JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyα Emitters
Michael V. Maseda, Zach Lewis, Jorryt Matthee, Joseph F. Hennawi, Leindert Boogaard, A. Feltre, Themiya Nanayakkara, Roland Bacon, A. J. Barger, J. Brinchmann, Marijn Franx, Takuya Hashimoto, Hanae Inami, Haruka Kusakabe, Floriane Leclercq, Lucie E. Rowland, Anthony J. Taylor, Christy Tremonti, T. Urrutia, Joop Schaye, Charlotte Simmonds, Eloïse Vitte
Abstract
Abstract Deep Very Large Telescope/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies ( M UV ≈ −15; 0.01 L ⋆ ) at z = 2.9−6.7 due to their strong Ly α emission with no detectable continuum. The implied Ly α 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 α emitters. Our spectroscopy covers the range 1.7−5.1 micron in order to target strong optical emission lines: H α , [O iii ], H β , 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 Ly α , in excess of 90 Å, have lower [N II]/H α (1.9 σ ) and [O iii ]/H β (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 Ly α equivalent width is an efficient technique for identifying younger, less chemically enriched systems.