Integral-field spectroscopy of a Lyman-break galaxy at z = 3.2: evidence for merging

¹ Observatoire de Paris, CNRS, Universite Denis Diderot, 5 Place Jules Janssen, 92190 Meudon, France e-mail: nicole.nesvadba@obspm.fr
² Marie-Curie Fellow, France
³ Max-Planck-Institut für Extraterrestrische Physik, Garching bei München, Germany
⁴ University of Oxford, Subdepartment of Astrophysics, Denys Wilkinson Building, Keble Road, Oxford, UK

Received: 27 August 2007
Accepted: 4 November 2007

Abstract

We present spatially-resolved, rest-frame optical spectroscopy of a Lyman-break galaxy (LBG), Q0347-383 C5, obtained with SINFONI on the VLT. This galaxy, among the ∼10% brightest LBGs, is only the second LBG observed with an integral-field spectrograph. It was first described by Pettini et al. (2001, ApJ, 554, 981), 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 ∼33 km s^-1. Velocity dispersions suggest that each blob has a mass of ∼10¹⁰ . Unlike Pettini et al. (2001), our spectra are deep enough to detect Hβ, and we derive star-formation rates of ∼  yr^-1, and use the Hβ/[OIII] ratio to crudely estimate an oxygen abundance , 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. 2006, ApJ, 650, 661), 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.