CECILIA: The Faint Emission Line Spectrum of z 2-3 Star-forming Galaxies

We present the first results from Chemical Evolution Constrained Using Ionized Lines in Interstellar Aurorae (CECILIA), a Cycle 1 JWST NIRSpec/MSA program that uses ultra-deep ~30 hr G235M/F170LP observations to target multiple electron temperature-sensitive auroral lines in the spectra of 33 galaxies at z ~ 1-3. Using a subset of 23 galaxies, we construct two ~600 object-hour composite spectra, both with and without the stellar continuum, and use these to investigate the characteristic rest-optical (λ _rest ≈ 5700-8500 Å) spectrum of star-forming galaxies at the peak epoch of cosmic star formation. Emission lines of eight different elements (H, He, N, O, Si, S, Ar, and Ni) are detected, with most of these features observed to be ≲3% the strength of Hα. We report the characteristic strength of three auroral features ([N II]λ5756, [S III]λ6313, and [O II]λ λ7322, 7332), as well as other semi-strong and faint emission lines, including forbidden [Ni II]λ λ7380, 7414 and permitted O I λ8449, some of which have never before been observed outside of the local Universe. Using these measurements, we find T _e [N II] = 13,630 ± 2540 K, representing the first measurement of electron temperature using [N II] in the high-redshift Universe. We also see evidence for broad line emission with a FWHM of ${536}_{-167}^{+45}$ km s^-1; the broad component of Hα is 6.01%-28.31% the strength of the narrow component and likely arises from star-formation-driven outflows. Finally, we briefly comment on the feasibility of obtaining large samples of faint emission lines using JWST in the future.