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

Abstract

We describe the properties of a system of red arcs discovered at z=4.04 around the cluster A2390 ($z=0.23$). These arcs are images of a single galaxy, where the lensing is compounded by an elliptical cluster member lying close to the critical curve of the cluster. The combined magnification is estimated to be $\sim 20$, using HST images, depending the gradient of the model potentials, implying an unlensed magnitude for the source of $I_{AB}=25.5$. Keck spectroscopy reveals a continuum well fitted by B-stars and attenuated by the Lyman-series forest with an opacity consistent with z$\sim$4 QSO spectra, making the arcs relatively red. Damped Ly$\alpha$ absorption is observed at the source redshift with a relatively high column, n$_{HI}=3\times 10^{21}$. Curiously, Ly$\alpha$ emission is spatially separated ($\sim 0.5$kpc) from the bright stellar continuum and lies $\sim$ 300Km/s redward of interstellar absorption lines at z=4.04. Similar redwardl 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.