Molecular gas in a z ∼ 2.5 triply-imaged, sub-mJy submillimetre galaxy typical of the cosmic far-infrared background

We present the results of observations from the Plateau de Bure IRAM interferometric array of the submillimetre (submm) galaxy SMM J16359+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 f(850 mu m) = 0.8 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 sigma 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 similar to 1.5 x 10(10) M. and an H-2 gas mass of 4.5 x 10(9) M-circle dot. We identify a spatial offset of similar to 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 similar to 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 alpha 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.