Abstract
We present imaging and spectroscopic observations of the gravitationally lensed arcs in the field of RX J1347.5-1145, the most X-ray luminous galaxy cluster known. Based on the detection of the [O II] λ3727 emission line, we confirm that the redshift of one of the arcs is z = 0.806. Its color and [O II] line strength are consistent with those of distant, actively star-forming galaxies. In a second arc, we tentatively identify a pair of absorption lines superposed on a red continuum; the lines are consistent with Ca II λ3933 (K) and Ca II λ3968 (H) at z = 0.785. We detected a faint blue continuum in two additional arcs, but no spectral line features could be measured. We establish lower limits to their redshifts based on the absence of [O II] emission, which we argue should be present and detectable in these objects. Redshifts are also given for a number of galaxies in the field of the cluster.
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