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. The implied
Lyman-α equivalent widths are in excess of 100-200 Angstrom, challenging
existing models of normal star formation and implying 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, [OIII], Hβ, and
[NII]. Individual measurements as well as stacks reveal line ratios consistent
with a metal poor nature (2−30% Z⊙) and intense ionizing radiation
fields. The galaxies with the highest equivalent widths of Lyman-α, in
excess of 120 Angstrom, have lower gas-phase metallicities than those with
lower equivalent widths. This implies a selection based on Lyman-α
equivalent width is an efficient technique for identifying younger, less
chemically enriched systems.Comment: 13 pages, 4 appendices; submitted to AAS Journal