Title: Magnifying the early episodes of star formation: super star clusters at cosmological distances,
Abstract: We study the spectrophotometric properties of a highly magnified (\mu~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~30.6), young (<100 Myr),
low-mass (<10^7 Msun), low-metallicity (12+Log(O/H)~7.7, or 1/10 solar) and
compact (30 pc effective radius) stellar systems separated by ~300pc, after
correcting for lensing amplification. We measured several rest-frame
ultraviolet and optical narrow (\sigma_v <~ 25 km/s) high-ionization lines.
These features may be the signature of very hot (T>50000 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 Lya
emission (e.g., CIV / Lya > 15), despite the fact that the Lya line flux is
expected to be 150 times brighter (inferred from the Hbeta flux). A
spatially-offset, strongly-magnified (\mu>50) Lya emission with a spatial
extent <~7.6 kpc^2 is instead identified 2 kpc away from the system. The origin
of such a faint emission can be the result of fluorescent Lya induced by a
transverse leakage of ionizing radiation emerging from the stellar systems
and/or can be associated to 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 Lya emission, suggesting that, at least in
some cases, a non-uniform covering factor of the neutral gas might hamper the
Lya detection.