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The exceptional spectra of the most luminous z>10 sources observed so far have challenged our understanding of early galaxy evolution, requiring a new observational benchmark for meaningful interpretation. As such, we construct spectroscopic templates representative of high-redshift, star-forming populations, using 482 confirmed sources at z=5. 0-12. 9 with JWST/NIRSpec prism observations, and report on their average properties. We find z=5-11 galaxies are dominated by blue UV continuum slopes (=-2. 3 to -2. 7) and inverse Balmer jumps, characteristic of dust-poor and young systems, with a shift towards bluer slopes and younger ages with redshift. The evolution is mirrored by ubiquitous CIII] detections across all redshifts (EW₀=5-14 A), which increase in strength towards early times. Rest-frame optical lines reveal elevated ratios (O32=7-31, R23=5-8, and Ne3O2=1-2) and subsolar metallicities (log O/H=7. 3-7. 9), typical of ionization conditions and metallicities rarely observed in z0 populations. Within our sample, we identify 57 Ly-emitters which we stack and compare to a matched sample of non-emitters. The former are characterized by more extreme ionizing conditions with enhanced CIII], CIV, and HeII+OIII line emission, younger stellar populations from inverse Balmer jumps, and a more pristine ISM seen through bluer UV slopes and elevated rest-frame optical line ratios. The novel comparison illustrates important intrinsic differences between the two populations, with implications for Ly visibility. The spectral templates derived here represent a new observational benchmark with which to interpret high-redshift sources, lifting our constraints on their global properties to unprecedented heights and extending out to the earliest of cosmic times.
Roberts-Borsani et al. (Mon,) studied this question.