Magnifying the Early Episodes of Star Formation: Super Star Clusters at Cosmological Distances

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.