Discovery of Red-selected Arcs at [CLC][ITAL]z[/ITAL][/CLC] = 4.04 behind Abell 2390

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Abstract
We describe the properties of three red arcs discovered at z=4.04 behind the cluster A2390 (z=0.23). We demonstrate that these arcs are images of a single galaxy where lensing is compounded by an elliptical cluster member near the critical curve of the cluster. The combined magnification is estimated to be ~20 using Hubble Space Telescope images, depending on the gradient of the model potentials, implying an unlensed magnitude for the source of IAB=25.5. Keck spectroscopy reveals a continuum that is well fitted by B stars and attenuated by the Lyman series forest with an opacity consistent with z ~4 QSO spectra, making the arcs relatively red. Damped Lyα absorption is observed at the source redshift corresponding to a high column density NH I=3 × 1021. Lyα emission is found to be spatially separated ( ~0.5 kpc) from the bright stellar continuum and lies ~300 km s-1 redward of interstellar absorption lines at z=4.04. Similar redward shifts are found in all high-redshift galaxies with good spectroscopy, indicating that outward flows of enriched gas are typical of young galaxies. Finally, we briefly comment on the notorious "straight arc" in A2390, which is resolved into two unrelated galaxies at z=0.913 and z=1.033.
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