Discovery of Red Selected Arcs at Z = 4.04 behind Abell 2390

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 I_AB=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 N_HI=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.