MUSE integral-field spectroscopy towards the Frontier Fields cluster Abell S1063. I. Data products and redshift identifications

We present the first observations of the Frontier Fields cluster Abell S1063 taken with the newly commissioned Multi Unit SpectroscopicExplorer (MUSE) integral field spectrograph. Because of the relativelylarge field of view (1 arcmin^2) MUSE is ideal tosimultaneously target multiple galaxies in blank and cluster fields overthe full optical spectrum. We analysed the four hours of data obtainedin the science verification phase on this cluster and measured redshiftsfor 53 galaxies. We confirm the redshift of five cluster galaxies anddetermine the redshift of 29 other cluster members. Behind the clusterwe find 17 galaxies at higher redshift including three previouslyunknown Lyman-α emitters at z> 3 and five multiply-lensedgalaxies. We report the detection of a new z = 4.113 multiply lensedgalaxy with images that are consistent with lensing model predictionsderived for the Frontier Fields. We detect C iii] C iv and He iiemission in a multiply lensed galaxy at z = 3.116 suggesting the likelypresence of an active galactic nucleus. We also created narrow-bandimages from the MUSE datacube to automatically search for additionalline emitters corresponding to high-redshift candidates but we couldnot identify any significant detections other than those found by visualinspection. With the new redshifts it will become possible to obtain anaccurate mass reconstruction in the core of Abell S1063 through refinedstrong lensing modelling. Overall our results illustrate the breadth ofscientific topics that can be addressed with a single MUSE pointing. Weconclude that MUSE is a very efficient instrument to observe galaxyclusters enabling their mass modelling and to perform a blind searchfor high-redshift galaxies.