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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. [ABSTRACT FROM AUTHOR]

We analyze a sample of z-dropout galaxies in the CANDELS GOODS South and UDS fields that have been targeted by a dedicated spectroscopic campaign aimed at detecting their Lyα line. Deep IRAC observations at 3.6 and 4.5 μm are used to determine the strength of optical emission lines affecting these bands at z ∼ 6.5–6.9 in order to (1) investigate possible physical differences between Lyα emitting and non-emitting sources; (2) constrain the escape fraction of ionizing photons; and (3) provide an estimate of the specific star formation rate at high redshifts. We find evidence of strong [O iii]+Hβ emission in the average (stacked) SEDs of galaxies both with and without Lyα emission. The blue IRAC [3.6]–[4.5] color of the stack with detected Lyα line can be converted into a rest-frame equivalent width EW([O iii]+Hβ) = Å assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of on the escape fraction of ionizing photons from Lyα detected objects. The objects with no Lyα line show less extreme EW([O iii]+Hβ) = Å, suggesting different physical conditions of the H ii regions with respect to Lyα-emitting ones, or a larger . The latter case is consistent with a combined evolution of and the neutral hydrogen fraction as an explanation of the lack of bright Lyα emission at z > 6. A lower limit on the specific star formation rate, SSFR > 9.1 Gyr−1 for galaxies at these redshifts can be derived from the spectroscopically confirmed sample. [ABSTRACT FROM AUTHOR]

Star-forming galaxies are considered to be the leading candidate sources dominating cosmic reionization at : the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently an active line of research. We have observed a star-forming galaxy at z = 3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730–890 Å rest-frame, and detected LyC emission. This galaxy is very compact and also has a large Oxygen ratio / (). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6 Ms X-ray Chandra observations). The measured escape fraction of ionizing radiation spans the range 50%–100%, depending on the intergalactic medium (IGM) attenuation. The LyC emission is measured at (with signal-to-noise ratio (S/N) = 10) and is spatially unresolved, with an effective radius of pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible to optically thin media. Irrespective of the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([O iii]λλ4959,5007 + Hβ) recently observed at is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at , allowing a direct comparison with the lower-redshift LyC emitters, such as that reported here. [ABSTRACT FROM AUTHOR]