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 30Ma, with an extinction of A_v = 0.5 mag and an extinction-corrected star formation rate SFR_UV of 200-300h^-1₇₀M_solara^-1. The Lyα line exhibits a damped profile in absorption produced by a column density of about N_HI = (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 (+830kms^-1 with respect to the low-ionization absorption lines from its interstellar medium), a clear signature of outflows with an expansion velocity of about 270kms^-1. A strong emission line from HeII λ164.04 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.

Based on observations made at the ESO VLT under programmes 74.A-0536 and 78.A-0240.

E-mail: remi.cabanac@ast.obs-mip.fr (RAC); david.valls-gabaud@obspm.fr (DV-G); clidman@eso.org (CL)