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Vanzella, E.; Castellano, M.; Meneghetti, M.; Mercurio, A.; Caminha, G. B.; Cupani, G.; Calura, F.; Christensen, L.; Merlin, E.; Rosati, P.; Gronke, M.; Dijkstra, M.; Mignoli, M.; Gilli, R.; Barros, S. de; Caputi, K.; Grillo, C.; Balestra, I.; Cristiani, S.; Nonino, M.; Giallongo, E.; Grazian, A.; Pentericci, L.; Fontana, A.; Comastri, A.; Vignali, C.; Zamorani, G.; Brusa, M.; Bergamini, P.; Tozzi, P. (2017): Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances. In: Astrophysical Journal, Vol. 842, No. 1, 47
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Abstract

We study the spectrophotometric properties of a highly magnified (mu similar or equal to 40-70) pair of stellar systems identified at z = 3.2222 behind the Hubble Frontier Field galaxy cluster MACS J0416. Five multiple images (out of six) have been spectroscopically confirmed by means of VLT/MUSE and VLT/X-Shooter observations. Each image includes two faint (m(UV) similar or equal to 30.6), young (less than or similar to 100 Myr), low-mass (< 10(7) M-circle dot), low-metallicity (12 + Log(O/H) similar or equal to 7.7, or 1/10 solar), and compact (30 pc effective radius) stellar systems separated by similar or equal to 300 pc after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (sigma(nu) less than or similar to 25 km s(-1)) high-ionization lines. These features may be the signature of very hot (T > 50,000 K) stars within dense stellar clusters, whose dynamical mass is likely dominated by the stellar component. Remarkably, the ultraviolet metal lines are not accompanied by Ly alpha emission (e.g., C IV/Ly alpha > 15), despite the fact that the Ly alpha line flux is expected to be 150 times brighter (inferred from the H beta flux). A spatially offset, strongly magnified (mu > 50) Ly alpha emission with a spatial extent less than or similar to 7.6 kpc(2) is instead identified 2 kpc away from the system. The origin of such a faint emission could be the result of fluorescent Ly alpha induced by a transverse leakage of ionizing radiation emerging from the stellar systems and/or may be associated with an underlying and barely detected object (with m(UV) > 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Ly alpha emission-suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Ly alpha detection.