EIGER II. first spectroscopic characterisation of the young stars and
ionised gas associated with strong Hβ and [OIII] line-emission in
galaxies at z=5-7 with JWST
We present emission-line measurements and physical interpretations for a
sample of 117 [OIII] emitting galaxies at z=5.33−6.93, using the first deep
JWST/NIRCam wide field slitless spectroscopic observations. Our 9.7-hour
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 microns. We
detect 133 [OIII] doublets, but merge pairs within ≈10 kpc and 600 km
s−1, motivated by their small scale clustering excess. We detect Hβ
in 68 and Hγ emission in two galaxies. The galaxies are characterised by
a UV luminosity MUV​∼−19.6 (−17.7 to −22.3), stellar mass
~108(106.8−10.1) M⊙​, Hβ and [OIII] EWs ≈ 850
Angstrom (up to 3000 Angstrom), young ages (~100 Myr), a highly excited
interstellar medium ([OIII]/Hβ≈6) 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 [OIII]4364​ which shows that the galaxies are
typically dust and metal poor (E(B-V)=0.1, 12+log(O/H)=7.4) with a high
electron temperature (2×104 K) and a production efficiency of ionising
photons (ξion​=1025.3 Hz erg−1). We further show the
existence of a strong mass-metallicity relation. The young highly ionising
stellar populations, moderately low metallicities, low dust attenuations and
high ionisation state in z~6 galaxies conspire to maximise the [OIII] output
from galaxies, yielding an [OIII] luminosity density at z~6 that is
significantly higher than at z~2, despite the order of magnitude decline in
cosmic star formation. Thus, [OIII] emission-line surveys with JWST prove a
highly efficient method to trace the galaxy density in the epoch of
reionization.Comment: Accepted for publication in ApJ. Main text 22 pages, 20 figures. Main
results in Figs 14 (Xi_ion), 15 (MEx diagram),17 (MZR), 19 ([OIII] luminosity
density