HST Emission Line Galaxies at z ~ 2: The Mystery of Neon

TL;DR

This study uses Hubble Space Telescope near-infrared spectroscopy to analyze neon emission lines in low-mass star-forming galaxies at z ~ 2, revealing unexpected line ratio enhancements that challenge existing models.

Contribution

It provides the first detailed analysis of [Ne~III] emission in z ~ 2 galaxies and explores potential explanations for the observed line ratio anomalies.

Findings

[Ne~III]/[O~II] ratios are marginally higher at z ~ 2 than in local galaxies.

[Ne~III]/[O~III] ratio is enhanced by approximately 0.2 dex.

Simple scenarios like oxygen depletion or denser HII regions do not fully explain the observations.

Abstract

We use near-IR grism spectroscopy from the Hubble Space Telescope to examine the strength of [Ne~III] 3869 relative to H-beta, [O~II] 3727 and [O~III] 5007 in 236 low mass (7.5 < log (Mstar/Msolar) < 10.5) star-forming galaxies in the redshift range 1.90 < z < 2.35. By stacking the data by stellar mass, we show that the [Ne~III]/[O~II] ratios of the z ~ 2 universe are marginally higher than those seen in a comparable set of local SDSS galaxies, and that [Ne~III]/[O~III] is enhanced by ~0.2 dex. We consider the possible explanations for this ~4-sigma result, including higher oxygen depletion out of the gas-phase, denser H~II regions, higher production of Ne22 via Wolf-Rayet stars, and the existence of a larger population of X-ray obscured AGN at z ~ 2 compared to z ~ 0. None of these simple scenarios, alone, are favored to explain the observed line ratios. We conclude by suggesting several avenues of future observations to further explore the mystery of enhanced [Ne~III] emission.