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

Free access article

Issue		A&A Volume 479, Number 1, February III 2008


Page(s)		67 - 73
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20078555

A&A 479, 67-73 (2008)
DOI: 10.1051/0004-6361:20078555

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

N. P. H. Nesvadba^{1, 2}, M. D. Lehnert¹, R. I. Davies³, A. Verma⁴, and F. Eisenhauer³

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 $z\sim 3$ Lyman-break galaxy (LBG), Q0347-383 C5, obtained with SINFONI on the VLT. This galaxy, among the $\sim$ 10% brightest LBGs, is only the second $z\sim 3$ 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 $\sim$ 5 kpc with a velocity offset of $\sim$ 33 km s^-1. Velocity dispersions suggest that each blob has a mass of $\sim$ 10¹⁰ $M_{\odot}$ . Unlike Pettini et al. (2001), our spectra are deep enough to detect H $\beta$ , and we derive star-formation rates of $\sim$ 20-40 $M_{\odot}$ yr^-1, and use the H $\beta$ /[OIII] ratio to crudely estimate an oxygen abundance $12+\rm [O/H]=7.9{-}8.6$ , 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. 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.

Key words: cosmology: observations -- galaxies: evolution -- galaxies: kinematics and dynamics -- infrared: galaxies

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