The MUSE Hubble Ultra Deep Field Survey

X. Lyα equivalent widths at 2.9 < z < 6.6

¹ Univ. Lyon, Univ. Lyon1, ENS de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, UMR 5574, 69230 Saint-Genis-Laval, France
² Department of Astronomy, Graduate School of Science, The University of Tokyo, 113-0033 Tokyo, Japan
³ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
⁴ College of General Education, Osaka Sangyo University, 3-1-1 Nakagaito, Daito, 574-8530 Osaka, Japan
e-mail: thashimoto@est.osaka-sandai.ac.jp
⁵ Observatoire de Genève, Université de Genève, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
⁶ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, rua das Estrelas, 4150-762 Porto, Portugal
⁸ Institute for Astronomy, Department of Physics, ETH Zürich, Wolfgang, Pauli, Strasse 27, 8093 Zürich, Switzerland
⁹ Department of Physics, University of Malta, Msida MSD 2080, Malta
¹⁰ Institute for Space Sciences & Astronomy, University of Malta, Msida MSD 2080, Malta
¹¹ Department of Astronomy, Stockholm University, AlbaNova University Centre, 106 91 Stockholm, Sweden
¹² Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

Received: 17 July 2017
Accepted: 21 October 2017

Abstract

We present rest-frame Lyα equivalent widths (EW₀) of 417 Lyα emitters (LAEs) detected with Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) at 2.9 <z< 6.6 in the Hubble Ultra Deep Field. Based on the deep MUSE spectroscopy and ancillary Hubble Space Telescope (HST) photometry data, we carefully measured EW₀ values taking into account extended Lyα emission and UV continuum slopes (β). Our LAEs reach unprecedented depths, both in Lyα luminosities and UV absolute magnitudes, from log (L_Lyα/erg s^-1) ~ 41.0 to 43.0 and from M_UV ~ −16 to −21 (0.01−1.0 L^*_z=3). The EW₀ values span the range of ~ 5 to 240 Å or larger, and their distribution can be well fitted by an exponential law N = N₀ exp(−EW₀/w₀). Owing to the high dynamic range in M_UV, we find that the scale factor, w₀, depends on M_UV in the sense that including fainter M_UV objects increases w₀, i.e., the Ando effect. The results indicate that selection functions affect the EW₀ scale factor. Taking these effects into account, we find that our w₀ values are consistent with those in the literature within 1σ uncertainties at 2.9 < z < 6.6 at a given threshold of M_UV and L_Lyα. Interestingly, we find 12 objects with EW₀> 200 Å above 1σ uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak Civλ1549 emission line. For the remaining 10 very large EW₀ LAEs, we find that the EW₀ values can be reproduced by young stellar ages (< 100 Myr) and low metallicities (≲ 0.02 Z_⊙). Otherwise, at least part of the Lyα emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.