EIGER. II. First Spectroscopic Characterization of the Young Stars and Ionized Gas Associated with Strong Hβ and [O III] Line Emission in Galaxies at z = 5-7 with JWST

Abstract

We present emission-line measurements and physical interpretations for a sample of 117 [O III] emitting galaxies at z = 5.33-6.93, using the first deep JWST/NIRCam wide-field slitless spectroscopic observations. Our 9.7 hr integration is centered upon the z = 6.3 quasar J0100+2802-the first of six fields targeted by the EIGER survey-and covers λ = 3-4 μm. We detect 133 [O III] doublets, but close pairs motivated by their small scale clustering excess. The galaxies are characterized by a UV luminosity M _UV ~ -19.6 (-17.7 to -22.3), stellar mass ~10⁸ (10^6.8-10.1) M _⊙, Hβ and [O III]_4960+5008 EWs ≈ 850 Å (up to 3000 Å), young ages, a highly excited interstellar medium, and low dust attenuations. These high EWs are very rare in the local universe, but we show they are ubiquitous at z ~ 6 based on the measured number densities. The stacked spectrum reveals Hγ and [O III]₄₃₆₄, which shows that the galaxies are typically dust- and metal-poor (E (B - V) = 0.1, $12+\mathrm{log}({\rm{O}}/{\rm{H}})=7.4$ ) with a high electron temperature (2 × 10⁴ K) and a production efficiency of ionizing photons (ξ _ion = 10^25.3 Hz erg^-1). We further show the existence of a strong mass-metallicity relation. The properties of the stars and gas in z ~ 6 galaxies conspire to maximize the [O III] output from galaxies, yielding an [O III] luminosity density at z ≈ 6 that is significantly higher than that at z ≈ 2. Thus, [O III] emission-line surveys with JWST prove a highly efficient method to trace the galaxy density in the Epoch of Reionization.