Ionizing the intergalactic medium by star clusters: the first empirical evidence

We present a VLT/X-Shooter spectroscopy of the Lyman continuum (LyC) emitting galaxy Ion2 at z = 3.2121 and compare it to that of the recently discovered strongly lensed LyC emitter at z = 2.37, known as the Sunburst arc. Three main results emerge from the X-Shooter spectrum: (a) the Ly alpha has three distinct peaks with the central one at the systemic redshift, indicating a ionized tunnel through which both Ly alpha and LyC radiation escape; (b) the large O32 oxygen index ([O III] lambda lambda 4959, 5007/[O II] lambda lambda 3727, 3729) of 9.18(-1.32)(+1.82) is compatible to those measured in local (z similar to 0.4) LyC leakers; (c) there are narrow nebular high-ionization metal lines with sigma(v) < 20 km s(-1), which confirms the presence of young hot, massive stars. The He II lambda 1640 appears broad, consistent with a young stellar component including Wolf-Rayet stars. Similarly, the Sunburst LyC emitter shows a triple-peaked Ly alpha profile and from VLT/MUSE spectroscopy the presence of spectral features arising from young hot and massive stars. The strong lensing magnification, (mu > 20), suggests that this exceptional object is a gravitationally bound star cluster observed at a cosmological distance, with a stellar mass M less than or similar to 10(7) M-circle dot and an effective radius smaller than 20 pc. Intriguingly, sources like Sunburst but without lensing magnification might appear as Ion2-like galaxies, in which unresolved massive star clusters dominate the ultraviolet emission. This work supports the idea that dense young star clusters can contribute to the ionization of the IGM through holes created by stellar feedback.