Keck spectroscopic survey of strongly lensed galaxies in Abell 1703: further evidence of a relaxed, unimodal cluster

Context. Strong gravitational lensing is a unique tool that can be used to accurately model the inner mass distribution of massive galaxy clusters. In particular, clusters with large Einstein radii provide a wealth of multiply imaged systems in the cluster core. Measuring the redshift of these multiple images provides strong constraints for precisely determining the shape of the central dark matter profile. Aims. This paper presents a spectroscopic survey of strongly lensed galaxies in the massive cluster lens Abell 1703, which displays a large Einstein radius (28 '' at z = 2.8) and numerous strongly-lensed systems including a central ring-like configuration. Methods. We used the LRIS spectrograph on Keck to target multiple images and lensed galaxy candidates, and used the measured spectroscopic redshifts to constrain the mass distribution of the cluster using a parametric model. Results. The spectroscopic data enable us to measure accurate redshifts for 7 sources at z > 2, all of which are in good agreement with their photometric redshifts. We update the identification of multiply imaged systems by discovering 3 new systems and identifying a radial counter image. We also report the discovery of a remarkably bright similar to 3.6 L* i-band dropout at z = 5.827 in our mask and it is only moderately magnified by the cluster (mu similar to 3.0 +/- 0.08). The improved parametric mass model, including 16 multiple systems with 10 spectroscopic redshifts, further constrains the smooth cluster-scale mass distribution with a generalized NFW profile of best-fit logarithmic slope alpha = 0.92 +/- 0.04, concentration c(200) = 4.72 +/- 0.40 and scale radius r(s) = 476 +/- 45 kpc. The overall rms in the image plane is 1.3 ''. Conclusions. Our strong-lensing model allows us to predict a large-scale shear signal that is consistent with weak-lensing measurements inferred from Subaru data out to 4 Mpc h(-1). Also considering the strong-lensing modeling requires a single dark matter clump, this suggests that Abell 1703 is a relaxed, unimodal cluster. This unique cluster could be probed further using deep X-ray, SZ, and dynamics analysis for a detailed study of the physics in a relaxed cluster.