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

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 A2218. 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 f850 μm = 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 3σ 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 × 1010 M_ and an H2 gas mass of 4.5 × 109 M_. 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 Hα 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.