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