Discovery of an exceptionally bright giant arc atz= 2.369, gravitationally lensed by thePlanckcluster PSZ1 G311.65−18.48

As part of an all-sky follow-up of the Planck catalogue of Sunyaev-Zeldovich (SZ) cluster candidates detected in the first 14 months of data, we are observing cluster candidates in the southern sky in the optical imaging and spectroscopy through an ESO Large Programme. Inspection of ESO New Technology Telescope (NTT) R-and z-band imaging data from our programme has revealed an unusually large and bright arc in the field of PSZ1 G311.65-18.48. We establish the basic photometric and morphological properties of the arc and provide conclusive evidence for the gravitational lensing nature of this object. Guided by the NTT images, we have obtained a long-slit spectrum with IMACS on the Magellan-I Baade Telescope, covering a part of the arc and the brightest cluster galaxy of PSZ1 G311.65-18.48. Our imaging data confirm the presence of a galaxy cluster coinciding (within 0 .6) with the position of the Planck SZ source. The arc is separated by 30 from the brightest cluster galaxy, which closely coincides with the center of curvature of the arc. A photometric analysis yields integrated (Vega) magnitudes of (R, z, J, K s ) = (17. 82, 17.38, 16.75, 15.43) for the arc, more than one magnitude brighter than any previously known lensed arc at z 2-3. The arc is a vigorously star-forming galaxy at z = 2.369, while the Planck SZ cluster lens is at z = 0.443. Even when allowing for lensing magnifications as high as = 100 still leads to the conclusion that the source galaxy is among the intrinsically most luminous normal (i.e., non-AGN) galaxies known at z 2-3.