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

Abstract

Abstract 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 to 5 μ m, and medium spectral resolution ( R ∼ 1000), AURORA includes detections of a large suite of nebular emission lines spanning a broad range in rest-frame wavelength. We investigate the locations of AURORA galaxies in multiple different emission-line diagrams, including traditional BPT diagrams of [O iii ] λ 5007/H β versus [N ii ] λ 6583/H α , [S ii ] λλ 6717, 6731/H α , and [O i ] λ 6300/H α , and the ionization–metallicity diagram of [O iii ] λ 5007/[O ii ] λ 3727 (O 32 ) versus ([O iii ] λ 5007+[O ii ] λ 3727)/H β ( R 23 ). We also consider a bluer rest-frame ionization–metallicity diagram introduced recently to characterize z > 10 galaxies, [Ne iii ] λ 3869/[O ii ] λ 3727 versus ([Ne iii ] λ 3869+[O ii ] λ 3727)/H δ , as well as longer-wavelength diagnostic diagrams extending into the rest-frame near-IR: [O iii ] λ 5007/H β versus [S iii ] λλ 9069, 9532/[S ii ] λλ 6717, 6731 (S 32 ), and He i λ 1.083 μ m/Pa γ and [S iii ] λ 9532/Pa γ versus [Fe ii ] λ 1.257 μ m/Pa β . With a significant boost in signal-to-noise ratio and large, representative samples of individual galaxy detections, the AURORA emission-line diagrams presented here definitively confirm a physical picture in which chemically young, α -enhanced, massive stars photoionize the interstellar medium (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 [O iii ] λ 5007/H β versus [N ii ] λ 6583/H α 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 [Fe ii ] λ 1.257 μ m, we must obtain better constraints on dust depletion in the high-redshift ISM.