The MOSDEF Survey: Neon as a Probe of ISM Physical Conditions at High Redshift

We present results on the properties of neon emission in z ∼ 2 star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly ionized neon ([Ne iii]λ3869) is detected at ≥3σ in 61 galaxies, representing ∼25% of the MOSDEF sample with Hα, Hβ, and [O iii]λ5007 detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [Ne iii]λ3869 detections and composite z ∼ 2 spectra binned by stellar mass. With no requirement of [Ne iii]λ3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [Ne iii]λ3869/[O ii]λ3727 ratio (Ne3O2) is anticorrelated with stellar mass in z ∼ 2 galaxies, as expected based on the mass–metallicity relation. It is also positively correlated with the [O iii]λ5007/[O ii]λ3727 ratio (O32), but z ∼ 2 line ratios are offset toward higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H ii regions. Despite the offset toward higher Ne3O2 at fixed O32 at z ∼ 2, biases in inferred Ne3O2-based metallicity are small. Accordingly, Ne3O2 may serve as an important metallicity indicator deep into the reionization epoch. Analyzing additional rest-optical line ratios including [Ne iii]λ3869/[O iii]λ5007 (Ne3O3) and [O iii]λ5007/Hβ (O3Hβ), we conclude that the nebular emission-line ratios of z ∼ 2 star-forming galaxies suggest a harder ionizing spectrum (lower stellar metallicity, i.e., Fe/H) at fixed gas-phase oxygen abundance, compared to systems at z ∼ 0. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission of an ionized interstellar medium in high-redshift star-forming galaxies irradiated by chemically young, α-enhanced massive stars.