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Abstract Deep Very Large Telescope/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies (M_UV≈ −15; 0.01L_⋆) atz= 2.9−6.7 due to their strong Lyαemission with no detectable continuum. The implied Lyαequivalent widths can be in excess of 100–200 Å, challenging existing models of normal star formation and indicating extremely young ages, small stellar masses, and a very low amount of metal enrichment. We use JWST/NIRSpec’s microshutter array to follow up 45 of these galaxies (11 hr in G235M/F170LP and 7 hr in G395M/F290LP), as well as 45 lower-equivalent width Lyαemitters. Our spectroscopy covers the range 1.7−5.1 micron in order to target strong optical emission lines: Hα, [Oiii], Hβ, and [N II]. Individual measurements as well as stacks reveal line ratios consistent with a metal-poor nature (2%−40%Z_⊙, depending on the calibration). The galaxies with the highest equivalent widths of Lyα, in excess of 90 Å, have lower [N II]/Hα(1.9σ) and [Oiii]/Hβ(2.2σ) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4σ. This implies a selection based on Lyαequivalent width is an efficient technique for identifying younger, less chemically enriched systems. 

Abstract We analyze the evolution of massive (log₁₀[M_⋆/M_⊙] > 10) galaxies atz∼ 1–4 selected from JWST Cosmic Evolution Early Release Survey (CEERS). We infer the physical properties of all galaxies in the CEERS NIRCam imaging through spectral energy distribution (SED) fitting withdense basisto select a sample of high-redshift massive galaxies. Where available we include constraints from additional CEERS observing modes, including 18 sources with MIRI photometric coverage, and 28 sources with spectroscopic confirmations from NIRSpec or NIRCam WFSS. We sample the recovered posteriors in stellar mass from SED fitting to infer the volume densities of massive galaxies across cosmic time, taking into consideration the potential for sample contamination by active galactic nuclei. We find that the evolving abundance of massive galaxies tracks expectations based on a constant baryon conversion efficiency in dark matter halos forz∼ 1–4. At higher redshifts, we observe an excess abundance of massive galaxies relative to this simple model, resulting in a shallower decline of observed volume densities of massive galaxies. These higher abundances can be explained by modest changes to star formation physics and/or the efficiencies with which star formation occurs in massive dark matter halos, and are not in tension with modern cosmology.