JWST view of three infant galaxies at z=8.3 and implications for reionization

New JWST/NIRCam wide-field slitless spectroscopy provides redshifts for two z > 8 galaxies located behind the lensing cluster MACS J0416.1-2403. Both galaxies are strong [O iii]λ5007 emitters. For one galaxy, "Y1", the existing redshift z = 8.31, based on ALMA measurements of [O iii] 88 μm and [C ii] 157.7 μm lines, is confirmed. JWST/NIRCam images resolve this galaxy into three components of similar colors, and the whole system extends over  3.4 kpc. The other galaxy, "JD", is at z = 8.34 instead of the previously claimed z = 9.28. It has a companion, "JD-N", at the same redshift with projected separation  2.3 kpc. All objects are only moderately magnified and have intrinsic MUV ranging from -19.66 to -20.85 mag. Their eight-band NIRCam spectral energy distributions show that the galaxies are all very young with ages ≲11 Myr and stellar masses about 108 M_⊙. These infant galaxies are actively forming stars at rates of a few tens to a couple of hundred M_⊙ yr^-1, but only one of them (JD) has a blue rest-frame UV slope. This slope indicates a high Lyman-continuum photon escape fraction that could contribute significantly to the cosmic hydrogen-reionizing background. The other two systems have much flatter slopes largely because their dust extinction is twice as high as JD's albeit only AV   0.90 mag. The much lower indicated escape fractions show that even very young, actively star-forming galaxies can have negligible contribution to reionization when they quickly form dust throughout their bodies.