2017ApJ...842...47V

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 (m_UV≃30.6), young ( 100 Myr), low-mass ( 10⁷M_☉), 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 (σ_v 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 (µ50) 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 m_UV 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.