  We present the first results from CECILIA, a Cycle 1 JWST NIRSpec/MSA program
that uses ultra-deep ~30 hour 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 (5700-8500 Angstrom) 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
H-alpha. We report the characteristic strength of three auroral lines
([NII]5756, [SIII]6313, and [OII]7322,7332), as well as other semi-strong and
faint emission lines, including forbidden [NiII]7380,7414 and the OI 8449
recombination line, some of which have never before been observed outside of
the local universe. Using these measurements, we find T_e[NII]=13630+/-2540 K,
representing the first measurement of electron temperature using [NII] in the
high-redshift universe. We also see evidence for broad line emission with a
FWHM of ~536 km/s; the broad component of H-alpha 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.
