A&A 434, 819-825 (2005)
DOI: 10.1051/0004-6361:20042034

Molecular gas in a z triply-imaged, sub-mJy submillimetre galaxy typical of the cosmic far-infrared background

J.-P. Kneib^{1, 2, 3}, R. Neri⁴, I. Smail⁵, A. Blain², K. Sheth², P. van der Werf⁶ and K. K. Knudsen<sup>6

1</sup>  Observatoire Midi-Pyrénées, CNRS-UMR5572, 14 avenue E. Belin, 31400 Toulouse, France
    e-mail: jean-paul.kneib@oamp.fr
²  Caltech-Astronomy, MC105-24, Pasadena, CA 91125, USA
³  OAMP, Laboratoire d'Astrophysique de Marseille, traverse du Siphon, 13012 Marseille, France
⁴  IRAM, 300 rue de la Piscine, 38640 Saint Martin d'Hères, France
⁵  Institute for Computational Cosmology, University of Durham, South Road, Durham DH1 3LE, UK
⁶  Leiden Observatory, PO Box 9513, NL - 2300 RA Leiden, The Netherlands

(Received 21 September 2004 / Accepted 10 January 2005)

Abstract
We present the results of observations from the Plateau de Bure IRAM interferometric array of the submillimetre (submm) galaxy SMMJ16359+6612 lying at z=2.516 behind the core of the massive cluster A 2218. The foreground gravitational lens produces three images with a total magnification of 45 of this faint submm galaxy, which has an intrinsic submm flux of just  mJy - placing it below the confusion limit of blank-field surveys. The substantial magnification provides a rare opportunity to probe the nature of a distant sub-mJy submm-selected galaxy, part of the population which produces the bulk of the cosmic far-infrared background at submm wavelengths. Our observations detect the CO(3-2) line in all three images, as well as the CO(7-6) line and the dust continuum at 1.3 mm for the brightest image but only at a 3 level. The velocity profile of the CO(3-2) line displays a double-peak profile which is well fit by two Gaussians with FWHM of 220 km s^-1 and separated by 280 km s^-1. We estimate the dynamical mass of the system to be ~1.5  and an H₂ gas mass of 4.5  . We identify a spatial offset of ~1'' between the two CO(3-2) velocity components, again benefiting from the magnification due to the foreground lens, modeling of which indicates that the offset corresponds to just ~3 kpc in projection at z=2.516. The spatial and velocity properties of these two components are closely related to features detected in previously published H spectroscopy. We propose that this source is a compact merger of two typical Lyman-break galaxies with a maximal separation between the two nuclei of about 3 kpc, although a dusty disk explanation is not excluded. This system is much less luminous and massive than other high-redshift submillimetre galaxies studied to date, but it bears a close similarity to similarly luminous, dusty starburst resulting from lower-mass mergers in the local Universe.

Key words: gravitational lensing -- cosmology: observations -- galaxies: clusters: individual: A2219 -- galaxies: high-redshift -- infrared: galaxies -- galaxies: individual: SMM J16359+6612

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© ESO 2005

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