http://arxiv.org/abs/2008.08831
We present results on the properties of ionized neon emission in $z\sim2$ star-forming galaxies drawn from the complete MOSFIRE Deep Evolution Field (MOSDEF) survey. Doubly-ionized neon ([NeIII]$\lambda3869$) is detected at $\geq3\sigma$ in 61 galaxies, representing $\sim25$% of the sample of MOSDEF galaxies with H$\alpha$, H$\beta$, and [OIII]$\lambda5007$ detections at similar redshifts. We consider the neon emission-line properties of both individual galaxies with [NeIII]$\lambda3869$ detections and composite $z\sim2$ spectra binned by stellar mass. With no requirement of [NeIII]$\lambda3869$ detection for inclusion in the composite spectra, the latter provide a more representative picture of neon emission-line properties in the MOSDEF sample. We find that the [NeIII]$\lambda3869$/[OII]$\lambda3727$ ratio (Ne3O2) is anti-correlated with stellar mass, as expected based on the mass-metallicity relation. We also find that Ne3O2 is positively correlated with the [OIII]$\lambda5007$/[OII]$\lambda3727$ ratio (O32) in $z\sim2$ galaxies, but that $z\sim2$ line ratios are offset towards higher Ne3O2 at fixed O32, compared with both local star-forming galaxies and individual HII regions. Analyzing additional rest-optical line ratios including [NeIII]$\lambda3869$/[OIII]$\lambda5007$ (Ne3O3) and [OIII]$\lambda5007$/H$\beta$ (O3H$\beta$) and comparing with both local HII regions and predictions from photoionization models, we conclude that the nebular emission-line ratios of $z\sim2$ 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 $z\sim0$. These new results based on neon lend support to the physical picture painted by oxygen, nitrogen, hydrogen, and sulfur emission, of an ionized ISM in high-redshift star-forming galaxies irradiated by chemically young, $\alpha$-enhanced massive stars.
M. Jeong, A. Shapley, R. Sanders, et. al.
Fri, 21 Aug 20
-1140/51
Comments: 7 pages, 5 figures, submitted to ApJ Letters
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