CECILIA: Multi-line Constraints on Excess Nebular Emission from Low-Ionization Gas at Cosmic Noon

Spectroscopy from JWST has offered unique insights into elemental abundance patterns at Cosmic Noon (z 2-3), most notably a growing census of sub-solar S/O. These new measurements, coupled with new observations of [O I]λ6302 at z>1, motivate an updated study of neutral and low-ionization emission within Cosmic Noon galaxies. In this letter, we present a new analysis of [O I]λ6302 and [S II]λλ6718,33 in 19 galaxies observed using ultra-deep JWST/NIRSpec spectroscopy as part of the CECILIA survey. We find that it is necessary to consider sub-solar abundance patterns in sulfur at high-z when analyzing galaxies on the S2-BPT, and that this correction reveals the potential of additional emission sources in CECILIA galaxies. Using the photoionization model emulator Cue, we find that 48% of observed [S II] and 55% of observed [O I] emission in CECILIA galaxies cannot be reproduced by conventional HII region models. We find no clear evidence for turbulence or shocks driving this excess emission when comparing to existing models. Our findings imply that this excess emission may originate in diffuse ionized gas outside of H II regions, though its nature may be different than that of low-z DIG. Further, we find that including emission from gas outside of star-forming regions can incur a small bias up to 0.1 dex in 12+log(S/H).