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

Abstract

We discovered a strongly lensed (μ ≳ 40) Ly α emission at z = 6.629 (S/N ≃ 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 α emitting region necessarily crosses the caustic. The arc-like shape of the Ly α extends 3 arcsec on the observed plane and is the result of two merged multiple images, each one with a de-lensed Ly α luminosity L ≲ 2.8 × 10<SUP>40</SUP> erg s<SUP>-1</SUP> arising from a confined region (≲150 pc effective radius). A spatially unresolved Hubble Space Telescope(HST) counterpart is barely detected at S/N ≃ 2 after stacking the near-infrared bands, corresponding to an observed (intrinsic) magnitude m<SUB>1500</SUB> ≳ 30.8 (≳35.0). The inferred rest-frame Ly α equivalent width is EW<SUB>0</SUB> > 1120 Šif the IGM transmission is T<SUB>IGM</SUB> < 0.5. The low luminosities and the extremely large Ly α EW<SUB>0</SUB> match the case of a Population III (Pop III) star complex made of several dozens stars (∼10<SUP>4</SUP> M<SUB>⊙</SUB>) that irradiate an H II region crossing the caustic. While the Ly α and stellar continuum are among the faintest ever observed at this redshift, the continuum and the Ly α emissions could be affected by differential magnification, possibly biasing the EW<SUB>0</SUB> estimate. The aforementioned tentative HST detection tends to favour a large EW<SUB>0</SUB>, making such a faint Pop III candidate a key target for the James Webb Space Telescope and Extremely Large Telescopes.