, Chile
⁸ Leiden Observatory, Leiden University, PO Box 9513, 2300, RA Leiden, The Netherlands
⁹ Department of Physics, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹⁰ Leibniz-Institut für Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
¹¹ Aix Marseille Université, CNRS, CNES, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France
¹² Department of Astronomy, University of Michigan, 1085 South University Ave, Ann Arbor, MI 48109, USA

Received: 17 December 2019
Accepted: 26 March 2020

Abstract

Context. The Lyα emitter (LAE) fraction, X_LAE, is a potentially powerful probe of the evolution of the intergalactic neutral hydrogen gas fraction. However, uncertainties in the measurement of X_LAE are still under debate.

Aims. Thanks to deep data obtained with the integral field spectrograph Multi Unit Spectroscopic Explorer (MUSE), we can measure the evolution of the LAE fraction homogeneously over a wide redshift range of z ≈ 3–6 for UV-faint galaxies (down to UV magnitudes of M₁₅₀₀ ≈ −17.75). This is a significantly fainter range than in former studies (M₁₅₀₀ ≤ −18.75) and it allows us to probe the bulk of the population of high-redshift star-forming galaxies.

Methods. We constructed a UV-complete photometric-redshift sample following UV luminosity functions and measured the Lyα emission with MUSE using the latest (second) data release from the MUSE Hubble Ultra Deep Field Survey.

Results. We derived the redshift evolution of X_LAE for M₁₅₀₀ ∈ [ − 21.75; −17.75] for the first time with a equivalent width range EW(Lyα) ≥ 65 Å and found low values of X_LAE ≲ 30% at z ≲ 6. The best-fit linear relation is X_LAE = 0.07^+0.06_−0.03z − 0.22^+0.12_−0.24. For M₁₅₀₀ ∈ [ − 20.25; −18.75] and EW(Lyα) ≥ 25 Å, our X_LAE values are consistent with those in the literature within 1σ 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 X_LAE on M₁₅₀₀ for EW(Lyα) ≥ 50 Å at z ≈ 3–4, in contrast with previous work. The differences in X_LAE mainly arise from selection biases for Lyman Break Galaxies (LBGs) in the literature: UV-faint LBGs are more easily selected if they have strong Lyα emission, hence X_LAE is biased towards higher values when those samples are used.

Conclusions. Our results suggest either a lower increase of X_LAE towards z ≈ 6 than previously suggested, or even a turnover of X_LAE 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 LAE fractions much better than models where SF is a smooth function of time.