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

Abstract

We present results on the properties of neon emission inz similar to 2 star-forming galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly ionized neon ([Neiii]lambda 3869) is detected at >= 3 sigma in 61 galaxies, representing similar to 25% of the MOSDEF sample with H alpha, H beta, and [Oiii]lambda 5007 detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [Neiii]lambda 3869 detections and compositez similar to 2 spectra binned by stellar mass. With no requirement of [Neiii]lambda 3869 detection, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. The [Neiii]lambda 3869/[Oii]lambda 3727 ratio (Ne3O2) is anticorrelated with stellar mass inz similar to 2 galaxies, as expected based on the mass-metallicity relation. It is also positively correlated with the [Oiii]lambda 5007/[Oii]lambda 3727 ratio (O32), butz similar to 2 line ratios are offset toward higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual H iiregions. Despite the offset toward higher Ne3O2 at fixed O32 atz similar to 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 [Neiii]lambda 3869/[Oiii]lambda 5007 (Ne3O3) and [Oiii]lambda 5007/H beta(O3H beta), we conclude that the nebular emission-line ratios ofz similar to 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 atz similar to 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,alpha-enhanced massive stars.