JWST/NIRSpec Measurements of Extremely Low Metallicities in High Equivalent Width Lyman-$\hat{I}\pm$ Emitters

Abstract

Deep VLT/MUSE optical integral field spectroscopy has recently revealed an abundant population of ultra-faint galaxies ($M_{UV}\approx -15$; 0.01 $L_{\star }$) at $z=$2.9$-$6.7 due to their strong Lyman-$\hat{I}\pm$ emission with no detectable continuum. The implied Lyman-$\hat{I}\pm$ 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-$\hat{I}\pm$ emitters. Our spectroscopy covers the range 1.7$-$5.1 micron in order to target strong optical emission lines: H$\hat{I}\pm$, [N II], [O III], and H${\hat{I}}^2$. Individual measurements as well as stacks reveal line ratios consistent with a metal poor nature (2$-$40% $Z_{\odot }$, depending on the calibration). The galaxies with the highest equivalent widths of Lyman-$\hat{I}\pm$, in excess of 90 Angstrom, have lower [N II]/H$\hat{I}\pm$ (1.9-$\ddot{I}ƒ$) and [O III]/H${\hat{I}}^2$ (2.2-$\ddot{I}ƒ$) ratios than those with lower equivalent widths, implying lower gas-phase metallicities at a combined significance of 2.4-$\ddot{I}ƒ$. This implies a selection based on Lyman-$\hat{I}\pm$ equivalent width is an efficient technique for identifying younger, less chemically enriched systems.

14 pages, 4 appendices; ApJ Accepted