CLEAR: The Ionization and Chemical-enrichment Properties of Galaxies at 1.1 < z < 2.3

We use deep spectroscopy from the Hubble Space Telescope Wide-Field-Camera 3 IR grisms combined with broadband photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at z similar to 1.1-2.3. The data stem from the CANDELS Ly alpha Emission At Reionization (CLEAR) survey. At these redshifts, the grism spectroscopy measure the [O II] lambda lambda 3727, 3729, [O III]lambda lambda 4959, 5008, and H beta strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line-flux measurements to predictions from updated photoionization models (MAPPINGS V; Kewley et al.), which include an updated treatment of nebular gas pressure, logP/k=n(e)T(e) . Compared to low-redshift samples (z similar to 0.2) at fixed stellar mass, logM(*)/M-circle dot= 9.4-9.8, the CLEAR galaxies at z = 1.35 (1.90) have lower gas-phase metallicity, Delta(logZ) = 0.25 (0.35) dex, and higher ionization parameters, Delta(logq) = 0.25 (0.35) dex, where U equivalent to q/c. We provide updated analytic calibrations between the [O III], [O II], and H beta emission-line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star formation rates, where Delta log q similar or equal to 0.4 x Delta(log sSFR) , derived from changes in the strength of galaxy H beta equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with a higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.