CLEAR: The Ionization and Chemical-Enrichment Properties of Galaxies at
  1.1 < z < 2.3
  We use deep spectroscopy from the Hubble Space Telescope (HST)
Wide-Field-Camera 3 (WFC3) IR grisms combined with broad-band photometry to
study the stellar populations, gas ionization and chemical abundances in
star-forming galaxies at $z\sim 1.1-2.3$. The data stem from the CANDELS
Lyman-$\alpha$ Emission At Reionization (CLEAR) survey. At these redshifts the
grism spectroscopy measure the [OII] 3727, 3729, [OIII] 4959, 5008, 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 (Kewley et al. 2019),
which include an updated treatment of nebular gas pressure, log P/k = $n_e
T_e$. Compared to low-redshift samples ($z\sim 0.2$) at fixed stellar mass,
llog M / M$_\odot$ = 9.4-9.8, the CLEAR galaxies at z=1.35 (z=1.90) have lower
gas-phase metallicity, $\Delta$(log Z) = 0.25 (0.35) dex, and higher ionization
parameters, $\Delta$(log q) = 0.25 (0.35) dex, where U = q/c. We provide
updated analytic calibrations between the [OIII], [OII], 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 (sSFRs), where $\Delta$ log q = 0.4
$\Delta$(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 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.
