Medium-resolution spectroscopy of FORJ0332−3557: probing the interstellar medium and stellar populations of a lensed Lyman-break galaxy at z= 3.77

We recently reported the discovery of FORJ0332−3557, a lensed Lyman-break galaxy at z= 3.77 in a remarkable example of strong galaxy–galaxy gravitational lensing. We present here a medium-resolution rest-frame ultraviolet (UV) spectrum of the source, which appears to be similar to the well-known Lyman-break galaxy MS1512−cB58 at z= 2.73. The spectral energy distribution is consistent with a stellar population of less than 30 Ma, with an extinction of Av= 0.5 mag and an extinction-corrected star formation rate SFRUV of 200–300 h−170 M⊙ a−1. The Lyα line exhibits a damped profile in absorption produced by a column density of about NH i= (2.5 ± 1.0) × 1021cm−2, superimposed on an emission line shifted both spatially (0.5 arcsec with respect to the UV continuum source) and in velocity space (+830 km s−1 with respect to the low-ionization absorption lines from its interstellar medium), a clear signature of outflows with an expansion velocity of about 270 km s−1. A strong emission line from He iiλ164.04 nm indicates the presence of Wolf–Rayet stars and reinforces the interpretation of a very young starburst. The metallic lines indicate subsolar abundances of elements Si, Al and C in the ionized gas phase.