JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyman-α Emitters
Abstract
Deep VLT/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies (MUV ≈ -15; 0.01 L) at z=2.9-6.7 due to their strong Lyman-α emission with no detectable continuum. The implied Lyman-α equivalent widths can be in excess of 100-200 Angstrom, 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 (11h in G235M/F170LP and 7h in G395M/F290LP), as well as 45 lower-equivalent width Lyman-α emitters. Our spectroscopy covers the range 1.7-5.1 micron in order to target strong optical emission lines: Hα, [N II], [O III], and Hβ. 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 Lyman-α, in excess of 90 Angstrom, have lower [N II]/Hα (1.9-σ) and [O III]/Hβ (2.2-σ) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4-σ. This implies a selection based on Lyman-α equivalent width is an efficient technique for identifying younger, less chemically enriched systems.