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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 present spatially resolved morphological properties of [CII] 158μm, [OIII] 88μm, dust, and rest-frame ultraviolet (UV) continuum emission for A1689-zD1, a strongly lensed, sub-L* galaxy atz= 7.13, by utilizing deep Atacama Large Millimeter/submillimeter Array (ALMA) and Hubble Space Telescope (HST) observations. While the [OIII] line and UV continuum are compact, the [CII] line is extended up to a radius ofr∼ 12 kpc. Using multi-band rest-frame far-infrared continuum data ranging from 52 to 400μm, we find an average dust temperature and emissivity index of $T_{\mathrm{dust}}={41}_{-14}^{+17}$K and $\beta ={1.7}_{-0.7}^{+1.1}$, respectively, across the galaxy. We find slight differences in the dust continuum profiles at different wavelengths, which may indicate that the dust temperature decreases with distance. We map the star formation rate (SFR) via IR and UV luminosities and determine a total SFR of 37 ± 1M_⊙yr⁻¹with an obscured fraction of 87%. While the [OIII] line is a good tracer of the SFR, the [CII] line shows deviation from the localL_[CII]-SFR relations in the outskirts of the galaxy. Finally, we observe a clear difference in the line profile between [CII] and [OIII], with significant residuals (∼5σ) in the [OIII] line spectrum after subtracting a single Gaussian model. This suggests a possible origin of the extended [CII] structure from the cooling of hot ionized outflows. The extended [CII] and high-velocity [OIII] emission may both contribute in part to the highL_[OIII]/L_[CII]ratios recently reported inz> 6 galaxies.