Abstract
Abstract We study the spectrophotometric properties of a highly magnified ( ) 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 ( ), young ( Myr), low-mass ( ), low-metallicity (12 + Log(O/H) ≃ 7.7, or 1/10 solar), and compact (30 pc effective radius) stellar systems separated by pc after correcting for lensing amplification. We measured several rest-frame ultraviolet and optical narrow ( km s−1) high-ionization lines. These features may be the signature of very hot ( 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 ), 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 ( ) Lyα emission with a spatial extent 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 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.
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