2013A&A...559L...1B

In the course of our 870µm APEX/LABOCA follow-up of the Herschel Lensing Survey we have detected a source in AS1063 (RXC J2248.7-4431) that has no counterparts in any of the Herschel PACS/SPIRE bands, it is a Herschel ``drop-out'' with S₈₇₀/S₅₀₀≥0.5. The 870µm emission is extended and centered on the brightest cluster galaxy, suggesting either a multiply imaged background source or substructure in the Sunyaev-Zel'dovich increment due to inhomogeneities in the hot cluster gas of this merging cluster. We discuss both interpretations with emphasis on the putative lensed source. Based on the observed properties and on our lens model we find that this source may be the first submillimeter galaxy (SMG) with a moderate far-infrared (FIR) luminosity (L_FIR<10¹²L_☉) detected so far at z>4. In deep HST observations we identified a multiply imaged z∼6 source and measured its spectroscopic redshift to be z=6.107 with VLT/FORS. This source may be associated with the putative SMG, but it is most likely offset spatially by 10-30kpc and they may be interacting galaxies. With a FIR luminosity in the range [5-15]x10¹¹L_☉ corresponding to a star formation rate in the range [80-260]M_☉/yr, this SMG would be more representative of the z>4 dusty galaxies than the extreme starbursts detected so far. With a total magnification of ∼25 it would open a unique window to the normal dusty galaxies at the end of the epoch of reionization.