The Neon Gap: Probing Ionization with Dwarf Galaxies at z~1

Abstract

We present measurements of [NeIII]{\lambda}3869 emission in z~1 low-mass galaxies taken from the Keck/DEIMOS spectroscopic surveys HALO7D and DEEPWinds. We identify 167 individual galaxies with significant [NeIII] emission lines, including 112 "dwarf" galaxies with log(M_{\star}/M_{\odot}) < 9.5, with 0.3 < z < 1.4. We also measure [NeIII] emission from composite spectra derived from all [OII]{\lambda}{\lambda}3727,3729 line emitters in this range. This provides a unique sample of [NeIII]-emitters in the gap between well-studied emitters at z = 0 and 2 < z < 3. To study evolution in ionization conditions in the ISM over this time, we analyze the log([NeIII]{\lambda}3869/[OII]{\lambda}{\lambda}3727,3729) ratio (Ne3O2) as a function of the stellar mass and of the log([OIII]{\lambda}{\lambda}4959,5007/[OII]{\lambda}{\lambda}3727,3729) ratio (O32). We find that the typical star-forming dwarf galaxy at this redshift, as measured from the composite spectra, shares the Ne3O2-M_{\star} relation with local galaxies, but have higher O32 at given Ne3O2. This finding implies that the ionization and metallicity characteristics of the z~1 dwarf population do not evolve substantially from z~1 to z=0, suggesting that the known evolution in those parameter from z~2 has largely taken place by z~1. Individual [NeIII]-detected galaxies have emission characteristics situated between local and z~2 galaxies, with elevated Ne3O2 and O32 emission potentially explained by variations in stellar and nebular metallicity. We also compare our dwarf sample to similarly low-mass z > 7 galaxies identified in JWST Early Release Observations, finding four HALO7D dwarfs with similar size, metallicity, and star formation properties.