JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyα Emitters

<jats:title>Abstract</jats:title> <jats:p>Deep Very Large Telescope/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies (<jats:italic>M</jats:italic> <jats:sub>UV</jats:sub> ≈ −15; 0.01 <jats:italic>L</jats:italic> <jats:sub>⋆</jats:sub>) at <jats:italic>z</jats:italic> = 2.9−6.7 due to their strong Ly<jats:italic>α</jats:italic> emission with no detectable continuum. The implied Ly<jats:italic>α</jats:italic> equivalent 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 Ly<jats:italic>α</jats:italic> emitters. Our spectroscopy covers the range 1.7−5.1 micron in order to target strong optical emission lines: H<jats:italic>α</jats:italic>, [O <jats:sc>iii</jats:sc>], H<jats:italic>β</jats:italic>, and [N II]. Individual measurements as well as stacks reveal line ratios consistent with a metal-poor nature (2%−40% <jats:italic>Z</jats:italic> <jats:sub>⊙</jats:sub>, depending on the calibration). The galaxies with the highest equivalent widths of Ly<jats:italic>α</jats:italic>, in excess of 90 Å, have lower [N II]/H<jats:italic>α</jats:italic> (1.9<jats:italic>σ</jats:italic>) and [O <jats:sc>iii</jats:sc>]/H<jats:italic>β</jats:italic> (2.2<jats:italic>σ</jats:italic>) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4<jats:italic>σ</jats:italic>. This implies a selection based on Ly<jats:italic>α</jats:italic> equivalent width is an efficient technique for identifying younger, less chemically enriched systems.</jats:p>