The MUSE Hubble Ultra Deep Field Survey-X. Lyα equivalent widths at 2.9< z< 6.6

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 (LLyα/erg s−1 ) ∼ 41.0 to 43.0 and from MUV ∼ −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 MUV, we find that the scale factor, w0, depends on MUV in the sense that including fainter MUV 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 MUV and LLyα. 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 Civ λ1549 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