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

We study the spectrophotometric properties of a highly magnified (μ ≤ 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 (mUV ≤ 30.6), young (≲100 Myr), low-mass (<107 M⊙), low-metallicity (12 + Log (O/H) ≤ 7.7, or 1/10 solar), and compact (30 pc effective radius) stellar systems separated by ≤ 300 pc after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow (σv ≲ 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 Lyα emission (e.g., C IV/Lyα > 15), despite the fact that the Lyα line flux is expected to be 150 times brighter (inferred from the Hβ flux). A spatially offset, strongly magnified (m > 50) Lyα emission with a spatial extent ≲7.6 kpc2 is instead identified 2 kpc away from the system. The origin of such a faint emission could be the result of fluorescent Lyα 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 mUV > 34 de-lensed). This is the first confirmed metal-line emitter at such low-luminosity and redshift without Lyα emission-suggesting that, at least in some cases, a non-uniform covering factor of the neutral gas might hamper the Lyα detection.