  We report the serendipitous discovery of a faint (M_UV > -12.2),
low-metallicity (Z ~ 0.02 Zsun), ionizing source (dubbed T2c) with a
spectroscopic redshift of z=6.146. T2c is part of a larger structure amplified
by the Hubble Frontier Field galaxy cluster MACSJ0416, and was observed with
JWST/NIRSpec IFU. Stacking the short-wavelength NIRCam data reveals no stellar
continuum detection down to a magnitude limit of m_UV ~ 31.0 (3 sigma).
However, prominent Hb, [OIII]4959,5007, and Ha emissions are detected, with
equivalent widths exceeding 200A, 800A, and 1300A (3 sigma), respectively. The
corresponding intrinsic (magnification-corrected x23 +/- 3) ultraviolet and
optical rest-frame magnitudes exceed 34.4 and 33.9 (corresponding to M_uv and
M_opt fainter than -12.2 and -12.8, at lambda_rest ~ 2000A and ~5000A,
respectively), suggesting a stellar mass lower than a few 10^4 Msun under an
instantaneous burst scenario. The inferred ionizing photon production
efficiency (xi_ion) is high, xi_ion >~ 26.08(25.86) 3(5)sigma, assuming no dust
attenuation and no Lyman continuum leakage, indicating the presence of massive
stars despite the low mass of the object. The very poor sampling of the initial
mass function at such low mass star-forming complex suggests that the formation
of very massive stars might be favored in very low metallicity environments.
T2c is surrounded by Balmer and weak oxygen emission on a spatial scale of a
few hundred parsecs after correcting for lensing effects. This system resembles
an HII region potentially powered by currently undetected, extremely efficient,
low-metallicity star complexes or clusters. We propose that massive O-type
stars populate this low-mass and metallicity high-redshift satellites, likely
caught in an early and short formation phase, contributing to the ionization of
the surrounding medium.
