Hubble Space Telescope emission-line galaxies at_z - 2: The mystery of neon

We use near-infrared grism spectroscopy from the Hubble Space Telescope to examine the strength of [Ne III] λ3869 relative to Hβ, [OII] λ3727, and [OIII] λ5007 in 236 low-mass (7.5 ≲ log(M∗/M⊙ ) ≲ 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 Sloan Digital Sky Survey galaxies, and that [Ne III]/[O III] is enhanced by -0.2 dex. We consider the possible explanations for this -4σ result, including higher oxygen depletion out of the gas phase, denser H ii regions, higher production of ²²Ne via Wolf-Rayet stars, and the existence of a larger population of X-ray obscured active galactic nuclei 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.