Massive star cluster formation under the microscope at z=6

We report on a superdense star-forming region with an effective radius (R-e) smaller than 13 pc identified at z = 6.143 and showing a star formation rate density Sigma(SFR) similar to 1000 M-circle dot yr(-1) kpc(-2) (or conservatively > 300 M-circle dot yr(-1) kpc(-2)). Such a dense region is detected with S/N greater than or similar to 40 hosted by a dwarf extending over 440 pc, dubbed D1. D1 is magnified by a factor 17.4(+/- 5.0) behind the Hubble Frontier Field galaxy cluster MACS J0416 and elongated tangentially by a factor 13.2 +/- 4.0 (including the systematic errors). The lens model accurately reproduces the positions of the confirmed multiple images with a rms of 0.35 arcsec. D1 is part of an interacting star-forming complex extending over 800 pc. The SED-fitting, the very blue ultraviolet slope (beta similar or equal to -2.5, F-lambda similar to lambda(beta)), and the prominent Lya emission of the stellar complex imply that very young (< 10-100 Myr), moderately dust-attenuated (E(B - V) < 0.15) stellar populations are present and organized in dense subcomponents. We argue that D1 (with a stellar mass of 2 x 10(7) M-circle dot) might contain a young massive star cluster of M less than or similar to 10(6) M-circle dot and M-UV similar or equal to -15.6 (or m(UV) = 31.1), confined within a region of 13 pc, and not dissimilar from some local super star clusters (SSCs). The ultraviolet appearance of D1 is also consistent with a simulated local dwarf hosting an SSC placed at z = 6 and lensed back to the observer. This compact system fits into some popular globular cluster formation scenarios. We show that future high spatial resolution imaging (e.g. E-ELT/MAORY-MICADO and VLT/MAVIS) will allow us to spatially resolve light profiles of 2-8 pc.