A multiply - imaged , submillimetre - selected ULIRG in a galaxy group at z ∼ 2 .

We present observations of a remarkable submillimetre-selected galaxy, SMMJ16359+6612. This distant galaxy lies behind the core of a massive cluster of galaxies, A 2218, and is gravitationally lensed by the foreground cluster into three discrete images which were identified in deep submillimetre maps of the cluster core at both 450 and 850μm. Subsequent follow-up using deep optical and near-infrared images identify a faint counterpart to each of the three images, with similar red optical–near-infrared colours and HST morphologies. By exploiting a detailed mass model for the cluster lens we estimate that the combined images of this galaxy are magnified by a factor of ∼45, implying that this galaxy would have un-lensed magnitudes Ks = 22.9 and I = 26.1, and an un-lensed 850-μum flux density of only 0.8mJy. Moreover, the highly constrained lens model predicted the redshift of SMMJ16359+6612 to be z = 2.6 ± 0.4. We confirm this estimate using deep optical and near-infrared Keck spectroscopy, measuring a redshift of z = 2.516. SMMJ16359+6612 is the faintest submm-selected galaxy so far identified with a precise redshift. Thanks to the large gravitational magnification of this source, we identify three sub-components in this submm galaxy, which are also seen in the NIRSPEC data, arguing for either a strong dust (lane) absorption or a merger. Interestingly, there are two other highly-amplified galaxies at almost identical redshifts in this field (although neither is a strong submm emitter). The three galaxies lie within a ∼ 100 kpc region on the background sky, suggesting this submm galaxy is located in a dense high-redshift group.