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 1.3mm 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 \sim 1.5 \times 10 ^ { 10 } M _ { \odot } and an H _ { 2 } gas mass of 4.5 \times 10 ^ { 9 } M _ { \odot } .
We identify a spatial offset of \sim 1 ^ { \prime \prime } 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 \sim 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 .