\n
We present detections of auroral emission lines of [O iii], [O ii], [S iii], and [S ii] in deep JWST/NIRSpec spectroscopy for 41 star-forming galaxies at z = 1.4–7.2 from the AURORA survey. We combine these new observations with 98 star-forming galaxies at z = 1.3–10.6 with detected auroral lines drawn from the literature to form a sample of 139 high-redshift galaxies with robust electron temperature and direct-method oxygen abundance determinations. This sample notably covers a wider dynamic range in metallicity than previous work, spanning 0.02–0.9 Z\u2299. We calibrate empirical relations between 19 emission-line ratios and oxygen abundance, providing a robust tool set to infer accurate gas-phase metallicities of high-redshift galaxies when auroral lines are not detected. While calibrations based on lines of α elements (O, Ne, S, Ar) appear reliable, we find significant scatter in calibrations involving lines of N driven by a high dispersion in N/O at fixed O/H, suggesting that N-based line ratios are less reliable tracers of the oxygen abundance at high redshift. These new high-redshift calibrations are notably offset from those based on typical z ∼ 0 galaxy and H ii region samples, and are better matched by samples of extreme local galaxies that are analogs of high-redshift sources. The new metallicity calibrations presented in this work pave the way for robust studies of galaxy chemical evolution in the early Universe, leading to a better understanding of baryon cycling and galaxy formation from Cosmic Noon through the Epoch of Reionization.
\n
\n
We present the nebular attenuation curves and dust covering fractions for 24 z = 1.5–4.4 star-forming galaxies using multiple Balmer and Paschen lines from the JWST/AURORA survey. Nebular reddening derived from Paschen lines exceeds that from Balmer lines for at least half the galaxies in the sample when assuming the commonly adopted Galactic extinction curve, implying the presence of heavily reddened star formation. The nebular attenuation curves exhibit a broad range of normalizations (RV \u2243 3.2–16.4). Motivated by the offsets in reddening deduced from the Balmer and Paschen lines, as well as the high RV values for the individual nebular attenuation curves, both of which suggest variations in the dust–star geometry, we propose a model with a sub-unity dust covering fraction (fcov). Fitting such a model to the H i recombination line ratios indicates fcov ∼ 0.6–1.0. The normalizations of the nebular attenuation curves, RV, are driven primarily by fcov and the mix of reddened and unreddened OB associations. Thus, the diversity of nebular attenuation curves can be accommodated by assuming dust grain properties similar to that of Milky Way sightlines but with a sub-unity covering fraction of dust. Integrated measurements of multiple Balmer and Paschen lines can be used to place novel constraints on the dust covering fraction towards OB associations. These, in turn, provide new avenues for exploring the role of the dust and gas covering fractions in a number of relevant aspects of high-redshift galaxies, including the impact of stellar feedback on ISM porosity and the escape of Lyα and Lyman continuum radiation.
\n
\n
We use JWST/NIRSpec observations from the Assembly of Ultradeep Rest-optical Observations Revealing Astrophysics survey to constrain the shape of the nebular attenuation curve of a star-forming galaxy at z = 4.41, GOODSN-17940. We utilize 12 H i recombination lines to derive the attenuation curve spanning optical to near-infrared wavelengths (3751–9550 Å). We then leverage a high signal-to-noise ratio spectroscopic detection of the rest-frame ultraviolet continuum in combination with rest-UV photometric measurements to constrain the shape of the curve at ultraviolet wavelengths. While this UV constraint is predominantly based on stellar emission, the large measured equivalent widths of Hα and Hβ indicate that GOODSN-17940 is dominated by an extremely young stellar population <10 Myr in age such that the UV stellar continuum experiences similar attenuation to that of the nebular emission. The resulting combined nebular attenuation curve spans 1400–9550 Å and has a shape that deviates significantly from commonly assumed dust curves in high-redshift studies. Relative to the Milky Way, SMC, and Calzetti curves, the new curve has a steeper slope at long wavelengths (λ > 5000 Å) while displaying a similar slope across blue-optical wavelengths (λ = 3750–5000 Å). In the ultraviolet, the new curve is shallower than the SMC and Calzetti curves and displays no significant 2175 Å bump. This work demonstrates that the most commonly assumed dust curves are not appropriate for all high-redshift galaxies. These results highlight the ability to derive nebular attenuation curves for individual high-redshift sources with deep JWST/NIRSpec spectroscopy, thereby improving the accuracy of physical properties inferred from nebular emission lines.
\n
\n
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 μm, 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]λ5007/Hβ versus [N ii]λ6583/Hα, [S ii]λλ6717, 6731/Hα, and [O i]λ6300/Hα, and the ionization–metallicity diagram of [O iii]λ5007/[O ii]λ3727 (O32) versus ([O iii]λ5007+[O ii]λ3727)/Hβ (R23). We also consider a bluer rest-frame ionization–metallicity diagram introduced recently to characterize z > 10 galaxies, [Ne iii]λ3869/[O ii]λ3727 versus ([Ne iii]λ3869+[O ii]λ3727)/Hδ, as well as longer-wavelength diagnostic diagrams extending into the rest-frame near-IR: [O iii]λ5007/Hβ versus [S iii]λλ9069, 9532/[S ii]λλ6717, 6731 (S32), and He iλ1.083 μm/Paγ and [S iii]λ9532/Paγ versus [Fe ii]λ1.257μm/Paβ. 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]λ5007/Hβ versus [N ii]λ6583/Hα 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]λ1.257 μm, we must obtain better constraints on dust depletion in the high-redshift ISM.
\n