2008A&A...479...67N

We present spatially-resolved, rest-frame optical spectroscopy of a z∼3 Lyman-break galaxy (LBG), Q0347-383 C5, obtained with SINFONI on the VLT. This galaxy, among the ∼10% brightest LBGs, is only the second z∼3 LBG observed with an integral-field spectrograph. It was first described by Pettini et al. (2001ApJ...554..981P), who obtained WFPC2 F702W imaging and longslit spectroscopy in the K-band. We find that the emission line morphology is dominated by two unresolved blobs at a projected distance of ∼5 kpc with a velocity offset of ∼33km/s. Velocity dispersions suggest that each blob has a mass of ∼10¹⁰M_☉. Unlike Pettini et al. (2001ApJ...554..981P), our spectra are deep enough to detect Hβ, and we derive star-formation rates of ∼20-40M_☉/yr, and use the Hβ/[OIII] ratio to crudely estimate an oxygen abundance 12+[O/H]=7.9-8.6, which is in the range typically observed for LBGs. We compare the properties of Q0347-383 C5 with what is found for other LBGs, including the gravitationally lensed ``arc+core'' galaxy (Nesvadba et al., 2006ApJ...650..661N), and discuss possible scenarios for the nature of the source, namely disk rotation, a starburst-driven wind, disk fragmentation, and merging of two LBGs. We favor the merging interpretation for bright, extended LBGs like Q0347-383 C5, in broad agreement with predicted merger rates from hierarchical models.