Between the Extremes: A JWST Spectroscopic Benchmark for High-redshift Galaxies Using ~500 Confirmed Sources at $z\geqslant5$

TL;DR

Addresses the need for a representative spectroscopic benchmark for high-redshift galaxies by compiling a large JWST/NIRSpec prism dataset to characterize population-level rest-frame UV–optical properties. Using 482 confirmed sources spanning redshifts up to 12.9, the study constructs high-SNR spectral templates and quantifies UV slopes, Balmer indices, and rest-frame optical line diagnostics. Key results include very blue UV continua with slopes around -2.3 to -2.7, reduced Balmer indices, ubiquitous CIII] with rest-frame EWs of 5–14 Å, elevated rest-frame optical line ratios (O32 7–31, R23 5–8, Ne3O2 1–2), and subsolar metallicities. Lyα emitters (57 objects) show more extreme ionizing conditions with stronger CIII], CIV, HeII+[OIII], younger Balmer-jump ages, and bluer continua, informing Lyα visibility in the early Universe.

Abstract

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 ($β=-2.3$ to $-2.7$) and reduced Balmer indices, 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 (rest-frame equivalent widths of $=5-14$ Å), 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 $z\sim0$ populations. Within our sample, we identify 57 Ly$α$ emitters, which we stack and compare to a matched sample of nonemitters. The former are characterized by more extreme ionizing conditions with enhanced CIII], CIV, and HeII+[OIII] line emission, younger stellar populations from Balmer jumps, and a more pristine interstellar medium 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$α$ 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.