Candidate Population III stellar complex at z = 6.629 in the MUSE Deep Lensed Field

We discovered a strongly lensed (\u3bc 73 40) Ly \u3b1 emission at z = 6.629 (S/N 43 18) in the MUSE Deep Lensed Field (MDLF) targeting the Hubble Frontier Field (HFF) galaxy cluster MACS J0416. Dedicated lensing simulations imply that the Ly \u3b1 emitting region necessarily crosses the caustic. The arc-like shape of the Ly \u3b1 extends 3 arcsec on the observed plane and is the result of two merged multiple images, each one with a de-lensed Ly \u3b1 luminosity L 72 2.8 7 1040 erg s 121 arising from a confined region ( 72150 pc effective radius). A spatially unresolved Hubble Space Telescope (HST) counterpart is barely detected at S/N 43 2 after stacking the near-infrared bands, corresponding to an observed (intrinsic) magnitude m1500 73 30.8 ( 7335.0). The inferred rest-frame Ly \u3b1 equivalent width is EW0 > 1120 \uc5 if the IGM transmission is TIGM < 0.5. The low luminosities and the extremely large Ly \u3b1 EW0 match the case of a Population III (Pop III) star complex made of several dozens stars ( 3c104 M&09) that irradiate an H II region crossing the caustic. While the Ly \u3b1 and stellar continuum are among the faintest ever observed at this redshift, the continuum and the Ly \u3b1 emissions could be affected by differential magnification, possibly biasing the EW0 estimate. The aforementioned tentative HST detection tends to favour a large EW0, making such a faint Pop III candidate a key target for the James Webb Space Telescope and Extremely Large Telescopes