The AURORA Survey: A New Era of Emission-line Diagrams with JWST/NIRSpec

We present results on the emission-line properties of z=1.4-7.5 star-forming galaxies in the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics (AURORA) Cycle 1 JWST/NIRSpec program. Based on its depth, continuous wavelength coverage from 1--5 microns, and medium spectral resolution (R~1000), AURORA includes detections of a large suite of nebular emission lines spanning a broad range in rest wavelength. We investigate the locations of AURORA galaxies in multiple different emission-line diagrams, including traditional "BPT" diagrams of OIII/Hbeta vs. NII/Halpha, SII/Halpha, and OI/Halpha, and the "ionization-metallicity" diagram of OIII/OII (O32) vs. (OIII+OII)/Hbeta (R23). We also consider a bluer rest-frame "ionization-metallicity" diagram introduced recently to characterize z>10 galaxies: NeIII/OII vs. (NeIII+OII)/Hdelta; as well as longer-wavelength diagnostic diagrams extending into the rest-frame near-IR: OIII/Hbeta vs. SIII/SII (S32); and HeI/Pagamma and SIII/Pagamma vs. FeII/Pabeta. With a significant boost in signal-to-noise and large, representative samples of individual galaxy detections, the AURORA emission-line diagrams presented here definitively confirm a physical picture in which chemically-young, alpha-enhanced, massive stars photoionize the ISM in distant galaxies with a harder ionizing spectrum at fixed nebular metallicity than in their z~0 counterparts. We also uncover previously unseen evolution prior to z~2 in the OIII/Hbeta vs. NII/Halpha diagram, which motivates deep NIRSpec observations at even higher redshift. Finally, we present the first statistical sample of rest-frame near-IR emission-line diagnostics in star-forming galaxies at high redshift. In order to truly interpret rest-frame near-IR line ratios including FeII, we must obtain better constraints on dust depletion in the high-redshift ISM.