2019MNRAS.483.3618V

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 Σ_SFR ∼ 1000 M_☉ yr^–1 kpc^–2 (or conservatively >300 M_☉ yr^–1 kpc^–2). Such a dense region is detected with S/N >= 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 (β ≃ -2.5, F_λ ∼ λ^β), and the prominent Lyα 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 10⁷ M_☉) might contain a young massive star cluster of M <= 10⁶ M_☉ and M_UV ≃ -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.