The nature of a gravitationally lensed submillimetre arc in MS0451.6−0305: two interacting galaxies at z∼ 2.9?

We present a new SCUBA image of the cluster MS 0451.6-0305, which exhibits strong, extended submillimetre flux at 850 μm. The most striking feature in the map is an elongated region of bright submillimetre emission, with a flux density of ∼10 mJy over several beam sizes. This region is apparently coincident with a previously known optical arc [which turns out to be a strongly lensed Lyman break galaxy (LBG) at z = 2.911], as well as with a newly identified multiply imaged ERO (extremely red object) pair predicted to be at a similar, if not identical redshift. By combining a detailed lensing model with deep images from the Hubble Space Telescope (HST), Chandra, Canada-France-Hawaii Telescope (CFHT), James Clerk Maxwell Telescope (JCMT) and spectra from the Very Large Telescope (VLT), we conclude that both the strongly lensed optical arc and ERO systems have properties consistent with known submillimetre emitters. Using a simple model for the two sources, we estimate that the multiply lensed EROs contribute the majority of the flux in the SCUBA lensed arc. Correcting for the lensing amplification, we estimate that the inherent 850-μm fluxes for both objects are ≤0.4 mJy. If the LBG and ERO pair are truly at the same redshift, then they are separated by only ∼10kpc in the source plane, and hence constitute an interacting system at z ∼ 2.9. Higher angular resolution observations in submillimetre/millimetre will permit us to separate the contribution more accurately from each candidate, and better understand the nature of this system.