2024ApJ...975...87M

New JWST/NIRCam wide-field slitless spectroscopy provides redshifts for four z > 8 galaxies located behind the lensing cluster MACS J0416.1-2403. Two of them, “Y1” and “JD,” have previously reported spectroscopic redshifts based on Atacama Large Millimeter/submillimeter Array measurements of [O III] 88 μm and/or [C II] 157.7 μm lines. Y1 is a merging system of three components, and the existing redshift z = 8.31 is confirmed. However, JD is at z = 8.34 instead of the previously claimed z = 9.28. JD's close companion, “JD-N,” which was a previously discovered z > 8 candidate, is now identified at the same redshift as JD. JD and JD-N form an interacting pair. A new candidate at z > 8, “f090d_018,” is also confirmed and is at z = 8.49. These four objects are likely part of an overdensity that signposts a large structure extending ∼165 kpc in projected distance and ∼48.7 Mpc in radial distance. They are magnified by less than 1 mag and have an intrinsic M_UV ranging from -19.57 to -20.83 mag. Their spectral energy distributions show that the galaxies are all very young with ages ∼ 4-18 Myr and stellar masses of about 10^7–8M_⊙. These infant galaxies have very different star formation rates ranging from a few to over a hundred solar masses per year, but only two of them (JD and f090d_018) have blue rest-frame UV slopes β < -2.0 indicative of a high Lyman-continuum photon escape fraction that could contribute significantly to the cosmic hydrogen-reionizing background. Interestingly, these two galaxies are the least massive and least active ones among the four. The other two systems have much flatter UV slopes largely because of their high dust extinction (A_V = 0.9-1.0 mag). Their much lower indicated escape fractions show that even very young, actively star-forming galaxies can have a negligible contribution to reionization when they quickly form dust throughout their bodies.