A multiply imaged, submillimetre-selected ultraluminous infrared galaxy in a galaxy group at z ∼ 2.5

We present observations of a remarkable submillimetre-selected galaxy, SMM J16359+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 850 mum. Subsequent follow-up investigations using deep optical and near-infrared (NIR) images identify a faint counterpart to each of the three images, with similar red optical-NIR colours and Hubble Space Telescope 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 similar to45, implying that this galaxy would have unlensed magnitudes K-s = 22.9 and I = 26.1, and an unlensed 850-mum flux density of only 0.8 mJy. Moreover, the highly constrained lens model predicted the redshift of SMM J16359+6612 to be z = 2.6 +/- 0.4. We confirm this estimate using deep optical and NIR Keck spectroscopy, measuring a redshift of z = 2.516. SMM J16359+6612 is the faintest submillimetre (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 Near Infrared Spectrograph (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 similar to100-kpc region on the background sky, suggesting this submm galaxy is located in a dense high-redshift group.