The MUSE Hubble Ultra Deep Field Survey

Abstract

We present rest-frame Ly α equivalent widths ( E W 0) of 417 Ly α 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 E W 0 values taking into account extended Ly α emission and UV continuum slopes ( β). Our LAEs reach unprecedented depths, both in Ly α luminosities and UV absolute magnitudes, from log ( L Ly α /erg s -1) ~ 41.0 to 43.0 and from M UV ~ −16 to −21 (0.01−1.0 L * z =3). The E W 0 values span the range of ~ 5 to 240 Å or larger, and their distribution can be well fitted by an exponential law N = N 0 exp(− E W 0 / w 0). Owing to the high dynamic range in M UV, we find that the scale factor, w 0, depends on M UV in the sense that including fainter M UV objects increases w 0, i.e., the Ando effect. The results indicate that selection functions affect the E W 0 scale factor. Taking these effects into account, we find that our w 0 values are consistent with those in the literature within 1 σ uncertainties at 2.9 z < 6.6 at a given threshold of M UV and L Ly α . Interestingly, we find 12 objects with E W 0 200 Å above 1 σ uncertainties. Two of these 12 LAEs show signatures of merger or AGN activity: the weak Civ λ 1549 emission line. For the remaining 10 very large E W 0 LAEs, we find that the E W 0 values can be reproduced by young stellar ages (< 100 Myr) and low metallicities (≲ 0.02 Z ⊙). Otherwise, at least part of the Ly α emission in these LAEs needs to arise from anisotropic radiative transfer effects, fluorescence by hidden AGN or quasi-stellar object activity, or gravitational cooling.